Since Fisher's work in the 1930s, there has been a method to calculate in advance how well a given experiment will be able to estimate model parameters. This `Fisher matrix' approach has been invaluable in experimental design. In the same spirit, we present here a method to predict how well a given experiment can distinguish between different models, regardless of their parameters. From a Bayesian viewpoint, this involves computation of the Evidence. In this paper, we generalize the Fisher matrix approach from the context of parameter fitting to that of model testing, and show how the expected Bayesian Evidence can be computed under the same simplifying assumptions as the Fisher matrix approach for parameter estimation. We find that all that is needed to compute the expected evidence is the Fisher matrix itself. We illustrate the method with a study of how well upcoming and planned experiments should perform at distinguishing between Dark Energy models and modified gravity theories. In particular we consider the combination of 3D weak lensing, for which planned and proposed wide-field multi-band imaging surveys will provide suitable data, and probes of the expansion history of the Universe, such as proposed Supernova and baryonic acoustic oscillations surveys. We find that proposed large-scale weak lensing surveys from space should be able readily to distinguish General Relativity from modified gravity models.
On 2006 May 5, a four second duration, low energy, ~10^49 erg, Gamma-Ray Burst (GRB) was observed, spatially associated with a z=0.0894 galaxy. Here, we report the discovery of the GRB optical afterglow and observations of its environment using Gemini-south, Hubble Space Telescope (HST), Chandra, Swift and the Very Large Array. The optical afterglow of this GRB is spatially associated with a prominent star forming region in the Sc-type galaxy 2dFGRS S173Z112. Its proximity to a star forming region suggests that the progenitor delay time, from birth to explosion, is smaller than about 10 Myr. Our HST deep imaging rules out the presence of a supernova brighter than an absolute magnitude of about -11 (or -12.6 in case of ``maximal'' extinction) at about two weeks after the burst, and limits the ejected mass of radioactive Nickel 56 to be less than about 2x10^-4 solar mass (assuming no extinction). Although it was suggested that GRB 060505 may belong to a new class of long-duration GRBs with no supernova, we argue that the simplest interpretation is that the physical mechanism responsible for this burst is the same as for short-duration GRBs.
The Mercury-Manganese star Phi Her is a well known spectroscopic binary that has been the subject of a recent study by Zavala et al. (2006), in which they resolved the companion using long-baseline interferometry. The total mass of the binary is now fairly well established, but the combination of the spectroscopy with the astrometry has not resulted in individual masses consistent with the spectral types of the components. The motion of the center of light of Phi Her was clearly detected by the Hipparcos satellite. Here we make use of the Hipparcos intermediate data (`abscissa residuals') and show that by combining them in an optimal fashion with the interferometry the individual masses can be obtained reliably using only astrometry. We re-examine and then incorporate existing radial-velocity measurements into the orbital solution, obtaining improved masses of 3.05 +/- 0.24 M_Sun and 1.614 +/- 0.066 M_Sun that are consistent with the theoretical mass-luminosity relation from recent stellar evolution models. These mass determinations provide important information for the understanding of the nature of this peculiar class of stars.
A new international Pilot Project for the re-determination of the ICRF was initiated by the International VLBI Service for Geodesy and Astrometry (IVS) in January 2005. The purpose of this project is to compare the individual CRF solutions and to analyze their systematic and random errors with focus on the selection of the optimal strategy for the next ICRF realization. Eight radio source catalogues provided by the IVS Analysis Centers GA, SHAO, DGFI, GIUB-BKG, JPL, MAO NANU, GSFC, USNO were analyzed. In present study, four analytical models were used to investigate the systematic differences between solutions: solid rotation, rotation and deformation (IERS method), and expansion in orthogonal functions: Legendre-Fourier polynomials and spherical functions. It was found that expansions by orthogonal function describe the differences between individual catalogues better than the two former models. Finally, the combined CRF was generated. Using the radio source positions from this combined catalogue for estimation of EOP has shown improvement of the uncertainty of the celestial pole offset time series.
We use a large cosmological N-body simulation to study the origin of possible correlations between the merging history and spin of cold dark matter halos. In particular, we examine claims that remnants of major mergers tend to have higher-than-average spins, and find that the effect is driven largely by unrelaxed systems: equilibrium dark matter halos show no significant correlation between spin and merger history. Out-of-equilibrium halos have, on average, higher spins than relaxed systems, suggesting that the virialization process leads to a net decrease in the value of the spin parameter. We find that this decrease is driven by the internal redistribution of mass and angular momentum that occurs during virialization, a process that is especially efficient during major mergers, when high angular momentum material is pushed beyond the virial radius of the remnant. Since such redistribution likely affects the angular momentum of baryons and dark matter unevenly, our findings question the common practice of identifying the specific angular momentum content of a halo with that of its embedded luminous component. Further work is needed to elucidate the true relation between the angular momentum content of baryons and dark matter in galaxy systems assembled hierarchically.
Recent proper motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil et al (2006a,b) suggest that the 3D velocities of the Clouds are substantially higher (~100 km/s) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we re-examine the orbital history of the Clouds using the new velocities and a LCDM-motivated MW model with virial mass Mvir = 1e12 Msun (e.g. Klypin et al 2002). We conclude that the Clouds are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances >200 kpc (i.e., Mvir > 2e12 Msun), that their orbital period and apogalacticon distance must be a factor of two larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of HI gas trailing the LMC and SMC that extends roughly 100 degrees across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.
Reverberation mapping methods have been used to measure masses in about three dozen AGNs. The consistency of the virial masses computed from line widths and time delays, the relationship between black hole mass and host-galaxy stellar bulge velocity dispersion, and the consistency with black hole masses estimated from stellar dynamics in the two cases in which such determinations are possible all indicate that reverberation mass measurements are robust and are accurate to typically a factor of a few. The reverberation-mapped AGNs are of particular importance because they anchor the scaling relationships that allow black hole mass estimation based on single spectra. We discuss potential sources of systematic error, particularly with regard to how the emission line widths are measured.
We study the UV and optical properties of 38 massive GCs in M87, imaged using the HST/STIS and WFPC2 instruments. The majority of these GCs appear extremely bright in the FUV - roughly a magnitude brighter than their Galactic counterparts with similar metallicities. The observed FUV flux is several times larger than predictions of canonical old stellar population models. These canonical models, which assume a fiducial helium enrichment parameter, dY/dZ=2, are able to reproduce the observed FUV fluxes only if ages are 3--5 Gyr larger than the WMAP age of the Universe, although the same models fit the UV photometry of Galactic and M31 GCs for ages less than the WMAP age. A similar discrepancy is found between the mass-weighted and UV-luminosity weighted ages of the massive Galactic GC omega Cen, whose CMD (including peculiar features on its well-populated horizontal branch) can be accurately reproduced by invoking a small super-He-rich (dY/dZ ~ 90) stellar component. By comparison to omega Cen, we propose that the majority of M87 GCs in our sample contain strong signatures of similarly minor super-He-rich sub-components. Although we cannot prove or disprove the extreme helium scenario at the moment, we show that the same phenomenon that causes the extended horizontal branch of omega Cen explains the UV brightness of our sample. If this is indeed due to the extreme helium, this study would be the first to find its signatures in extragalactic objects.
Shock waves driven in the intergalactic medium during the merging of galaxy clusters have been observed in X-ray imaging and spectroscopy. Fluid motions inferred from the shock strength and morphology can be compared to the cold dark matter (CDM) distribution inferred from gravitational lensing. A detailed reconstruction of the CDM kinematics, however, must take into account the nontrivial response of the fluid intracluster medium to the collisionless CDM motions. We have carried out two-dimensional simulations of gas dynamics in cluster collisions. We analyze the relative motion of the clusters, the bow shock wave, and the contact discontinuity and relate these to X-ray data. We focus on the "bullet cluster," 1E 0657-56, a near head-on collision of unequal-mass clusters, for which the gas density and temperature jumps across the prominent bow shock imply a high shock velocity 4,700 km/s. The velocity of the fluid shock has been widely interpreted as the relative velocity of the CDM components. This need not be the case, however. An illustrative simulation finds that the present relative velocity of the CDM halos is 16% lower than that of the shock. While this conclusion is sensitive to the detailed initial mass and gas density profile of the colliding clusters, such a decrease of the inferred halo relative velocity would increase the likelihood of finding 1E 0657-56 in a LambdaCDM universe.
We present a comprehensive study of the dust and gas properties in the
after-head-on-collision UGC12914/15 galaxy system using multi-transition CO
data and SCUBA sub-mm continuum images at both 450 and 850$\mu$m. CO(3-2) line
emission was detected in the disks of UGC 12914 and UGC 12915 as well as in a
bridge connecting the two galaxies. Dust emission at 450$\mu$m was detected for
the first time in the two galactic disks and in the connecting bridge. Using an
LVG excitation analysis model we have obtained good estimates of the physical
parameters in different regions of this system and the amount of molecular gas
was found to be 3-4 times lower than that estimated by other investigators
using the standard Galactic CO-to-H2 conversion factor. Comparing with the dust
mass derived from the SCUBA data, we found that the gas-to-dust ratio was
comparable to the Galactic value in the two galaxy disks but a factor of ~3
higher in the bridge. The physical condition of the molecular gas in the bridge
is comparable to that in the diffuse clouds in our Galaxy. Our result is
consistent with the scenario that the bridge molecular gas originated from the
disk molecular clouds and has been drawn out of the galactic disks due to
direct cloud-cloud collision.
Our data indicate that the global star formation efficiency (SFE) in UGC
12915 is comparable to that of normal spiral galaxies, and the SFE is 40% lower
in UGC 12914 than in UGC 12915. Little star formation activity was found in the
bridge except in an HII region adjacent to the disk of UGC 12915.
Origin of turbulence in cold accretion disks, particularly in 3D, which is expected to be hydrodynamic but not magnetohydrodynamic, is a big puzzle. While the flow must exhibit some turbulence in support of the transfer of mass inward and angular momentum outward, according to the linear perturbation theory it should always be stable. We demonstrate that the 3D secondary disturbance to the primarily perturbed disk which exhibits elliptical vortices into the system solves the problem. This result is essentially applicable to the outer region of accretion disks in active galactic nuclei where the gas is significantly cold and neutral in charge and the magnetic Reynolds number is smaller than 10^4.
In this paper we study the sound speed, c_s^2, directly related to the classical perturbations, of the dynamical dark energy (DE) especially with an equation of state crossing the cosmological constant boundary in details and show its implications on Cosmic Microwave Background (CMB) Anisotropy. With the present observational data of CMB, Type Ia Supernova (SNIa) and galaxy clustering, we perform a global analysis to constrain the sound speed of dark energy using the Markov Chain Monte Carlo method. We find that the sound speed of dark energy is weakly constrained by current observations thus the futuristic precision measurements of CMB on very large angular scale (low multipoles) are necessary.
The theory of polar magnetic burial in accreting neutron stars predicts that a mountain of accreted material accumulates at the magnetic poles of the star, and that, as the mountain spreads equatorward, it is confined by, and compresses, the equatorial magnetic field. Here, we extend previous, axisymmetric, Grad-Shafranov calculations of the hydromagnetic structure of a magnetic mountain up to accreted masses as high as $\Ma = 6\times 10^{-4}\Msun$, by importing the output from previous calculations (which were limited by numerical problems and the formation of closed bubbles to $\Ma < 10^{-4}\Msun$) into the time-dependent, ideal-magnetohydrodynamic code ZEUS-3D and loading additional mass onto the star dynamically. The rise of buoyant magnetic bubbles through the accreted layer is observed in these experiments. We also investigate the stability of the resulting hydromagnetic equilibria by perturbing them in ZEUS-3D. Surprisingly, it is observed that the equilibria are marginally stable for all $\Ma\leq 6\times 10^{-4}\Msun$; the mountain oscillates persistently when perturbed, in a combination of Alfv\'en and acoustic modes, without appreciable damping or growth, and is therefore not disrupted (apart from a transient Parker instability initially, which expels $< 1 %$ of the mass and magnetic flux).
We report multifrequency radio continuum and hydrogen radio recombination
line observations of HII regions near l=24.8d b=0.1d using the Giant Metrewave
Radio Telescope (GMRT) at 1.28 GHz (n=172), 0.61 GHz (n=220) and the Very Large
Array (VLA) at 1.42 GHz (n=166). The region consists of a large number of
resolved HII regions and a few compact HII regions as seen in our continuum
maps, many of which have associated infrared (IR) point sources. The largest
HII region at l=24.83d and b=0.1d is a few arcmins in size and has a shell-type
morphology. It is a massive HII region enclosing ~ 550 solar mass with a linear
size of 7 pc and an rms electron density of ~ 110 cm^-3 at a kinematic distance
of 6 kpc. The required ionization can be provided by a single star of spectral
type O5.5.
We also report detection of hydrogen recombination lines from the HII region
at l=24.83d and b=0.1d at all observed frequencies near Vlsr=100 km/s. We model
the observed integrated line flux density as arising in the diffuse HII region
and find that the best fitting model has an electron density comparable to that
derived from the continuum. We also report detection of hydrogen recombination
lines from two other HII regions in the field.
We report here discovery of Quasi Periodic Oscillations (QPOs) in the High Mass X-ray Binary (HMXB) Pulsar XTE J0111.20-7317 during a transient outburst in this source in December 1998. Using observations made with the proportional counter array of the Rossi X-ray Timing Explorer during the second peak and the declining phase of this outburst we have discovered a QPO feature at a frequency of 1.27 Hz. We have ruled out the possibility that the observed QPOs can instead be from the neighbouring bright X-ray pulsar SMC X-1. This is the highest frequency QPO feature ever detected in any HMXB pulsar. In the absence of a cyclotron absorption feature in the X-ray spectrum, the QPO feature, along with the pulse period and X-ray flux measurement measurement helps us to constrain the magnetic field strength of the neutron star.
The weakly non-linear interaction of slow magnetoacoustic and torsional Alfven waves propagating along an unperturbed poloidal magnetic field in a stellar interior is studied in the high plasma beta limit. It is shown that slow magnetoacoustic waves parametrically drive torsional modes with the half frequency and wave number. Therefore global slow magnetoacoustic waves which have a radial velocity polarization, may amplify the torsional oscillations. Possible applications of this mechanism to the Sun and binary stars are briefly discussed.
This is the first of a series of papers in which the kinematics of disk galaxies over a range of scales is scrutinised employing spectroscopy. A fundamental aspect of these studies is presented here: the new publicly available software tool TiRiFiC (this http URL) enables a direct fit of a ``tilted-ring model'' to spectroscopic data cubes. The algorithm generates model data cubes from the tilted-ring parametrisation of a rotating disk, which are automatically adjusted to reach an optimum fit via a chi-squared minimisation method to an observed data cube. The structure of the new software, the shortcomings of the previously available programs to produce a tilted-ring model, and the performance of TiRiFiC are discussed. Our method is less affected by the well-known problem of beam smearing that occurs when fitting to the velocity field. Since with our method we fit many data points in a data cube simultaneously, TiRiFiC is sensitive to very faint structures and can hence be used to derive tilted-ring models significantly extending in radius beyond those derived from a velocity field. The software is able to parametrise HI disks of galaxies that are intersected by the line-of-sight twice or more, i.e. if the disks are heavily warped, and/or with a significant shift of the projected centre of rotation, and/or if seen edge-on. Furthermore, our method delivers the surface-brightness profile of the examined galaxy in addition to the orientational parameters and the rotation curve. In order to derive kinematic and morphological models of disk galaxies, especially reliable rotation curves, a direct-fit method as implemented in our code should be the tool of choice.
We present metallicity measurements based on GIRAFFE@VLT spectra of 80 subgiant-branch stars of the Galactic globular cluster Omega Centauri. The VLT spectroscopic data are complemented by ACS/HST and WFI@ESO2.2m high-accuracy color-magnitude diagrams. We have obtained the [Fe/H] abundance for each of the 80 target stars, and the abundances of C, N, Ca, Ti, and Ba for a subset of them, by comparison with synthetic spectra. We show that stars with [Fe/H]<-1.25 have a large magnitude spread on the flat part of the SGB. We interpret this as empirical evidence for an age spread. We have identified four distinct stellar groups within the SGB region: (i) an old, metal-poor group ([Fe/H]~-1.7); (ii) an old, metal-rich group ([Fe/H]~-1.1); (iii) a young (up to 4--5 Gyr younger than the old component) metal-poor group ([Fe/H]~-1.7); (iv) a young, intermediate-metallicity ([Fe/H]~-1.4) group, on average 1--2 Gyr younger than the old metal-poor population, and with an age spread that we cannot properly quantify with the present sample. In addition, a group of SGB stars are spread between the intermediate-metallicity and metal-rich branches of the SGB. The spread in age within each population establishes that the progenitor of Omega Cen system must have had a composite nature.
The standard model of GRB afterglows assumes that the radiation observed as a delayed emission is of synchrotron origin, which requires the shock magnetic field to be relatively homogeneous on small scales. An alternative mechanism -- jitter radiation, which traditionally has been applied to the prompt emission -- substitutes synchrotron when the magnetic field is tangled on a microscopic scale. Such fields are produced at relativistic shocks by the Weibel instability. Here we explore the possibility that small-scale fields populate afterglow shocks. We derive the spectrum of jitter radiation under the afterglow conditions. We also derive the afterglow lightcurves for the ISM and Wind profiles of the ambient density. Jitter self-absorption is calculated here for the first time. We find that jitter radiation can produce afterglows similar to synchrotron-generated ones, but with some important differences. We compare the predictions of the two emission mechanisms. By fitting observational data to the synchrotron and jitter afterglow lightcurves, it can be possible to discriminate between the small-scale vs large-scale magnetic field models in afterglow shocks.
We present a quantitative analysis of the astrophysical and cosmological information that can be extracted from the many important wide-area, shallow surveys that will be carried out in the next few years. Our calculations combine the predictions of the physical model by Granato et al. (2004) for the formation and evolution of spheroidal galaxies with up-to-date phenomenological models for the evolution of starburst and normal late-type galaxies and of radio sources. We compute the expected number counts and the redshift distributions of these source populations separately and then focus on proto-spheroidal galaxies. For the latter objects we predict the counts and redshift distributions of strongly lensed sources at 250, 350, 500, and 850 micron, the angular correlation function of sources detected in the surveys considered, the angular power spectra due to clustering of sources below the detection limit in Herschel and Planck surveys. An optimal survey for selecting strongly lensed proto-spheroidal galaxies is described, and it is shown how they can be easily distinguished from the other source populations. We also discuss the detectability of the imprints of the 1-halo and 2-halo regimes on angular correlation functions and clustering power spectra, as well as the constraints on cosmological parameters that can be obtained from the determinations of these quantities. The novel data relevant to derive the first sub-millimeter estimates of the local luminosity functions of starburst and late-type galaxies, and the constraints on the properties of rare source populations, such as blazars, are also briefly described.
We study the stellar and wind properties of massive stars in the central cluster of the Galaxy. We use non-LTE atmosphere models including winds and line-blanketing to fit their H and K band spectra obtained with the 3D spectrograph SINFONI on the VLT. We derive the main stellar (Teff, L, abundances, ionizing flux) and wind (mass loss rate, terminal velocity) properties. They are found to be similar to other galactic massive stars. We show that a direct evolutionary link between Ofpe/WN9, WN8 and WN/C stars exists. Using individual SEDs for each massive star, we construct the total spectral energy distribution of the cluster and use it to compute photoionization models. We show that the nebular properties of the central HII region are well reproduced. We conclude that, contrary to previous claims, standard stellar evolution and atmosphere models are well suited to explain the properties of the central cluster. Our results indicate that massive stars in the central cluster do not have a peculiar evolution as could be expected from their proximity to the supermassive black hole SgrA*.
We investigate the enhancement of star formation efficiency in galaxy interactions and mergers, by numerical simulations of several hundred galaxy collisions. All morphological types along the Hubble sequence are considered in the initial conditions of the two colliding galaxies, with varying bulge-to-disk ratios and gas mass fractions. Different types of orbits are simulated, direct and retrograde, according to the initial relative energy and impact parameter, and the resulting star formation history is compared to that occuring in the two galaxies when they are isolated. Our principal results are: (1) retrograde encounters have a larger star formation efficiency (SFE) than direct encounters; (2) the amount of gas available in the galaxy is not the main parameter governing the SFE in the burst phase; (3) there is an anticorrelation between the amplitude of the star forming burst and the tidal forces exerted per unit of time, which is due to the large amount of gas dragged outside the galaxy by tidal tails in strong interactions; (4) globally, the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies, interacting pairs and mergers; (5) the enhanced star formation is essentially occurring in nuclear starbursts, triggered by inward gas flows driven by non-axisymmetries in the galaxy disks. Direct encounters develop more pronounced asymmetries than retrograde ones. Based on these statistical results, we derive general laws for the enhancement of star formation in galaxy interactions and mergers, as a function of the main parameters of the encounter.
Physical dust models are presented for 65 galaxies in the SINGS survey that
are strongly detected in the four IRAC bands and three MIPS bands. For each
galaxy we estimate (1) the total dust mass, (2) the fraction of the dust mass
contributed by PAHs, and (3) the intensity of the starlight heating the dust
grains. We find that spiral galaxies have dust properties resembling the dust
in the local region of the Milky Way, with similar dust-to-gas ratio, and
similar PAH abundance. The observed SEDs, including galaxies with SCUBA
photometry, can be reproduced by dust models that do not require "cold" (T<10K)
dust.
The dust-to-gas ratio is observed to be dependent on metallicity. In the
interstellar media of galaxies with A_O=12+log(O/H)>8.1, grains contain a
substantial fraction of interstellar Mg, Si and Fe. Galaxies with A_O<8.1 and
extended HI envelopes in some cases appear to have global dust-to-gas ratios
that are low for their measured oxygen abundance, but in the regions where
infrared emission is detected, the dust-to-gas ratio generally appears to be
consistent with a substantial fraction of interstellar Mg, Si, and Fe being
contained in dust. The PAH index q_PAH -- the fraction of the dust mass in the
form of PAHs -- correlates with metallicity. The nine galaxies in our sample
with A_O<8.1 have a median q_PAH=1.0%, whereas galaxies with A_O>8.1 have a
median q_PAH=3.55%. The derived dust masses favor a value X_CO approx 4e20
cm^{-2}(K kms)^{-1} for the CO to H_2 conversion factor. Except for some
starbursting systems (Mrk33, Tolo89, NGC3049), dust in the diffuse ISM
dominates the IR power.
(Abridged) High-redshift ultra luminous infrared galaxies contribute the bulk of the cosmic IR background and are the best candidates for very massive galaxies in formation at z>1.5. We present Keck/LRIS optical spectroscopy of 35 z>1.4 luminous IR galaxies in the Spitzer Wide-area Infra-Red Extragalactic survey (SWIRE) northern fields (Lockman Hole, ELAIS-N1, ELAIS-N2). The primary targets belong to the ``IR-peak'' class of galaxies, having the 1.6 micron (restframe) stellar feature detected in the IRAC Spitzer channels.The spectral energy distributions of the main targets are thoroughly analyzed, by means of spectro-photometric synthesis and multi-component fits (stars + starburst dust + AGN torus). The IR-peak selection technique is confirmed to successfully select objects above z=1.4, though some of the observed sources lie at lower redshift than expected. Among the 16 galaxies with spectroscopic redshift, 62% host an AGN component, two thirds being type-1 and one third type-2 objects. The selection, limited to r'<24.5, is likely biased to optically-bright AGNs. The SEDs of non-AGN IR-peakers resemble those of starbursts (SFR=20-500 Msun/yr) hosted in massive (M>1e11 Msun) galaxies. The presence of an AGN component provides a plausible explanation for the spectroscopic/photometric redshift discrepancies, as the torus produces an apparent shift of the peak to longer wavelengths. These sources are analyzed in IRAC and optical-IR color spaces. In addition to the IR-peak galaxies, we present redshifts and spectral properties for 150 objects, out of a total of 301 sources on slits.
(4) Vesta and (9) Metis are large main-belt asteroids with high albedos. With millimetre-observations at 93.0 and 95.5 GHz we characterised the emission properties of the surface material. The coverage of the full rotation period allowed a detailed study of the heterogeneity of the surface. The rotationally averaged fluxes are explained very well by our thermophysical model techniques when using an emissivity in the mm-range of about 0.6 for (4) Vesta and about 0.7 for (9) Metis. The mm-lightcurves follow for a large fraction of the rotation period the shape-introduced variations. The rotational phases with clear deviations are connected to structures which are visible in the HST images of (4) Vesta and the Keck AO-images of (9) Metis. The observed lightcurve amplitudes are peak-to-peak ~30% for (4) Vesta and ~25% for (9) Metis, while the shape-related amplitudes are only 5 and 4%, respectively. The emissivities at mm-wavelengths are lower than in the far-IR, confirming that particles with sizes of about 100 mikron influence the mm-behaviour. The 3-mm observations are very powerful for the study of asteroid surface heterogeneities.
We present a comprehensive study that applies the Fourier transform to a
sample of O and early B-type stars (either dwarfs, giants, or supergiants) to
determine their projected rotational velocities, compare with previous values
obtained with other methods, and seek for evidence of extra broadening in the
spectral lines
The Fourier technique, extensively used in the study of cooler stars, has
remained only marginally applied for the case of early-type stars. The
comparison of \vsini values obtained through the \ft and \fwhm methods shows
that the \fwhm technique must be used with care in the analysis of OB giants
and supergiants, and when it is applied to \ion{He}{i} lines. Contrarily, the
\ft method appears to be a powerful tool to derive reliable projected
rotational velocities, and separate the effect of rotation from other
broadening mechanisms present in these stars.
The analysis of the sample of OB stars shows that while dwarfs and giants
display a broad range of projected rotational velocities, from less than 30 up
to 450 \kms, supergiants have in general values close to or below 100 \kms. The
analysis has also definitely shown that while the effect of extra broadening is
negligible in OB dwarfs, it is clearly present in supergiants. When examining
the behavior of the projected rotational velocities with the stellar parameters
and across the HR diagram, we conclude, in agreement with previous researchers,
that the rotational velocity should decrease when the stars evolve. On the
contrary, macroturbulence may be constant, resulting therefore in an increasing
importance as compared to rotation when the stars evolve.
We have undertaken a program to observe emission lines of SIV 10.5, NeII 12.8, NeIII 15.6, & SIII 18.7 um in a number of extragalactic HII regions with the Spitzer Space Telescope. We report our results for the nearly face-on spiral galaxy M83. The nebulae selected cover a wide range of galactocentric radii (R_G). The observations were made with the Infrared Spectrograph in the short wavelength, high dispersion configuration. The above set of 4 lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne++/Ne+, S3+/S++, and S++/Ne+ and find that there is a correlation of increasingly higher ionization with larger R_G. By sampling the dominant ionization states of Ne and S for HII regions, Ne/S ~ (Ne+ + Ne++)/(S++ + S3+). Our findings of ratios that exceed the benchmark Orion value are more likely due to other effects than a true gradient in Ne/S. Both Ne and S are primary elements produced in alpha- chain reactions. It is expected that Ne/S remains relatively constant throughout a galaxy. This type of observation and method of analysis has the potential for accurate measurements of Ne/S, particularly for HII regions with lower metallicity & higher ionization than those here, such as those in M33. Our observations may also be used to test the predicted ionizing spectral energy distribution of various stellar atmosphere models. We compare the fractional ionization ratios <Ne++>/<S++> & <Ne++>/<S3+> vs. <S3+>/<S++> with predictions made from our photoionization models using several state-of-the-art stellar atmosphere model grids. A second paper using Spitzer observations of HII regions in the galaxy M33 will follow.
We obtained a high-resolution VLT-UVES spectrum of the quasar Q2348-011 over a wavelength range that covers most of the prominent metal and molecular absorption lines from the log N(HI)=20.50+-0.10 damped Lyman-alpha system at zabs=2.43. From the column density ratios and the relative populations of H2 rotational and CI fine-structure levels, we derive the physical conditions (relative abundances, dust-depletion, particle density, kinetic temperature and ionizing flux) and discuss physical conditions in the neutral phase. Molecular hydrogen is detected in seven components in the first four rotational levels (J = 0-3) of the vibrational ground state. Absorption lines of H2 J = 4 (resp. J = 5) are also detected in six (resp. two) of these components. This leads to a total molecular fraction of log f = -1.69+0.37-0.58. We confirm the earlier findings that there is a correlation between N(FeII)/N(SII) and N(SiII)/N(SII) indicative of a dust-depletion pattern. Surprisingly, however, the depletion of metals onto dust in the H2 components is not large in this system: [Fe/S] = -0.8 to -0.1. The gas in H2-bearing components is found to be cold but still hotter than similar gas in our Galaxy (T > 130 K, instead of typically 80 K). and dense (n=100-200 cm^-3). There is an anti-correlation between the logarithm of the photo-absorption rate, and log N(H2)/N(CI) derived for each H2 component. We show that this is mostly due to shielding effects and implies that the photo- absorption rate is a good indicator of the physical conditions in the gas. We find that the gas is immersed in a intense UV field, about one order of magnitude higher than in the solar vicinity. The results suggest that the gas in H2-bearing DLAs is clumpy, and star-formation occurs in the associated object.
Relativistic black-hole jet sources are leading candidates for high energy (>> TeV) neutrino production. The relations defining (a) efficient photopion losses of cosmic ray protons on target photons and (b) gamma-gamma opacity of gamma rays through that same target photon field imply clear multiwavelength predictions for when and at what energies blazars and GRBs should be most neutrino bright and gamma-ray dim.
Comets can be divided into two groups: type I, characterized by high gas/dust ratio, low polarization, and a weak or absent 10 micron silicate feature, and type II, for which a low gas/dust ratio, high polarization, and strong silicate feature are typical. We show that the low polarization is the apparent result of depolarization by gas contamination at low dust concentration, which, in turn, results from the dust in type I comets being concentrated near the nucleus. The simulations of thermal emission show that for more porous particles (BCCA), the silicate feature is more pronounced than more compact ones (BPCA), for which it even vanishes as the particles become larger. We also show that in both types of comets the main contribution to light scattering and emission comes from particles larger than 10 micron. Conclusions: .The strength of the silicate feature in the cometary infrared spectra suggests that the dust in type II comets consists of high-porosity aggregates, whereas the dust of type I comets contains low-porosity ones. This is consistent with the polarimetric features of these comets, which indicate that the dust in type I comets tends to concentrate near the nucleus. This may result from the predominance of highly processed particles in type I comets, whereas in type II comets we see pristine or slightly-processed dust. This conclusion is in accordance with the orbital characteristics of the comets. We have found that the strength of the silicate feature correlates with the semi-major axis of periodic comets and, for short-period comets, with the perihelion distance. Thus, the silicate feature weakens due to compaction of aggregate particles if a comet spends more time in the vicinity of the Sun, which allows the comet to accumulate a mantle on the surface of its nucleus.
Comet C/1999 S4 (LINEAR) was exceptional in many respects. Its nucleus underwent multiple fragmentations culminating in the complete disruption around July 20, 2000. We present circular polarization measurements along the cuts through the coma and nucleus of the comet during three separate observing runs, in June 28 - July 2, July 8 - 9, and July 21 - 22, 2000. The circular polarization was detected at a rather high level, up to 0.8%. The left-handed as well as right-handed polarization was observed over the coma with the left circularly polarized light systematically observed in the sunward part of the coma. During our observations the phase angle of the comet varied from 61 up to 122 deg., which allowed us to reveal variations of circular polarization with the phase angle. Correlation between the degree of circular polarization, visual magnitude, water production rate, and linear polarization of Comet C/1999 S4 (LINEAR) during its final fragmentation in July 2000 was found. The mechanisms that may produce circular polarization in comets and specifically in Comet C/1999 S4 (LINEAR) are discussed and some tentative interpretation is presented.
We present a progress report of a project to study the quantitative star formation history (SFH) in different parts of the Small Magellanic Cloud (SMC). We use the information in [(B-R), R] color-magnitude diagrams (CMDs), which reach down to the oldest main-sequence turnoffs and allow us to retrieve the SFH in detail. We show the first results of the SFH in a SMC field located in the Southern direction (at $\thicksim$1 kpc from the SMC center). This field is particularly interesting because in spite of being located in a place in which the HI column density is very low, it still presents a recent enhancement of star formation.
An analysis of the interaction between a spherical relativistic blast-wave shell and a stationary cloud with a spherical cap geometry is performed assuming that the cloud width << x, where x is the distance of the cloud from the GRB explosion center. The interaction is divided into three phases: (1) a collision phase with both forward and reverse shocks; (2) a penetration phase when either the reverse shock has crossed the shell while the forward shock continues to cross the cloud, or vice versa; and (3) an expansion phase when, both shocks having crossed the cloud and shell, the shocked fluid expands. Temporally evolving spectral energy distributions (SEDs) are calculated for the problem of the interaction of a blast-wave shell with clouds that subtend large and small angles compared with the Doppler(-cone) angle 1/\Gamma_0, where \Gamma_0 is the coasting Lorentz factor. The Lorentz factor evolution of the shell/cloud collision is treated in the adiabatic limit. Behavior of the light curves and SEDs on, e.g., \Gamma_0, shell-width parameter \eta, and properties and locations of the cloud is examined. Short timescale variability (STV) in GRB light curves, including ~100 keV \gamma-ray pulses observed with BATSE and delayed ~1 keV X-ray flares found with Swift, can be explained by emissions from an external shock formed by the GRB blast wave colliding with small density inhomogeneities in the "frozen pulse" approximation (\eta --> 0, where \Delta_0 + \eta x/\Gamma_0^2 is the blast-wave shell width), and perhaps in the thin-shell approximation (\eta ~ 1/\Gamma_0), but not when \eta ~ 1. If this approximation is valid, then external shock processes could make the dominant prompt and afterglow emissions in GRB light curves, consistent with short delay two-step collapse models for GRBs.
We present the results of a comprehensive Spitzer survey of 69 radio galaxies across 1<z<5.2. Using IRAC (3.6-8.0um), IRS (16um) and MIPS (24-160um) imaging, we decompose the rest-frame optical to infrared spectral energy distributions into stellar, AGN, and dust components and determine the contribution of host galaxy stellar emission at rest-frame H-band. Stellar masses derived from rest-frame near-IR data, where AGN and young star contributions are minimized, are significantly more reliable than those derived from rest-frame optical and UV data. We find that the fraction of emitted light at rest-frame H-band from stars is >60% for ~75% the high redshift radio galaxies. As expected from unified models of AGN, the stellar fraction of the rest-frame H-band luminosity has no correlation with redshift, radio luminosity, or rest-frame mid-IR (5um) luminosity. Additionally, while the stellar H-band luminosity does not vary with stellar fraction, the total H-band luminosity anti-correlates with the stellar fraction as would be expected if the underlying hosts of these radio galaxies comprise a homogeneous population. The resultant stellar luminosities imply stellar masses of 10^{11-11.5}Msun even at the highest redshifts. Powerful radio galaxies tend to lie in a similar region of mid-IR color-color space as unobscured AGN, despite the stellar contribution to their mid-IR SEDs at shorter-wavelengths. The mid-IR luminosities alone classify most HzRGs as LIRGs or ULIRGs with even higher total-IR luminosities. As expected, these exceptionally high mid-IR luminosities are consistent with an obscured, highly-accreting AGN. We find a weak correlation of stellar mass with radio luminosity.
We present deep ground-based {\it B} and {\it R} observations of 12 fields in
the Small Magellanic Cloud (SMC). The resulting color-magnitude diagrams (CMDs)
reach the oldest main-sequence (MS) turnoff at M$_{R}$$\thicksim$3.5 and reveal
the stellar population differences between the part of the galaxy facing the
Large Magellanic Cloud (LMC) and an area on the opposite side. In the Southern
part of the galaxy, we found that there are still intermediate-age stars as far
as 4 kpc from the SMC center.
The Chemical Enrichment History (CEH) in one of our SMC fields is also
presented.
Central gravitational image detection is very important for the study of the mass distribution of the inner parts ($\sim 100$ pc) of lens galaxies. However, the detection of such images is extremely rare and difficult. We present a 1.7-GHz High Sensitivity Array (HSA) observation of the double-image radio lens system B1030+074. The data are combined with archive VLBA and global-VLBI observations, and careful consideration is given to the effects of noise, {\sc clean}ing and self-calibration. An upper limit is derived for the strength of the central image of 180 $\mu$Jy (90% confidence level), considerably greater than would have been expected on the basis of a simple analysis. This gives a lower limit of $\sim 10^3$ for the ratio of the brightest image to the central image. For cusped models of lens mass distributions, we have made use of this non-detection to constrain the relation between inner power-law slope $\beta$ of the lensing galaxy mass profile, and its break radius $r_b$. For $r_b>130$ pc the power-law slope is required to be close to isothermal ($\beta>1.8$). A flatter inner slope is allowed if a massive black hole is present at the centre of the lensing galaxy, but the effect of the black hole is small unless it is $\sim 10$ times more massive than that implied by the relation between black hole mass and stellar velocity dispersion. By comparing four epochs of VLBI observations, we also detected possible superluminal motion in the jet in the brighter A image. The B jet remains unresolved, as expected from a simple lens model of the system.
Large wavelength shifts of infrared triplet lines of Ca II have been observed
in the spectra of HgMn and magnetic Ap stars. They have been attributed to the
heavy calcium isotopes, including Ca-48. One member of the triplet,
$\lambda$8542, had been either unavailable, or of poor quality in earlier
spectra. The present material shows conclusively that the stellar $\lambda$8542
shifts are consistent with an interpretation in terms of Ca-48. We find no
relation between isotopic shifts of the Ca II triplet lines, and those of Hg II
$\lambda$3984. There is a marginal indication that the shifts are {\it
anticorrelated} with the surface field strengths of the magnetic stars. We see
sparse evidence for Ca-48 in other chemically peculiar stars, e.g. Am's,
metal-poor stars, or chemically peculiar red giants. However, the sample is
still very small, and the wavelengths of all three triplet lines, including
those in the Sun, show slight positive shifts with respect to terrestrial
positions.
Some profiles of the Ca II infrared triplet in the magnetic stars show
extensive wings beyond a well-defined core. We can obtain reasonable fits to
these profiles using a stratified calcium abundance similar to that used by
previous workers. There is no indication that either the stratification or the
Zeeman effect significantly disturbs the measurement of isotope shifts.
We report the discovery of resolved magnetically split lines in two chemically peculiar stars, the SrEuCr star HD 92499 and the Bp SiCr star HD 157751. From FEROS spectra, we have measured a mean magnetic field modulus of 8.5 kG for HD 92499 and a mean magnetic field modulus of 6.6 kG for HD 157751. Both stars have small projected rotational velocities: v sin i = 3.0 km/s and 8.5 km/s, respectively. Our preliminary abundance analysis reveals ionisation imbalance of rare earths in HD 92499 indicating the abundance pattern typical of rapidly oscillating Ap stars. Cr and Fe are found strongly overabundant in HD 157751.
The nature of non-variable high-excitation emission lines detected in the
optical spectra of normal late-B type and chemically peculiar HgMn and PGa
stars is still poorly understood.
To better understand the origin of the weak emission lines in B type stars it
is especially important to investigate the spectra of a variety of stars to
search for correlations between the emergence of these lines and fundamental
stellar parameters.
We have acquired high resolution UVES spectra for the sharp-lined magnetic
helium-variable star a Cen over the rotation period of 8.82 d to search for the
presence of weak emission lines.
For the first time we present observational evidence for the appearance of
variable high-excitation Si II, Mn II and Fe II emission lines in a magnetic Bp
star. Si II emissions are the strongest at the phase corresponding to the
maximum strength of He I lines. Mn II and Fe II emissions vary in antiphase to
the He I lines. A correlation is found between the probable location of Mn and
Fe surface spots and the strength of the emission lines. On the basis of the
currently available data it seems possible that the same kind of selective
excitation process is working in the atmospheres of objects within a broad
parameter space which could be defined by age, effective temperature, chemical
composition, rotational velocity, and magnetic field. Neutral iron lines
previously reported to appear broad and shallow at certain phases are not
detected in our spectra, although two of them are identified as He I forbidden
lines, showing maximum strength at the phase of the passage of the He rich
region across the visible disk.
For most of the Ultra High Energy Cosmic Ray (UHECR) experiments and projects (HiRes, AUGER, TA, JEM-EUSO, TUS,...), the detection technique of Extensive Air Showers (EAS) is based, at least, on the measurement of the air fluorescence induced signal. The knowledge of the Fluorescence Light Yield (FLY) is of paramount importance for the UHECR energy reconstruction. The MACFLY experiment was designed to perform such FLY measurements. In this paper we will present the results of dry air FLY induced by 50 GeV electromagnetic showers as a function of shower age and as a function of the pressure. The experiment was performed at CERN using an SPS electron test beam line. It is shown that the FLY is proportional to deposited energy in air (E_d) and that the ratio FLY/E_d and its pressure dependence remain constant independently of shower age and more generally independently of the excitation source used (single electron track or air shower).
Although indirect evidence for the presence of magnetic fields in high-mass stars is regularly reported in the literature, the detection of these fields remains an extremely challenging observational problem. We review the recent discoveries of magnetic fields in different types of massive stars and briefly discuss strategies for spectropolarimetric observations to be carried out in the future.
Deep Chandra exposures of the hot galaxy cluster 1E0657-56 have revealed that the cluster is observed shortly after the first core-passage of a massive infalling subcluster, which is preceded by a prominent bow shock with Mach number M~3. The inferred shock velocity of ~4700 km/s has been commonly interpreted as the velocity of the `bullet' subcluster itself. This velocity is unexpectedly high in the LCDM cosmology, which may require non-trivial modifications in the dark sector to be accommodated if taken at face value. Here we present explicit hydrodynamical toy models of galaxy cluster mergers which very well reproduce the observed dynamical state of 1E0657-56 and the mass models inferred from gravitational lensing observations. However, despite a shock speed of 4500 km/s, the subcluster's mass centroid is moving only with 2600 km/s in the rest frame of the system. The difference arises in part due to a gravitationally induced inflow velocity of the gas ahead of the shock, which amounts to ~1100 km/s for our assumed 10:1 mass ratio of the merger. A second effect is that the shock front moves faster than the subcluster itself. A generic LCDM collision model, where a bullet subcluster with concentration c=7.2 merges with a parent cluster with concentration c=3 on a zero-energy orbit, reproduces all the main observational features seen in 1E0657-56 with good accuracy, suggesting that 1E0657-56 is well in line with expectations from standard cosmological models. In theories with an additional 5th-force in the dark sector, the subcluster can be accelerated beyond the velocity reached in LCDM, and the spatial offset between the X-ray peak and the mass centroid of the subcluster can be significantly enlarged. (abridged)
We present a short review of strange quark matter in supernovae and related explosions, with particular attention to the issue of the propagation of the combustion in the dense stellar environment. We discuss the instabilities affecting the flame and present some new results of application to the turbulent regime. The transition to the distributed regime and further deflagration-to-detonation mechanism are addressed. Finally we show that magnetic fields may be important for this problem, because they modify the flame through the dispersion relations which characterize the instabilities. A tentative classification of explosive phenomena according to the value of the average local magnetic field affecting the burning and the type of stellar system in which this conversion is taking place is presented. As a general result, we conclude that ``short'' conversion timescales are always favored, since the burning falls in either the turbulent Rayleigh-Taylor (or even the distributed) regime, or perhaps in the detonation one. In both cases the velocity is several orders of magnitude larger than $v_{lam}$, and therefore the latter is irrelevant in practice for this problem. Interesting perspectives for the study of this problem are still open and important issues need to be addressed.
We searched for an anisotropic background of gravitational waves using data
from the LIGO S4 science run and a method that is optimized for point sources.
This is appropriate if, for example, the gravitational wave background is
dominated by a small number of distinct astrophysical sources. No signal was
seen. Upper limit maps were produced assuming two different power laws for the
source strain power spectrum. For an f^-3 power law and using the 50 Hz to 1.8
kHz band the upper limits on the source strain power spectrum vary between
1.2e-48 Hz^-1 (100 Hz/f)^3 and 1.2e-47 Hz^-1 (100 Hz /f)^3, depending on the
position in the sky. Similarly, in the case of constant strain power spectrum,
the upper limits vary between 8.5e-49 Hz^-1 and 6.1e-48 Hz^-1.
As a side product a limit on an isotropic background of gravitational waves
was also obtained. All limits are at the 90% confidence level. Finally, as an
application, we focused on the direction of Sco-X1, the closest low-mass X-ray
binary. We compare the upper limit on strain amplitude obtained by this method
to expectations based on the X-ray luminosity of Sco-X1.
The present work investigates the parametric instability of parallel propagating circularly polarized Alfven(pump) waves in a weakly ionized molecular cloud. It is shown that the relative drift between the plasma particles gives rise to the Hall effect resulting in the modified pump wave characteristics. Although the linearized fluid equations with periodic coefficients are difficult to solve analytically, it is shown that a linear transformation can remove the periodic dependence. The resulting linearized equations with constant coefficients are used to derive an algebraic dispersion relation. The growth rate of the parametric instability is a sensitive function of the amplitude of the pump wave as well as to the ratio of the pump and the modified dust-cyclotron frequencies. The instability is insensitive to the plasma-beta The results are applied to the molecular clouds.
We study the impact of astrophysical processes on the gamma-ray background produced by the annihilation of dark matter particles in cosmological halos, with particular attention to the consequences of the formation of supermassive black holes. In scenarios where these objects form adiabatically from the accretion of matter on small seeds, dark matter is first compressed into very dense ``spikes'', then its density progressively decreases due to annihilations and scattering off of stellar cusps. With respect to previous analyses, based on non-evolving halos, the predicted annihilation signal is higher and significantly distorted at low energies, reflecting the large contribution to the total flux from unevolved spikes at high redshifts. The peculiar spectral feature arising from the specific redshift distribution of the signal, would discriminate the proposed scenario from more conventional astrophysical explanations. We discuss how this affects the prospects for detection and demonstrate that the gamma-ray background from DM annihilations might be detectable even in absence of a signal from the Galactic center.
The core-accretion and disk instability models have so far been used to explain planetary formation. These models have different conditions, such as planet mass, disk mass, and metallicity for formation of gas giants. The core-accretion model has a metallicity condition ([Fe/H] > −1.17 in the case of G-type stars), and the mass of planets formed is less than 6 times that of the Jupiter mass MJ. On the other hand, the disk instability model does not have the metallicity condition, but requires the disk to be 15 times more massive compared to the minimum mass solar nebulae model. The mass of planets formed is more than 2MJ. These results are compared to the 161 detected planets for each spectral type of the central stars. The results show that 90% of the detected planets are consistent with the core-accretion model regardless of the spectral type. The remaining 10% are not in the region explained by the core-accretion model, but are explained by the disk instability model. We derived the metallicity dependence of the formation probability of gas giants for the core-accretion model. Comparing the result with the observed fraction having gas giants, they are found to be consistent. On the other hand, the observation cannot be explained by the disk instability model, because the condition for gas giant formation is independent of the metallicity. Consequently, most of planets detected so far are thought to have been formed by the core-accretion process, and the rest by the disk instability process.
An extreme Kerr black hole (BH) surrounded by a precessing disk is invoked to explain the light curves of gamma-ray bursts (GRBs) based on the coexistence of the Blandford-Znajek (BZ) and the magnetic coupling (MC) processes. The overall shape of the light curves and the duration of GRBs are interpreted by the evolution of the half-opening angle of the magnetic flux on the BH horizon, and the complex temporal structures are modulated by the precession and nutation of the jet powered by the BZ process. It turns out that the time profile of the emission exhibits a fast rise and a slow decay due to the effect of the evolution of the half-opening angle. The light curves of several GRBs are well fitted by virtue of this model with only six free parameters.
We use the Chandra X-ray Observatory to study the region in the Tycho supernova remnant between the blast wave and the shocked ejecta interface or contact discontinuity. This zone contains all the history of the shock-heated gas and cosmic-ray acceleration in the remnant. We present for the first time evidence for significant spatial variations of the X-ray synchrotron emission in the form of spectral steepening from a photon index of 2.6 right at the blast wave to a value of 3.0 several arcseconds behind. We interpret this result along with the profiles of radio and X-ray intensity using a self-similar hydrodynamical model including cosmic ray backreaction that accounts for the observed ratio of radii between the blast wave and contact discontinuity. Two different assumptions were made about the post-shock magnetic field evolution: one where the magnetic field (amplified at the shock) is simply carried by the plasma flow and remains relatively high in the post-shock region [synchrotron losses limited rim case], and another where the amplified magnetic field is rapidly damped behind the blast wave [magnetic damping case]. Both cases fairly well describe the X-ray data, however both fail to explain the observed radio profile. The projected synchrotron emission leaves little room for the presence of thermal emission from the shocked ambient medium. This can only be explained if the pre-shock ambient medium density in the vicinity of the Tycho supernova remnant is below 0.6 cm-3.
We present modeling results for the reprocessed radiation expected from magnetic flares above AGN accretion disks. Relativistic corrections for the orbital motion of the flare and for the curved space-time in the vicinity of the black hole are taken into account. We investigate the local emission spectra, as seen in a frame co-orbiting with the disk, and the observed spectra at infinity. We investigate long-term flares at different orbital phases and short-term flares for various global parameters of the accreting black hole. Particular emphasis is put on the relation between the iron Kalpha line and the Compton hump as these two features can be simultaneously observed by the Suzaku satellite and later by Simbol-X.
With its relatively long observation time per pointing, the Parkes multibeam survey was effective in detecting nulling pulsars. We have made 2-hour observations of 23 pulsars which showed evidence for pulse nulling or mode changing in the survey data. Because of the low flux density of these pulsars, in most cases averaging times of between 10 s and 60 s were necessary and so this analysis is insensitive to very short nulls. Seven of the pulsars had null fractions of more than 40% with the largest having a lower limit of 95%. Mode changes were observed in six pulsars with clear relationships between nulling and mode changing in some cases. Combined with earlier results, the data suggest that large null fractions are more related to large characteristic age than to long pulse period. The observations suggest that nulling and mode changing are different manifestations of the same phenomenon.
We examine the idea that dynamical parameters can be estimated by identifying locations in the solar neighbourhood where simulated velocity distributions match the observed local distribution. Here, the dynamical influence of both the Galactic bar and the outer spiral pattern are taken into account. The Milky Way disc is stirred by analytical potentials that are chosen to represent the two perturbations, the ratio of pattern speeds of which is explored, rather than held constant. The velocity structure of the final configuration is presented as heliocentric velocity distributions at different locations. These model velocity distributions are compared to the observed distribution in terms of a goodness-of-fit parameter that has been formulated here. We monitor the spatial distribution of the maximal value of this parameter, in order to constrain the solar position from a model. Efficiency of a model is based on a study of this distribution as well as on other independent dynamical considerations. We reject the bar only and spiral only models and arrive at the following bar parameters from the bar+spiral simulations: bar pattern speed of 57.4^{+2.8}_{-3.3} km/s/kpc and a bar angle in [0^\circ$, 30^\circ$]. However, extracting information in this way is no longer viable when the dynamical influence of the spiral pattern does not succumb to that of the bar. Orbital analysis indicates that even though the basic bimodality in the local velocity distribution can be attributed to scattering off the Outer Lindblad Resonance of the bar, it is the interaction of irregular orbits and orbits of other resonant families, that is responsible for the other moving groups; it is realised that such interaction increases with the warmth of the background disk.
We present an analysis of a devised sample of Rotation Curves (RCs), aimed at checking the consequences of a modified f(R) gravity in galactic scales. Originally motivated by the the dark energy mystery, this theory may serve as a possibility of explaining the observed non-Keplerian profiles of galactic RCs in terms of a break-down of the Einstein General Relativity. We show that in general the power-law f(R) version could fit well the observations with reasonable values for the mass model parameters, encouraging further investigation on R^n gravity from both observational and theoretical points of view.
The Mira AB system is a nearby (~107 pc) example of a wind accreting binary star system. In this class of system, the wind from a mass-losing red giant star (Mira A) is accreted onto a companion (Mira B), as indicated by an accretion shock signature in spectra at ultraviolet and X-ray wavelengths. Using novel imaging techniques, we report the detection of emission at mid-infrared wavelengths between 9.7 and 18.3 $\mu$m from the vicinity of Mira B but with a peak at a radial position about 10 AU closer to the primary Mira A. We interpret the mid-infrared emission as the edge of an optically-thick accretion disk heated by Mira A. The discovery of this new class of accretion disk fed by M-giant mass loss implies a potential population of young planetary systems in white-dwarf binaries which has been little explored, despite being relatively common in the solar neighborhood.
Recent direct $H(z)$ data indicate that the parameter
$H(z)$ may wiggle with respect to
$z$. On the other hand the luminosity distance data of supernovae flatten the
wiggles of $H(z)$ because of integration effect. It is expected that the
fitting results can be very different in a model permitting a wiggling $H(z)$
because the data of supernovae is highly degenerated to such a model. As an
example the natural phantom dark energy is investigated in this paper. The
dynamical property of this model is studied. The model is fitted by the direct
$H(z)$ data set and the SNLS data set, respectively. And the results are quite
different, as expected.
We have searched for persistent radio pulsations, bright single pulses, and bursts from four Southern anomalous X-ray pulsars (AXPs). Deep observations were conducted at 1.4 GHz in 1999 July and August with the Parkes 64-m telescope. For all of the target AXPs, the upper limits on integrated pulsed emission are ~0.02 mJy, while the limits on the flux of single pulses are ~1 Jy or better. The corresponding radio luminosity limits are significantly below the observed luminosities of the majority of the observed radio pulsars, and they are significantly below the radio luminosity of XTE J1810-197, the only AXP known thus far to emit radio pulsations, at the epoch when radio pulses were first detected from this source. Our null results support the hypothesis that pulsed radio emission from AXPs is only present in conjunction with X-ray outburst activity. However, we cannot rule out the possibility that pulsed radio emission is present, but that it is too weak to be detected in our observations, or that unfavorable viewing geometries prevent this emission from being seen by terrestrial observers. Given the possible association between the radio emission and transient X-ray behavior of XTE J1810-197, continued radio searches of these and other AXPs at different epochs are warranted, particularly after periods of X-ray burst activity.
Using the AAOmega instrument of the Anglo-Australian Telescope, we have obtained medium-resolution near-infrared spectra of 10,500 stars in two-degree fields centered on the galactic globular clusters 47 Tuc, NGC 288, M12, M30 and M55. Radial velocities and equivalent widths of the infrared Ca II triplet lines have been determined to constrain cluster membership, which in turn has been used to study the angular extent of the clusters. From the analysis of 140-1000 member stars in each cluster, we do not find extended structures that go beyond the tidal radii. For three cluster we estimate a 1% upper limit of extra-tidal red giant branch stars. We detect systemic rotation in 47 Tuc and M55.
Experimental results on the formation of molecular hydrogen on amorphous silicate surfaces are presented for the first time and analyzed using a rate equation model. The energy barriers for the relevant diffusion and desorption processes are obtained. They turn out to be significantly higher than those obtained earlier for polycrystalline silicates, demonstrating the importance of grain morphology. These barriers are used in order to evaluate the efficiency of molecular hydrogen formation on amorphous silicate grains under interstellar conditions. It is found that unlike polycrystalline silicates, amorphous silicate grains are efficient catalysts of H_2 formation within a temperature range which is relevant to diffuse interstellar clouds (but not to photo-dissociation regions, where grain temperatures are higher). The results also indicate that the hydrogen molecules are thermalized with the surface and desorb with low kinetic energy. Thus, they are unlikely to occupy highly excited states.
We propose a method to analyze the clustering property of samples of discrete sources. With that method one can determine the probability of forming a cluster with a given number of galaxy members and a certain number density by chance. A relative number density quantity, called the neighborhood function, is introduced so that the crowded nature of the neighborhood of individual sources could be described. To avoid those effects arising from the distance of objects, we compare the real number density concerned with the mean number density measured in the corresponding local area rather than the mean of the whole sample. The scale used to sort out clustering sources is determined by that mean local number density, and thus it is a redshift-dependent quantity. The method is applied to a sample drawn from the 2dF survey, analyzed in redshift space. We find from the analysis that the probability of forming the resulting large-scale structures by chance is very small; the phenomenon of clustering is dominant in the local universe; within the $3\sigma $ confidence level, a coherent cluster with its scale as large as $357h^{-1}Mpc$ and another with its number of galaxy members as large as 12966 are identified from the sample; there exist some galaxies which are not affected by the gravitation of clusters and hence are suspected to rest on the co-moving frame of the universe; voids are likely the volumes within which no very crowded sources are present and they are likely formed in embryo by fluctuation in the very early epoch of the universe; large-scale structures are coral-like and they are likely made up of smaller ones; sources with large values of the neighborhood function are mainly distributed within the structure of prominent clusters and it is them who form the frame of the large-scale structure.
A 3-D numerical model of comet nuclei is presented. An implicit numerical scheme was developed for the thermal evolution of a spherical nucleus composed of a mixture of ice and dust. The model was tested against analytical solutions, simplified numerical solutions, and 1-D thermal evolution codes. The 3-D code was applied to comet 67P/Churyumov-Gerasimenko; surface temperature maps and the internal thermal structure was obtained as function of depth, longitude and hour angle. The effect of the spin axis tilt on the surface temperature distribution was studied in detail. It was found that for small tilt angles, relatively low temperatures may prevail on near-pole areas, despite lateral heat conduction. A high-resolution run for a comet model of 67P/Churyumov-Gerasimenko with low tilt angle, allowing for crystallization of amorphous ice, showed that the amorphous/crystalline ice boundary varies significantly with depth as a function of cometary latitude.
For inviscid, rotational accretion flows driven by a general pseudo-Newtonian potential on to a Schwarzschild black hole, the only possible fixed points are saddle points and centre-type points. For the specific choice of the Newtonian potential, the flow has only two critical points, of which the outer one is a saddle point while the inner one is a centre-type point. A restrictive upper bound is imposed on the admissible range of values of the angular momentum of sub-Keplerian flows through a saddle point. These flows are very unstable to any deviation from a necessarily precise boundary condition. The difficulties against the physical realisability of a solution passing through the saddle point have been addressed through a temporal evolution of the flow, which gives a non-perturbative mechanism for selecting a transonic solution passing through the saddle point. An equation of motion for a real-time perturbation about the stationary flows reveals a very close correspondence with the metric of an acoustic black hole, which is also an indication of the primacy of transonicity.
Abridged: We present a detailed multi-wavelength study of the bright-rimmed cloud SFO 75, including 1.3cm and 1.2mm continuum, and 13CO and ammonia spectral line observations. The 13CO and 1.2 mm emission reveals the presence of a dense core located behind the bright rim of the cloud which is approximately coincident with that of the IRAS point source. From an analysis of the IRAS and 1.2mm fluxes we derive a dust temperature of ~30 K, a luminosity of 1.6x10^4 L\odot and estimate the core mass to be ~570 M\odot. The higher resolution ammonia observations resolve the 1.2mm core into two distinct cores, one directly behind the cloud's rim (Core A) and the second located slightly farther back (Core B). Comparing the morphology of Core A with that of the photon-dominated region and ionised boundary layer leaves little doubt that it is being strongly affected by the ionisation front. 2MASS and GLIMPSE archive data which reveal a small cluster of three deeply embedded high- and intermediate-mass young stellar objects towards Core A leads us to conclude that the star formation found towards this core has been triggered. In stark contrast, Core B appears to have a much simpler, almost spherical, morphology. No stars are found towards Core B. The scenario that emerges from our analysis is one where the two ammonia cores pre-date the arrival of the ionisation front. Since its arrival the over-pressure of the ionised gas at the surface of the cloud has driven shocks into the surface layers of the cloud. The propagation of these shocks through Core A have triggered the formation of a small cluster of massive stars, however, the shock front has not yet propagated deeply enough into the cloud to have affected the evolution of Core B.
We study the cross-correlation between galaxies of different luminosities and colors, using a sample selected from the SDSS Dr 4. Galaxies are divided into 6 samples according to luminosity, and each of these samples is divided into red and blue subsamples. Projected auto-correlation and cross-correlation is estimated for these subsample. At projected separations r_p > 1\mpch, all correlation functions are roughly parallel, although the correlation amplitude depends systematically on luminosity and color. On r_p < 1\mpch, the auto- and cross-correlation functions of red galaxies are significantly enhanced relative to the corresponding power laws obtained on larger scales. Such enhancement is absent for blue galaxies and in the cross-correlation between red and blue galaxies. We esimate the relative bias factor on scales r > 1\mpch for each subsample using its auto-correlation function and cross-correlation functions. The relative bias factors obtained from different methods are similar. For blue galaxies the luminosity-dependence of the relative bias is strong over the luminosity range probed (-23.0<M_r < -18.0),but for red galaxies the dependence is weaker and becomes insignificant for luminosities below L^*. To examine whether a significant stochastic/nonlinear component exists in the bias relation, we study the ratio R_ij= W_{ii}W_{jj}/W_{ij}^2, where W_{ij} is the projected correlation between subsample i and j. We find that the values of R_ij are all consistent with 1 for all-all, red-red and blue-blue samples, however significantly larger than 1 for red-blue samples. For faint red - faint blue samples the values of R_{ij} are as high as ~ 2 on small scales r_p < 1 \mpch and decrease with increasing r_p.
The compact dark objects with very large masses residing at the centres of galaxies are believed to be black holes. Due to the gravitational lensing effect, they would cast a shadow larger than their horizon size over the background, whose shape and size can be calculated. For the supermassive black hole candidate Sgr A*, this shadow spans an angular size of about 50 micro arc second, which is under the resolution attainable with the current astronomical instruments. Such a shadow image of Sgr A* will be observable at about 1 mm wavelength, considering the scatter broadening by the interstellar medium. By simulating the black hole shadow image of Sgr A* with the radiatively inefficient accretion flow model, we demonstrate that analyzing the properties of the visibility function can help us determine some parameters of the black hole configuration, which is instructive to the sub-millimeter VLBI observations of Sgr A* in the near future.
We present the spectral energy distributions (SEDs) of a hard X-ray selected sample. The sample contains 136 sources with F(2-10 keV)>10^-14 erg/cm^2/s and 132 are AGNs. The sources are detected in a 1 square degree area of the XMM-Newton-Medium Deep Survey where optical data from the VVDS, CFHTLS surveys, and infrared data from the SWIRE survey are available. Based on a SED fitting technique we derive photometric redshifts with sigma(1+z)=0.11 and 6% of outliers and identify AGN signatures in 83% of the objects. This fraction is higher than derived when a spectroscopic classification is available. The remaining 17+9-6% of AGNs shows star-forming galaxy SEDs (SF class). The sources with AGN signatures are divided in two classes, AGN1 (33+6-1%) and AGN2 (50+6-11). The AGN1 and AGN2 classes include sources whose SEDs are fitted by type 1 and type 2 AGN templates, respectively. On average, AGN1s show soft X-ray spectra, consistent with being unabsorbed, while AGN2s and SFs show hard X-ray spectra, consistent with being absorbed. The analysis of the average SEDs as a function of X-ray luminosity shows a reddening of the IR SEDs, consistent with a decreasing contribution from the host galaxy at higher luminosities. The AGNs in the SF classes are likely obscured in the mid-infrared, as suggested by their low L(3-20micron)/Lcorr(0.5-10 keV) ratios. We confirm the previously found correlation for AGNs between the radio luminosity and the X-ray and the mid-infrared luminosities. The X-ray-radio correlation can be used to identify heavily absorbed AGNs. However, the estimated radio fluxes for the missing AGN population responsible for the bulk of the background at E>10 keV are too faint to be detected even in the deepest current radio surveys.
The question "Are supernovae important sources of dust?" is a contentious one. Observations with the Infrared Astronomical Satellite (IRAS) and the Infrared Space Observatory (ISO) only detected very small amounts of hot dust in supernova remnants. Here, we review observations of two young Galactic remnants with the Submillimetre Common User Bolometer Array (SCUBA), which imply that large quantities of dust are produced by supernovae. The association of dust with the Cassiopeia A remnant is in question due to the contamination of foreground material. In this article, we compare the emission from cold dust with CO emission towards Kepler's supernova remnant. We detect very little CO at the location of the submillimetre peaks. A comparison of masses from the CO and the dust clouds are made, and we estimate the 3 sigma upper limit on the gas-to-dust ratios to range from 25 - 65 suggesting that we cannot yet rule out freshly-formed or swept up circumstellar dust in Kepler's supernova remnant.
We analyse the implications of superfluid turbulence for neutron star physics. We begin by extending our previous results for the mutual friction force for a straight vortex array to account for the self-induced flow which arises when the vortices are curved. We then discuss Vinen's phenomenological model for isotropic turbulence, and derive the associated (Gorter-Mellink) form for the mutual friction. We compare this derivation to a more recent analysis of Schwarz, which sheds light on various involved issues. Having discussed isotropic turbulence, we argue that this case is unlikely to be relevant for neutron stars. Instead we expect a rotating neutron star to exhibit polarised turbulence, where relative flow drives the turbulence and rotation counteracts it. Based on recent results for superfluid Helium, we construct a phenomenological model that should have the key features of such a polarised turbulent system.
In the generally accepted, but poorly documented model, silicate J-type
C-stars are binary objects for which the silicate emission originates from a
circumbinary or a circumcompanion disc. We aim at testing this hypothesis by a
thorough spectral and spatial observational study of one object:
IRAS18006-3213.
We obtained, analysed and modeled high spatial resolution interferometric
VLTI/MIDI observations on multiple baselines ranging from 45 m to 100 m. All
observations resolved the object and show the very compact nature of the N-band
emission (~30 mas). In addition, the highest spatial resolution data show a
significant differential phase jump around 8.3 micron. This demonstrates the
asymmetric nature of the N-band emission. Moreover, the single telescope N-band
spectrum shows the signature of highly processed silicate grains. These data
are used to confirm the model on silicate J-type C-stars for IRAS18006-3213. We
show that the most favourable model of the dust geometry is a stable
circumbinary disc around the system, seen under an intermediate inclination.
The data presented on the silicate J-type C-star IRAS18006-3213 provide
evidence that the oxygen rich dust is trapped in a circumbinary disc. The
formation of this disc is probably linked to the binary nature of the central
star.
We consider the general Lagrangian of k-essence models and propose a new method to study and classify them through variables connected to the fluid equation of state parameter w. This allows to find solutions around which the scalar field is able to describe a mixture of dark matter and cosmological constant-like dark energy, an example being the purely kinetic model proposed by Scherrer. Making the stronger assumption that the scalar field Lagrangian is constant along solutions of the equation of motion, we find a general class of k-essence models whose classical trajectories directly describe a unified dark matter/dark energy (cosmological constant) fluid. While the simplest case of a scalar field with canonical kinetic term unavoidably leads to an effective sound speed c_s=1, thereby inhibiting the growth of matter inhomogeneities, more general non-canonical k-essence models allow for the possibility that c_s << 1 whenever matter dominates.
We present the PolEMICA [Aumont & Macias-Perez 2007] (Polarized Expectation-Maximization Independent Component Analysis) algorithm which is an extension to polarization of the SMICA [Delabrouille et al. 2003] temperature component separation method. This algorithm allows us to estimate blindly in harmonic space multiple physical components from multi-detectors polarized sky maps. Assuming a linear noisy mixture of components we are able to reconstruct jointly the electromagnetic spectra of the components for each mode T, E and B, as well as the temperature and polarization spatial power spectra, TT, EE, BB, TE, TB and EB for each of the physical components and for the noise on each of the detectors. This has been tested using full sky simulations of the Planck satellite polarized channels for a 14-months nominal mission assuming a simple linear sky model including CMB, and optionally Galactic synchrotron and dust emissions.
We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at $z>0.3$, which cover a temperature range of $3> kT > 15$ keV. Our analysis is aimed at measuring the iron abundance in the ICM out to the highest redshift probed to date. We find that the emission-weighted iron abundance measured within $(0.15-0.3) R_{vir}$ in clusters below 5 keV is, on average, a factor of $\sim2$ higher than in hotter clusters, following $Z(T)\simeq 0.88 T^{-0.47} Z_\odot$, which confirms the trend seen in local samples. We made use of combined spectral analysis performed over five redshift bins at $0.3> z > 1.3$ to estimate the average emission weighted iron abundance. We find a constant average iron abundance $Z_{Fe}\simeq 0.25 Z_\odot$ as a function of redshift, but only for clusters at $z>0.5$. The emission-weighted iron abundance is significantly higher ($Z_{Fe}\simeq0.4 Z_\odot$) in the redshift range $z\simeq0.3-0.5$, approaching the value measured locally in the inner $0.15 R_{vir}$ radii for a mix of cool-core and non cool-core clusters in the redshift range $0.1<z<0.3$. The decrease in $Z_{Fe}$ with $z$ can be parametrized by a power law of the form $\sim(1+z)^{-1.25}$. The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of $\sim2$ larger than at $z\simeq 1.2$. We confirm that the ICM is already significantly enriched ($Z_{Fe}\simeq0.25 Z_\odot$) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM.
Based on the observed paucity of the dwarf spheroidal (dSph) satellites of the Milky Way at small Galactocentric distances, we put forward the hypothesis that subsequent to the formation of the Milky Way and its satellites, those dSphs that had orbits with small perigalacticons were tidally disrupted, leaving behind a population that now has a relatively larger value of its average perigalacticon to apogalacticon ratio and consequently a larger value of its r.m.s. transverse to radial velocities ratio compared to their values at the time of formation of the dSphs. We analyze the implications of this hypothesis for the phase space distribution of the dSphs and that of the dark matter (DM) halo of the Galaxy within the context of a self-consistent model in which the functional form of the phase space distribution of DM particles follows the King model i.e. the `lowered isothermal' distribution and the potential of the Galaxy is determined self-consistently by including the gravitational cross-coupling between visible matter and DM particles. This analysis, coupled with virial arguments, yields an estimate of $\gsim$ 270 km/s for the circular velocity of any test object at galactocentric distances of $\sim$ 100 kpc, the typical distances of the dSphs. The corresponding self-consistent values of the relevant DM halo model parameters, namely, the local (i.e., the solar neighbourhood) values of the DM density and velocity dispersion in the King model and its truncation radius, are estimated to be $\sim$ 0.3 GeV/cm^3, >350 km/s and $\gsim$ 150 kpc, respectively. Similar self-consistent studies with other possible forms of the DM distribution function will be useful in assessing the robustness of our estimates of the Galaxy's DM halo parameters.
Our goal is to probe the populations of obscured and unobscured AGN investigating their optical-IR and X-ray properties as a function of X-ray flux, luminosity and redshift within a hard X-ray selected sample of 136 X-ray sources in the XMM Medium Deep Survey (XMDS) with wide multiwavelength coverage. The XMDS area is covered with optical photometry from the VVDS and CFHTLS surveys and infrared Spitzer data. Based on the X-ray luminosity and X-ray to optical ratio, 132 sources are likely AGN, of which 122 have unambiguous optical - IR identification. The observed optical and IR spectral energy distributions of sources are fitted with AGN/galaxy templates in order to classify them and compute photometric redshifts. 70% of the AGN are fitted by a type 2 AGN or a star forming galaxy template and are grouped together in a single class of ``optically obscured'' AGN. They have ``red'' optical colors and generally show significant X-ray absorption from X-ray spectra or hardness ratios (N$_H > 10^{22}$ cm$^{-2}$). Sources with SEDs typical of type 1 AGN have ``blue'' optical colors and exhibit X-ray absorption in about 30% of cases. We performed a stacking analysis for obscured and type 1 AGN. The stacked X-ray spectrum of obscured AGN is flatter than that of type 1 AGN and has an average spectral slope of Gamma = 1.6. The subsample of objects fitted by a galaxy template has an even harder stacked spectrum, with Gamma = 1.2 - 1.3. The obscured fraction is larger at lower fluxes, lower redshifts and lower luminosities. X-ray absorption is less common than ``optical'' obscuration and its incidence is nearly constant with redshift and luminosity. This implies that X-ray absorption is not necessarily related to optical obscuration.
We present a new radiative transfer method Short-N Characteristics that is a hybrid between the standard long characteristic methods and the standard short characteristic methods. We have implemented the numerical method within the SPECT3D imaging and spectral analysis application. Numerical experiments have shown that the short-N method is capable of reproducing the accuracy of the long characteristic methods, while delivering the CPU time efficiency of the short characteristic methods. We apply the short-N method to 2.5D cylindrical coordinates, i.e., we assume a 3D problem under azimuthal symmetry leading to a 2D mathematical problem with r-z as the independent spatial coordinates (2DRZ). We are currently working to extend this new method to other geometries.
Formation of relativistic jets in the magnetosphere of collapsing stars is considered. These jets will be formed in the polar caps of magnetosphere of collapsing star, where the stellar magnetic field increases during the collapse and the charged particles are accelerated. The jets will generate non-thermal radiation. The analysis of dynamics and emission of particles in the stellar magnetosphere under collapse shows that collapsing stars can by powerful sources of relativistic jets.
At the largest angular scales the presence of a number of unexpected features has been confirmed by the latest measurements of the cosmic microwave background (CMB). Among them are the anomalous alignment of the quadrupole and octopole with each other as well as the stubborn lack of angular correlation on scales >60deg. We search for correlations between these two phenomena and demonstrate their absence. A Monte Carlo likelihood analysis confirms previous studies and shows that the joint likelihood of both anomalies is incompatible with the best-fit Lambda Cold Dark Matter model at >99.95% C.L. At the same time, a presumed special axis (the `Axis of Evil') identified on the microwave sky demands additional contributions to multipole power on top of the primordial standard inflationary ones. We find that the notion of a preferred axis in the CMB is misleading and inconsistent with three-year data from the Wilkinson Microwave Anisotropy Probe (WMAP). Rather the data require a preferred plane, whereupon the axis is just the normal direction to that plane. Rotational symmetry within that plane is inconsistent with the observations and is ruled out at high confidence.
We study the non--linear evolution of magnetic fields in neutron star crusts with special attention to the influence of the Hall drift. Our goal is to understand the conditions for fast dissipation due to the Hall term in the induction equation. We study the interplay of Ohmic dissipation and Hall drift in order to find a timescale for the overall crustal field decay. We solve numerically the Hall induction equation by means of a hybrid method (spectral in angles but finite differences in the radial coordinate). The microphysical input consists of the most modern available crustal equation of state, composition and electrical conductivities. We present the first long term simulations of the non--linear magnetic field evolution in realistic neutron star crusts with a stratified electron number density and temperature dependent conductivity. We show that Hall drift influenced Ohmic dissipation takes place in neutron star crusts on a timescale of 1 Myr. When the initial magnetic field has magnetar strength, the fast Hall drift results in an initial rapid dissipation stage that lasts 10-50 kyr. The interplay of the Hall drift with the temporal variation and spatial gradient of conductivity tends to favor the displacement of toroidal fields toward the inner crust, where stable configurations can last for 1 Myr. We show that the thermally emitting isolated neutron stars, as the Magnificent Seven, are very likely descendants of neutron stars born as magnetars.
We develop a technique to measure radial velocities of stars from spectra that present four sets of lines. The algorithm is an extension of the two-dimensional cross-correlation method TODCOR to four dimensions. It computes the correlation of the observed spectrum against a combination of four templates with all possible shifts, and allows also for the derivation of the light ratios of the components. After testing the algorithm and demonstrating its ability to measure Doppler shifts accurately even under conditions of heavy line blending, we apply it to the case of the quadruple-lined system HD 110555. The primary and secondary components of this previously known visual binary (separation about 0.4 arcsec) are each shown to be double-lined spectroscopic binaries with periods of 57 days and 76 days, respectively, making the system a hierarchical quadruple. The secondary in the 76-day subsystem contributes only 2.5% to the total light, illustrating the ability of the method to measure velocities of very faint components.
We present the first X-ray observations of three sources belonging to a new AGN class: the naked AGNs. Based on optical spectroscopic studies, these sources appear as classical type 2 (obscured) AGNs, with only narrow emission lines. However, long-term optical monitoring campaigns, carried out over more than two decades, show that the same sources are strongly variable, like type 1 (un-obscured) AGNs. Based on short Chandra observations, the sources appear to be fairly bright in the X-rays, with typical Seyfert 1s values for the photon index (Gamma~1.8) and without significant intrinsic absorption, supporting the conclusion that some bright AGNs may genuinely lack a broad line region. Future, broad-band studies as well as deeper X-ray observations, probing both the spectral and the temporal properties of the naked AGNs, are crucial to shed light on the central engine of these sources, which may be representative of a large class of AGNs.
Context. NGC 300 X-1 is the second extragalactic candidate, after IC 10 X-1, in the rare class of Wolf-Rayet/compact object X-ray binary systems exemplified in the Galaxy by Cyg X-3. From a theoretical point of view, accretion onto a black hole in a detached system is possible for large orbital periods only if the mass of the relativistic object is high or the velocity of the accreted wind is low. Aims. We analysed a 2 week SWIFT XRT light curve of NGC 300 X-1 and searched for periodicities. Methods. Period searches were made using Lomb-Scargle periodogram analysis. We evaluated the confidence level using Monte Carlo simulations. Results. A period of 32.8+-0.4h (3 sigma error) was found for NGC 300 X-1 with a confidence level >99%. Furthermore, we confirm the high irregular variability during the high flux level, as already observed in the XMM-Newton observations of the source. A folded XMM-Newton light curve is shown, with a profile that is in agreement with SWIFT. The mean absorbed X-ray luminosity in the SWIFT observations was 1.5x10^38 erg/s, close to the value derived from the XMM-Newton data. Conclusions. While Cyg X-3 has a short period of 4.8 h, the period of NGC 300 X-1 is very close to that of IC 10 X-1 (34.8+-0.9 h). These are likely orbital periods. Possibility of formation of accretion disk for such high orbital periods strongly depends on the terminal velocity of the Wolf-Rayet star wind and black-hole mass. While low masses are possible for wind velocities < 1000 km/s, these increase to several tens of solar masses for velocities > 1600 km/s and no accretion disk may form for terminal velocities larger than 1900 km/s.
We present survey results which suggest rotation signatures at the base of T-Tauri jets. Observations were conducted with the Hubble Space Telescope Imaging Spectrograph at optical and near ultraviolet wavelengths (NUV). Results are presented for the approaching jet from DG Tau, CW Tau, HH 30 and the bipolar jet from TH 28. Systematic asymmetries in Doppler shift were detected across the jet, within 100 AU from the star. At optical wavelengths, radial velocity differences were typically 10 to 25 (+/-5) km/s, while differences in the NUV range were consistently lower at typically 10 (+/-5) km/s. Results are interpreted as possible rotation signatures. Importantly, there is agreement between the optical and NUV results for DG Tau. Under the assumption of steady magnetocentrifugal acceleration, the survey results lead to estimates for the distance of the jet footpoint from the star, and give values consistent with earlier studies. In the case of DG Tau, for example, we see that the higher velocity component appears to be launched from a distance of 0.2 to 0.5 AU from the star along the disk plane, while the lower velocity component appears to trace a wider part of the jet launched from as far as 1.9 AU. The results for the other targets are similar. Therefore, if indeed the detected Doppler gradients trace rotation within the jet then, under the assumption of steady MHD ejection, the derived footpoint radii support the existence of magnetized disk winds. However, since we do not resolved the innermost layers of the flow, we cannot exclude the possibility that there also exists an X-wind or stellar wind component.
We present the first detection of the near-infrared CN absorption band in the nuclear spectra of active galactic nuclei (AGN). This feature is a recent star formation tracer, being particularly strong in carbon stars. The equivalent width of the CN line correlates with that of the CO at 2.3 microns, as expected in stellar populations (SP) with ages between ~ 0.2 and ~ 2 Gyr. The presence of the 1.1 microns CN band in the spectra of the sources is taken as an unambiguous evidence of the presence of young/intermediate SP close to the central source of the AGN. Near-infrared bands can be powerful age indicators for star formation connected to AGN, the understanding of which is crucial in the context of galaxy formation and AGN feedback.
We have used the very large JVAS/CLASS 8.4-GHz surveys of flat-spectrum radio sources to obtain a large, uniformly observed and calibrated, sample of radio source polarizations. These are useful for many investigations of the properties of radio sources and the interstellar medium. We discuss comparisons with polarization measurements from this survey and from other large-scale surveys of polarization in flat-spectrum sources.
The Burst Alert Telescope (BAT) on board Swift has accumulated extensive light curves for 265 sources (not including GRBs) in the energy range 14 to 200 keV. We present here a summary of searches for periodic modulation in the flux from X-ray binaries. Our results include: determination of the orbital periods of IGR J16418-4532 and IGR J16320-4751; the disappearance of a previously known 9.6 day period in 4U 2206+54; the detection of a 5 hour period in the symbiotic X-ray binary 4U 1954+31, which might be the slowest neutron star rotation period yet discovered; and the detection of flares in the supergiant system 1E 1145.1-6141 which occur at both periastron and apastron passage with nearly equal amplitude. We compare techniques of weighting data points in power spectra and present a method related to the semi-weighted mean which, unlike conventional weighting, works well over a wide range of source brightness.
We study the dynamical structure of a self-gravitating disc with corona around a super-massive black hole. Assuming that the magneto-rotational-instability (MRI) responsible for generating the turbulent stresses inside the disc is also the source for a magnetically dominated corona, a fraction of the power released when the disc matter accretes is transported to and dissipated in the corona. This has major effect on the structure of the disc and its gravitational (in)stability according to our analytical and self-consistent solutions. We determine the radius where the disc crosses the inner radius of gravitational instability and forms the first stars. Not only the location of this radius which may extend to very large distances from the central black hole, but also the mass of the first stars highly depends on the input parameters, notably the viscous coefficient, mass of the central object and the accretion rate. For accretion discs around quasi-stellar objects (QSOs) and the Galactic center, we determine the self-gravitating radius and the mass of the first clumps. Comparing the cases with a corona and without a corona for typical discs around QSOs or the Galactic center, when the viscosity coefficient is around 0.3, we show that the self-gravitating radius decreases by a factor of approximately 2, but the mass of the fragments increases with more or less the same factor. Existence of a corona implies a more gravitationally unstable disc according to our results. The effect of a corona on the instability of the disc is more effective when the viscosity coefficient increases.
(Abridged) This work explores the existence of high redshift massive galaxies unveiled with Spitzer+IRAC, but missed by conventional selection techniques based on optical and near-infrared observations. To this end, we use the multi-wavelength imaging data available for the GOODS-South field, and select a flux-limited sample from the IRAC 3.6um image to m(AB)<23.26. We confine our study to the galaxies undetected by the optical HST+ACS imaging and close to the detection limit of the K-band image (K>23.5 AB). Our selection unveiled 20 galaxies on which we performed a detailed photometric analysis. For each galaxy, we built an SED based on optical-to-8um photometry to estimate the photo-z and to derive the main galaxies physical properties. The majority of the sample sources show degenerate/bimodal solutions for the photometric redshifts (Abridged). These can either be heavily dust-enshrouded (Av~2-4) starbursts at 2<z<3 with bolometric luminosities L(IR)>10^12 Lsun, or massive post-starburst galaxies in the redshift interval 4<z<9 with stellar masses of 10^11 Msun. One galaxy, the only source in our sample with both an X-ray and a 24um detection, might be an extremely massive object at z~8 detected during a post-starburst phase with concomitant QSO activity (although a lower-z solution is not excluded). Our investigation of Spitzer-selected galaxies with very red SEDs and completely undetected in the optical reveals a potential population of massive galaxies at z>4 which appear to include significant AGN emissions. These sources may be the oldest stellar systems at z~4. These, previously unrecognized, optically obscured objects might provide an important contribution to the massive-end (M>10^11 sun) of the high-z stellar mass function and they would almost double it (Abridged).
A statistical analysis of the space-time distribution of absorption-line systems (ALSs) observed in QSO spectra within the cosmological redshift interval $z$=0.0--3.7 is carried out on the base of our catalog of absorption systems (Ryabinkov et al. 2003). We confirm our previous conclusion that the $z$-distribution of absorbing matter contains non-uniform component displaying a pattern of statistically significant alternating maxima (peaks) and minima (dips). Using the wavelet transformation we determine the positions of the maxima and minima and estimate their statistical significance. The positions of the maxima and minima of the $z$-distributions obtained for different celestial hemispheres turn out to be weakly sensitive to orientations of the hemispheres. The data reveal a regularity (quasi-periodicity) of the sequence of the peaks and dips with respect to some rescaling functions of $z$. The same periodicity was found for the one-dimensional correlation function calculated for the sample of the ALSs under investigation. We assume the existence of a regular structure in the distribution of absorption matter, which is not only spatial but also temporal in nature with characteristic time varying within the interval 150--650 Myr for the cosmological model applied.
In this paper we report detection of multiple component structures in a Chandra X-ray image obtained in March 2001 of the nearby symbiotic interacting binary system CH Cyg. These components include a compact central source, an arc-like structure or a loop extending to 1.5'' (400 AU) from the central source associated with the 1997 jet activity, and possibly a newly formed jet extending to about 150 AU from the central source. The structures are also visible in VLA and HST images obtained close in time to the Chandra observations. The emission from the loop is consistent with optically thin thermal X-ray emission originating from a shock resulting from interaction of the jet ejecta with the dense circumbinary material. The emission from the central source originates within less then 50 AU region, and is likely associated with the accretion disk around the white dwarf. CH Cyg is only the second symbiotic system with jet activity detected at X-ray wavelengths, and the Chandra high-angular resolution image, combined with the VLA and HST images, provides the closest view of the region of jet formation and interaction with the circumbinary material in a symbiotic binary.
We have conducted an HI 21 cm emission-line survey using the Parkes 20cm multibeam instrument and the Australia Telescope Compact Array (ATCA) of six loose groups of galaxies chosen to be analogs to the Local Group. The goal of this survey is to make a census of the HI-rich galaxies and high-velocity clouds (HVCs) within these groups and compare these populations with those in the Local Group. The Parkes observations covered the entire volume of each group with a rms M(HI) sensitivity of 4-10x10^5 M(sun) per 3.3 km/s channel. All potential sources detected in the Parkes data were confirmed with ATCA observations at ~2' resolution and the same M(sun) sensitivity. All the confirmed sources have associated stellar counterparts; no starless HI clouds--HVC analogs--were found in the six groups. In this paper, we present a description of the survey parameters, its sensitivity and completeness. Using the population of compact HVCs (CHVCs) around the Milky Way as a template coupled with the detailed knowledge of our survey parameters, we infer that our non-detection of CHVC analogs implies that, if similar populations exist in the six groups studied, the CHVCs must be clustered within 90 kpc of group galaxies, with average M(HI) < 4x10^5 M(sun) at the 95% confidence level. The corollary is that the same must apply to Milky Way CHVCs. This is consistent with our previous results from a smaller sample of groups, and in accordance with recent observational and theoretical constraints from other authors. These results confirm that there is very little neutral matter around galaxies, and that any substantial reservoir of baryons must be in other phases.
A model of the universe as proposed by Allen Rothwarf based upon a degenerate Fermion fluid composed of polarizable particle-antiparticle pairs leads to a big bang model of the universe where the velocity of light varies inversely with the square root of cosmological time, t. This model is here extended to predict a decelerating expansion of the universe and to derive the Tully-Fisher law describing the flat rotation curves of spiral galaxies. The estimated critical acceleration parameter, aoR, is compared to the experimental, critical modified Newtonian Dynamics (MOND) cosmological acceleration constant, obtained by fitting a large number of rotation curves. The present estimated value is much closer to the experimental value than that obtained with the other models. This model for aR(t) allows the derivation of the time dependent radius of the universe as a function of red shift Other cosmological parameters such as the velocity of light, Hubble's constant, the Tully-Fisher relation, and the index of refraction of the aether can also be expressed in terms of redshift. This is compared with the statistical fitting for Veron-Cetty data (2006) for quasar red shifts and good agreement is found. This model also determines the time and/or redshift dependence of certain electromagnetic parameters, i.e., the permittivity; the permeability ; and index of refraction of free space. These are found to be useful in various cosmological theories dealing with light passing through media in motion.
We perform a MCMC (Monte Carlo Markov Chains) analysis of the available CMB and LSS data (including the three years WMAP data) with single field slow-roll new inflation and chaotic inflation models. We do this within our approach to inflation as an effective field theory in the Ginsburg-Landau spirit with fourth degree trinomial potentials in the inflaton field phi. We derive explicit formulae and study in detail the spectral index n_s of the adiabatic fluctuations, the ratio r of tensor to scalar fluctuations and the running index dn_s/dln k . We use these analytic formulas in the MCMC analysis. Our results are as follow: (i) The data strongly indicate the breaking (whether spontaneous or explicit) of the phi -> - \phi symmetry of the inflaton potentials both for new and for chaotic inflation. (ii) Trinomial new inflation naturally satisfies this requirement and provides an excellent fit to the data. (iii) Trinomial chaotic inflation produces the best fit in a very narrow corner of the parameter space. (iv) The chaotic symmetric trinomial potential is almost certainly ruled out (at 95% CL). In trinomial chaotic inflation the MCMC runs go towards a potential in the boundary of the parameter space and which ressembles a spontaneously symmetry broken potential of new inflation. (v) The above results and further physical analysis here lead us to conclude that new inflation gives the best description of the data. (vi) We find a lower bound for r within trinomial new inflation potentials: r > 0.02 (95% CL) and r > 0.07 (68% CL). (vii) The preferred new inflation trinomial potential is a double well, even function of the field with a moderate quartic coupling yielding as most probable values: n_s ~ 0.958, r ~ 0.1 .
Some short-period exoplanets (hot Jupiters) are observed by their transits to have anomalously large radii. It has been suggested that these planets are in a resonance involving persistent misalignment and synchronous precession of their spin and orbital angular momenta, a Cassini state, and that the attendant tidal heating inflates the planet. We argue against this. Using explicit tidal integrations, we show that although an oblique Cassini state can dissipate many times the rotational energy of the planet, the rate of dissipation must be much less than hypothesized. Dissipation causes the planetary spin to lie at an angle to the plane containing the orbital and total angular momenta. If dissipation is too rapid, this angle becomes so large that Cassini equilibrium is lost. A separate consideration limits the total energy that can be extracted from the orbit. The source of the torque on the orbit, either an oblique parent star or an inclined third body, aligns with the orbit as it absorbs the angular momentum shed by the planet. Alignment removes the orbital precession required by the Cassini state. In combination with observational bounds on the mass and semimajor axis of a possible second planet and with bounds on the stellar rotation and obliquity, these constraints make it very unlikely that obliquity tides can be the explanation for inflated hot Jupiters, especially HD 209458b.
We have searched for Gravitational Waves (GWs) associated with the SGR 1806-20 hyperflare of 27 December 2004. This event, originating from a Galactic neutron star, displayed exceptional energetics. Recent investigations of the X-ray light curve's pulsating tail revealed the presence of Quasi-Periodic Oscillations (QPOs) in the 30 - 2000 Hz frequency range, most of which coincides with the bandwidth of the LIGO detectors. These QPOs, with well-characterized frequencies, can plausibly be attributed to seismic modes of the neutron star which could emit GWs. Our search targeted potential quasi-monochromatic GWs lasting for tens of seconds and emitted at the QPO frequencies. We have observed no candidate signals above a pre-determined threshold and our lowest upper limit was set by the 92.5 Hz QPO observed in the interval from 150 s to 260 s after the start of the flare. This bound corresponds to a (90% confidence) root-sum-squared amplitude h_rssdet^90% = 4.5e-22 strain Hz^-1/2 on the GW waveform strength in the detectable polarization state reaching our Hanford (WA) 4 km detector. We illustrate the astrophysical significance of the result via an estimated characteristic energy in GW emission that we would expect to be able to detect. The above result corresponds to 7.7e46 erg (= 4.3e-8 M_sun c^2), which is of the same order as the total (isotropic) energy emitted in the electromagnetic spectrum. This result provides a means to probe the energy reservoir of the source with the best upper limit on the GW waveform strength published and represents the first broadband asteroseismology measurement using a GW detector.
Relativistic electrons moving into a highly tangled magnetic field emit jitter radiation. We present a detailed computation of the jitter radiation spectrum for electrons inside Weibel shock generated magnetic fields, including self-absorption. We apply our results to the case of the prompt and afterglow emission of gamma-ray bursts. We show that jitter emission can reproduce most of the observed features with some important differences with respect to standard synchrotron, especially between the self-absorption and the peak frequencies. We discuss the similarities and differences between jitter and synchrotron and discusss experiments that can disentangle the two mechanisms.
We present an extinction map of the Large Magellanic Cloud (LMC), using 204,502 stars from the Two Micron All Sky Survey point source catalog. We first use the NICE method to determine the reddening distribution, \ehk and \ejh, which we compare to the HI distribution to find a near-infrared reddening law of $\ejh/\ehk=1.20\pm 0.04$. A visual extinction map ($\sim 6^\circ\times 6^\circ$) of the LMC is created using the NICER method; at 4 arcmin resolution, a mean value of $\av=0.38$ mag is found. We derive the LMC CO-to-H$_2$ conversion factor, $\x{LMC}$, independent of assumptions about the virialization of giant molecular clouds, by comparing the NICER extinction map with NANTEN $^{12}$CO observations. In regions where $\av>1$ mag and $^{12}$CO emission is $\ge$ 2 \counits, we measure $\x{LMC}=9.3\pm 0.4\times 10^{20} \xunits$. In the same regions, the LMC contains a total molecular mass of $(4.5\pm 0.2)\times 10^7 \msun$.
We analyze a 96 ks Chandra/HETGS observation of the single G-type giant HR 9024. The high flux allows us to examine spectral line and continuum diagnostics at high temporal resolution, to derive plasma parameters. A time-dependent 1D hydrodynamic model of a loop with half-length $L = 5 \times 10^{11}$ cm ($\sim R_{\star}/2$), cross-section radius $r = 4.3 \times 10^{10}$ cm, with a heat pulse of 15 ks and $2 \times 10^{11}$~erg cm$^{-2}$ s$^{-1}$ deposited at the loop footpoints, satisfactorily reproduces the observed evolution of temperature and emission measure, derived from the analysis of the strong continuum emission. For the first time we can compare predictions from the hydrodynamic model with single spectral features, other than with global spectral properties. We find that the model closely matches the observed line emission, especially for the hot ($\sim 10^8$ K) plasma emission of the FeXXV complex at $\sim 1.85$\AA. The model loop has $L/R_{\star} \sim 1/2$ and aspect ratio $r/L \sim 0.1$ as typically derived for flares observed in active stellar coronae, suggesting that the underlying physics is the same for these very dynamic and extreme phenomena in stellar coronae independently on stellar parameters and evolutionary stage.
Sulphur is an element which is formed in the alpha-process and is easily measured in the gaseous phase in external galaxies. Since it does not form dust, it is the preferred indicator for alpha-elements, rather than Si or Mg, for which dust corrections are necessary. The measurement of the sulphur abundance in stars is not an easy task, relying mainly on high excitation lines with non-negligible deviations from LTE. The 1082 nm sulphur forbidden transition is less sensitive to departures from LTE and is less dependent on temperature uncertainties than other sulphur lines usually employed as abundance indicators. Therefore it should provide a more robust abundance diagnostics. aims: To derive the solar photospheric abundance of sulphur from the 1082 nm [SI] line and to investigate 3D effects present in G- and F-type atmospheres at solar and lower metallicity. method: High-resolution, high signal-to-noise solar intensity and flux spectra were used to measure the sulphur abundance from the [SI] 1082 nm line. co5bold hydrodynamical model atmospheres were applied to predict 3D abundance corrections for the [SI] line. results: The solar sulphur abundance is derived to be 7.15+- (0.01)stat +- (0.05)sys, where the statistical uncertainty represents the scatter in the determination using four different solar spectra and the systematic uncertainty is due to the modelling of the blending lines. Sulphur abundances obtained from this line are insensitive to the micro-turbulence. 3D abundance corrections, found from strictly differential comparisons between 1D and 3D models, are negligible in the Sun, but become sizable for more metal-poor dwarfs.
We report the discovery of a companion to the high proper motion star LHS 1589, a nearby high-velocity, low-mass subdwarf. The companion (LHS 1589B) is located 0.220"+/-0.004" to the southwest of the primary (LHS 1589A), and is 0.5 magnitude fainter than the primary in the K_s band. The pair was resolved with the IRCAL infrared camera at Lick Observatory, operating with the Laser Guide Star Adaptive Optics system. A low-resolution spectrum obtained at MDM observatory confirms that the system consists of a pair of low-mass subdwarfs, with a composite spectral type sdK7.5. A photometric distance estimate places the system at a distance d=78+/-18 parsecs from the Sun. We also measure a radial velocity V_rad=75+/-25 km/s which, together with the proper motion and estimated distance, suggests that the star is roaming the inner Galactic halo on a highly eccentric orbit. With a projected orbital separation s=17.2+/-4.8 AU, we estimate the orbital period of the system to be in the range 95 yr < P < 370 yr. This suggests that the dynamical mass of the system could be derived astrometrically, after monitoring the orbital motion over a decade or so. The LHS 1589AB system could thus provide a much needed constraint to the mass-luminosity relationship of metal-poor, low-mass stars.
Context: The spatial distribution of the stellar populations inside a spheroidal system and their kinematical properties supply important informations about the formation process. Aims: We have performed a detailed stellar population analysis using long slit spectroscopic observations up to almost one effective radius of two different early-type galaxies of low density regions of the local Universe: NGC 1052, a E4 Liner prototype of a loose group that has a stellar rotating disc, and NGC 7796, a E1 of the field which shows a kinematically distinct core. The mean luminosity-weighted stellar age, metallicity, and alpha/Fe ratio along both photometric axes of them have been obtained in order to reconstruct the star formation history in their kinematically distinct subsystems. Methods: We have measured Lick indices and computed their radial gradients. They were compared with the predicted ones of simple stellar population models. We have also applied a stellar population synthesis. Results: The star characteristics are associated with their kinematics: they are older and alpha-enhanced in the bulge of NGC 1052 and core of NGC 7796, while they show a strong spread of alpha/Fe and age along the disc of NGC 1052 and an outwards radial decreasing of them outside the core of NGC 7796. The age variation is possibly connected to the alpha/Fe one. Conclusions: Both galaxies were formed by processes in which the star formation occurred firstly at the bulge (NGC 1052) and nucleus (NGC 7796) 12-15 Gyr ago on short timescales (0.1-1 Gyr) providing an efficient chemical enrichment by SN-II. In the disc of NGC 1052, there is some spread of age and formation timescales around its stars. In NGC 7796, the star formation timescale had some outwards radial increasing along both axes.
We present recent work undertaken by the Evolution and Seismic Tools Activity (ESTA) team of the CoRoT Seismology Working Group. The new ESTA-Task 3 aims at testing, comparing and optimising stellar evolution codes which include microscopic diffusion of the chemical elements resulting from pressure, temperature and concentration gradients. The results already obtained are globally satisfactory, but some differences between the different numerical tools appear that require further investigations.
Results from gamma-ray observations by the H.E.S.S. telescope array in the direction of the young stellar cluster Westerlund 2 are presented. Stereoscopic imaging of Cherenkov light emission of gamma-ray induced showers in the atmosphere is used to study the celestial region around the massive Wolf-Rayet (WR) binary WR 20a. Spectral and positional analysis is performed using standard event reconstruction techniques and parameter cuts. The detection of a new gamma-ray source is reported from H.E.S.S. observations in 2006. HESS J1023-575 is found to be coincident with the young stellar cluster Westerlund 2 in the well-known HII complex RCW 49. The source is detected with a statistical significance of more than 9 sigma, and shows extension beyond a point-like object within the H.E.S.S. point-spread function. The differential gamma-ray spectrum of the emission region is measured over approximately two orders of magnitude in flux. The spatial coincidence between HESS J1023-575 and the young open cluster Westerlund 2, hosting e.g. the massive WR binary WR 20a, requires a look into a variety of potential models to account for the observed very-high-energy (VHE) gamma-ray emission. Considered emission scenarios include emission from the colliding wind zone of WR 20a, collective stellar winds from the extraordinary ensemble of hot and massive stars in the stellar cluster Westerlund 2, diffusive shock acceleration in the wind-blown bubble itself, and supersonic winds breaking out into the interstellar medium (ISM). The observed source extension argues against a single star origin of the observed VHE emission.
The purpose of this work is to search the Yangbajing neutron monitor data obtained between 1998 October and 2000 June for solar neutrons associated with solar flares. Using the onset times of 166 BATSE-detected flares with the GOES peak flux (1 -- 8 \AA) higher than $1.0 \times 10^{-5}$ $\mathrm{Wm^{-2}}$, we prepare for each flare a light curve of the Yangbajing neutron monitor, spanning $\pm$ 1.5 hours from the BATSE onset time. Based on the light curves, a systematic search for solar neutrons in energies above 100 MeV from the 166 flares was performed. No statistically significant signals due to solar neutrons were found in the present work. Therefore, we put upper limits on the $>$ 100 MeV solar-neutron flux for 18 events consisting of 2 X and 16 M class flares. The calculation assumed a power-law shaped neutron energy spectrum and three types of neutron emission profiles at the Sun. Compared with the other positive neutron detections associated with X-class flares, typical 95% confidence level upper limits for the two X-class flares are found to be comparable to the lowest and second lowest neutron fluxes at the top of the atmosphere.In addition, the upper limits for M-class flares scatter in the range of $10^{-2}$ to 1 neutrons $\mathrm{cm^{-2}s^{-1}}$. This provides the first upper limits on the solar-neutron flux from M-class solar flares, using space observatories as well as ground-based neutron monitors.
The energy densities of matter and the vacuum are currently observed to be of the same order of magnitude: $(\Omega_{m 0} \approx 0.3) \sim (\Omega_{\Lambda 0} \approx 0.7)$. The cosmological window of time during which this occurs is relatively narrow. Thus, we are presented with the cosmological coincidence problem: Why, just now, do these energy densities happen to be of the same order? Here we show that this apparent coincidence can be explained as a temporal selection effect produced by the age distribution of terrestrial planets in the Universe. We find a large ($\sim 68 %$) probability that observations made from terrestrial planets will result in finding $\Omega_m$ at least as close to $\Omega_{\Lambda}$ as we observe today. Hence, we, and any observers in the Universe who have evolved on terrestrial planets, should not be surprised to find $\Omega_m \sim \Omega_{\Lambda}$. This result is relatively robust if the time it takes an observer to evolve on a terrestrial planet is less than $\sim 10$ Gyr.
After explaining the motivation for this article, we briefly recapitulate the methods used to determine the rotation curves of our Galaxy and other spiral galaxies in their outer parts, and the results of applying these methods. We then present the essential Newtonian theory of (disk) galaxy rotation curves. The next two sections present two numerical simulation schemes and brief results. Finally, attempts to apply Einsteinian general relativity to the dynamics are described. The article ends with a summary and prospects for further work in this area. Recent observations and models of the very inner central parts of galaxian rotation curves are omitted, as also attempts to apply modified Newtonian dynamics to the outer parts.
Correlations between 1-10 Hz quasi-periodic oscillations (QPOs) and spectral power law index have been reported for black hole (BH) candidate sources and one neutron star source, 4U 1728-34. An examination of QPO frequency and index relationships in Sco X-1 is reported herein. We discovered that Sco X-1, representing Z-source groups, can be adequately modeled by a simple two-component model of Compton up-scattering with a soft photon electron temperature of about 0.4 keV, plus an Iron K-line. The results show a strong correlation between spectral power law index and kHz QPOs. Because Sco X-1 radiates near the Eddington limit, one can infer that the geometrical configuration of the Compton cloud (CC) is quasi-spherical because of high radiation pressure in the CC. Thus, we conclude that the high Thomson optical depth of the Compton cloud, in the range of ~5-6 from the best-fit model parameters, is consistent with the neutron star's surface being obscured by material. Moreover, a spin frequency of Sco X-1 is likely suppressed due to photon scattering off CC electrons. Additionally, we demonstrate how the power spectrum evolves when Sco X-1 transitions from the horizontal branch to the normal branch.
The NSF Science and Technology Center for Adaptive Optics (CfAO) is supporting a major scientific legacy project called the CfAO Treasury Survey (CATS). CATS is obtaining near-infrared AO data in deep HST survey fields, such as GEMS, GOODS-N, & EGS. Besides summarizing the main objectives of CATS, we highlight some recent imaging work on the study of distant field galaxies, AGNs, and a redshift z = 1.32 supernova. CATS plans the first data release to the community in early 2007 (check this http URL for more details on CATS and latest updates).
The existence of an anti-correlation between the Equivalent Width (EW) of the neutral narrow core of the iron Kalpha emission line and the 2-10 keV luminosity (the so-called `X-ray Baldwin' or `Iwasawa-Taniguchi' effect) has been debated in the last years. We aim at testing this claim on the largest catalogue of radio quiet AGN high-quality X-ray spectra ever published. The final sample comprises 157 objects. We search for a relation of the iron line EW not only with the X-ray luminosity, but also with the Black Hole mass, the Eddington ratio and the cosmological distance. The data presented here were analyzed homogeneously, all spectra are from the same instrument and with high Signal-to-Noise Ratio. A linear censored fit on the EW versus 2-10 keV luminosity is highly significant and yields $\log(EW_{Fe}) = (1.73\pm0.03) + (-0.17\pm0.03) \log(L_{X,44})$, where $EW_{Fe}$ is the EW of the neutral iron Kalpha line in eV and $L_{x,44}$ is the 2-10 keV X-ray luminosity in units of $10^{44}$ erg s$^{-1}$. The anti-correlation with the Eddington ratio is also very significant, while no dependence of the iron EW on the BH mass is apparent.
gamma-ray bursts (GRBs) are likely sources of ultra-high energy, >10^{19} eV, protons and high energy, >1 TeV, neutrinos. Large volume detectors of ultra high energy cosmic rays (UHECRs) and high energy neutrinos, which are already operating and are being expanded, may allow to test in the coming few years the predictions of the GRB model for high energy proton and neutrino production. Detection of the predicted signals will allow to identify the sources of UHECRs and will provide a unique probe, which may allow to resolve open questions related to the underlying physics of GRB models. Moreover, detection of GRB neutrinos will allow to test for neutrino properties (e.g., flavor oscillations for which tau's would be a unique signature, and coupling to gravity) with an accuracy many orders of magnitude better than is currently possible.
We simulate the buildup of galaxies by spherical gas accretion through dark matter haloes, subject to the development of virial shocks. We find that a uniform cosmological accretion turns into a rapidly varying disc buildup rate. The generic sequence of events (Shocked-Accretion Massive Burst & Shutdown: SAMBA) consists of four distinct phases: a) continuous cold accretion while the halo is below a threshold mass Msh~10^12Mo, b) tentative quenching of gas supply for ~2Gyr, starting once the halo is ~Msh and growing a rapidly expanding shock, c) a massive burst due to the big crunch of ~10^11Mo gas in \~0.5Gyr, when the heated gas cools and joins new infalling gas, and d) a long-term shutdown, enhanced by a temporary shock instability in late SAMBAs, those that quench at z~2, burst at z~1 and end up quenched in 10^12-13Mo haloes today. These events occur at all redshifts in galaxies of baryonic mass \~10^11Mo and involve a substantial fraction of this mass. They arise from rather smooth accretion, or minor mergers, which, unlike major mergers, may leave the disc intact while being built in a rapid pace. The early bursts match observed maximum starbursting discs at z>~2, predicted to reside in <~10^13Mo haloes. The late bursts resemble discy LIRGs at z<~1. The tentative quenching gives rise to a substantial population of ~10^11Mo galaxies with a strongly suppressed star-formation rate at z~1-3. The predicted long-term shutdown leads to red & dead galaxies in groups. A complete shutdown in more massive clusters requires an additional quenching mechanism, as may be provided by clumpy accretion. Alternatively, the SAMBA bursts may trigger the AGN activity that couples to the hot gas above Msh and helps the required quenching. The SAMBA phenomenon is yet to be investigated using cosmological simulations.
Large HI shells, with diameters of hundreds of pc and expansion velocities of 10-20kms-1 are well observed features of local gas rich galaxies. These shells could well be predicted as a result of the impact of OB associations on the ISM, but doubt has been cast on this scenario by the apparent absence of OB stars close to the centres of a large fraction of these shells in recent observations of the SMC. Using Fabry-Perot scanned Halpha emission line mapping of nearby galaxy discs we have detected, in all the HII regions where the observations yield sufficient angular resolution and S:N ratio, dominant Halpha shells with radii a few tens of pc, expanding at velocities of 50-100kms-1. We have applied a simple dynamically consistent framework in which we can extrapolate the properties of the observed Halpha shells to a few 10^7yr after the formation of the OB stars. The framework includes the dynamical inputs of both winds and SNe on the surrounding ISM. The results give quantitative statistical support to the hypothesis that the Halpha emitting shells are generic progenitors of the HI shells. During the time taken for an expanding shell to reach the size of a typical HI shell, the OB association may well lose its most luminous stars so the absence of such stars near the centres of many of the HI shells is well explained in this scenario.
Higher resolution telescopes as well as 3D numerical simulations will require the development of detailed 3D radiative transfer calculations. Building upon our previous work we extend our method to include both continuum and line transfer. We present a general method to calculate radiative transfer including scattering in the continuum as well as in lines in 3D static atmospheres. The scattering problem for line transfer is solved via means of an operator splitting (OS) technique. The formal solution is based on a long-characteristics method. The approximate $\Lambda$ operator is constructed considering nearest neighbors {\em exactly}. The code is parallelized over both wavelength and solid angle using the MPI library. We present the results of several test cases with different values of the thermalization parameter and two choices for the temperature structure. The results are directly compared to 1D spherical tests. With our current grid setup the interior resolution is much lower in 3D than in 1D, nevertheless the 3D results agree very well with the well-tested 1D calculations. We show that with relatively simple parallelization that the code scales to very large number of processors which is mandatory for practical applications. Advances in modern computers will make realistic 3D radiative transfer calculations possible in the near future. Our current code scales to very large numbers of processors, but requires larger memory per processor at high spatial resolution.
Recombination of the primordial helium plasma (HeII->HeI, $z \simeq 1500 - 3000$) is considered. This process has an effect on the CMBR anisotropy and CMBR spectrum distortion. In this work an influence of neutral hydrogen on kinetics of HeII->HeI recombination is investigated in the frame of the standard cosmological model. It is shown that small amount of neutral hydrogen ($10^{-5} - 10^{-2}$ of total number of hydrogen ions and atoms) leads to acceleration of HeII->HeI recombination at $z\lesssim 2000$ and at $z\lesssim 1600$ quasi-equilibrium HeII->HeI recombination (according to the Saha formula) becomes valid.
Following the Chandra Orion Ultradeep Project (COUP) observation, we have studied the chemical composition of the hot plasma in a sample of 146 X-ray bright pre-main sequence stars in the Orion Nebula Cluster. We report measurements of individual element abundances for a subsample of 86 slightly-absorbed and bright X-ray sources, using low resolution X-ray spectra obtained from the Chandra ACIS instrument. The X-ray emission originates from a plasma with temperatures and elemental abundances very similar to those of active coronae in older stars. A clear pattern of abundances vs. First Ionization Potential (FIP) is evident if solar photospheric abundances are assumed as reference. The results are validated by extensive simulations. The observed abundance distributions are compatible with a single pattern of abundances for all stars, although a weak dependence on flare loop size may be present. The abundance of calcium is the only one which appears to vary substantially between stars, but this quantity is affected by relatively large uncertainties. The ensemble properties of the X-ray bright COUP sources confirm that the iron in the emitting plasma is underabundant with respect to both the solar composition and to the average stellar photospheric values. Comparison of the present plasma abundances with those of the stellar photospheres and those of the gaseous component of the nebula, indicates a good agreement for all the other elements with available measurements, and in particular for the high-FIP elements (Ne, Ar, O, and S) and for the low-FIP element Si. We conclude that there is evidence of a significant chemical fractionation effect only for iron, which appears to be depleted by a factor 1.5--3 with respect to the stellar composition.
We use group size haloes identified with a ``friends of friends'' (FOF) algorithm in a concordance $\Lambda \rm{CDM}$ GADGET2 (dark matter only) simulation to investigate the dependence of halo properties on the environment at $z=0$. The study is carried out using samples of haloes at different distances from their nearest massive {\em cluster} halo. We find that the fraction of haloes with substructure typically increases in high density regions. The halo mean axial ratio $<c/a>$ also increases in overdense regions, a fact which is true for the whole range of halo mass studied. This can be explained as a reflection of an earlier halo formation time in high-density regions, which gives haloes more time to evolve and become more spherical. Moreover, this interpretation is supported by the fact that, at a given halo-cluster distance, haloes with substructure are more elongated than their equal mass counterparts with no substructure, reflecting that the virialization (and thus sphericalization) process is interrupted by merger events. The velocity dispersion of low mass haloes with strong substructure shows a significant increase near massive clusters with respect to equal mass haloes with low-levels of substructure or with haloes found in low-density environments. The alignment signal between the shape and the velocity ellipsoid principal axes decreases going from lower to higher density regions, while such an alignment is stronger for haloes without substructure. We also find, in agreement with other studies, a tendency of halo major axes to be aligned and of minor axes to lie roughly perpendicular with the orientation of the filament within which the halo is embedded, an effect which is stronger in the proximity of the massive clusters.
It is well established that timing and spectral properties of Galactic Black Hole (BH) X-ray binaries (XRB) are strongly correlated. In particular, it has been shown that low frequency Quasi-Periodic Oscillation (QPO) frequency - photon index correlation curves have a specific pattern. In a number of sources the shape of the index-low frequency QPO correlations are self-similar with a position offset in the QPO frequency-Gamma plane. Titarchuk & Fiorito presented strong theoretical and observational arguments that the QPO frequency values in this QPO frequency-Gamma correlation should be inversely proportional to M_{BH}. A simple translation of the correlation for a given source along the frequency axis leads to the observed correlation for another source. As a result of this translation one can obtain a scaling factor which is simply a BH mass ratio for these particular sources. This property of the correlations offers a fundamentally new method for BH mass determination in XRBs. Here we use the observed QPO-index correlations observed in three BH sources: GRO J1655-40, GRS 1915+105 and Cyg X-1. The BH mass of (6.3 +/- 0.5) M_Sun in GRO J1655-40 is obtained using optical observations. RXTE observations during the recent 2005 outburst yielded sufficient data to establish the correlation pattern during both rise and decay of the event. We use GRO J1655-40 as a standard reference source to measure the BH mass in Cyg X-1. We also revisit the GRS 1915+105 data as a further test of our scaling method. We infer the value of BH mass of (15.6 +/- 1.5) M_Sun in this source which is consistent with the previous BH mass estimate in GRS 1915 of (13.3 +/- 4) M_Sun. We obtain the BH mass in Cyg X-1 in the range (8.7 +/- 0.8) M_Sun.
The ANTARES Neutrino Telescope is a water Cherenkov detector currently under construction in the Mediterranean Sea. It is also designed to serve as a platform for investigations of the deep-sea environment. In this context, the ANTARES group at the University of Erlangen will integrate acoustic sensors within the infrastructure of the experiment. With this dedicated setup, tests of acoustic particle detection methods and deep-sea acoustic background studies shall be performed. The aim of this project is to evaluate the feasibility of a future acoustic neutrino telescope in the deep sea operating in the ultra-high energy regime. In these proceedings, the implementation of the project is described in the context of the premises and challenges set by the physics of acoustic particle detection and the integration into an existing infrastructure.
We have examined the orientations of early-type galaxies in the Coma cluster to see whether the well-established tendency for brightest cluster galaxies to share the same major axis orientation as their host cluster also extends to the rest of the galaxy population. We find no evidence of any preferential orientations of galaxies within Coma or its surroundings. The implications of this result for theories of the formation of clusters and galaxies (particularly the first-ranked members) are discussed.
The acoustic detection method is a promising option for future neutrino telescopes operating in the ultra-high energy regime. It utilises the effect that a cascade evolving from a neutrino interaction generates a sound wave, and is applicable in different target materials like water, ice and salt. Described here are the developments in and the plans for the research on acoustic particle detection in water performed by the ANTARES group at the University of Erlangen within the framework of the ANTARES experiment in the Mediterranean Sea. A set of acoustic sensors will be integrated into this optical neutrino telescope to test acoustic particle detection methods and perform background studies.
We present a further development of a method for accelerating the calculation of CMB power spectra, matter power spectra and likelihood functions for use in cosmological Bayesian inference. The algorithm, called {\sc CosmoNet}, is based on training a multilayer perceptron neural network. We compute CMB power spectra (up to $\ell=2000$) and matter transfer functions over a hypercube in parameter space encompassing the $4\sigma$ confidence region of a selection of CMB (WMAP + high resolution experiments) and large scale structure surveys (2dF and SDSS). We work in the framework of a generic 7 parameter non-flat cosmology. Additionally we use {\sc CosmoNet} to compute the WMAP 3-year, 2dF and SDSS likelihoods over the same region. We find that the average error in the power spectra is typically well below cosmic variance for spectra, and experimental likelihoods calculated to within a fraction of a log unit. We demonstrate that marginalised posteriors generated with {\sc CosmoNet} spectra agree to within a few percent of those generated by {\sc CAMB} parallelised over 4 CPUs, but are obtained 2-3 times faster on just a \emph{single} processor. Furthermore posteriors generated directly via {\sc CosmoNet} likelihoods can be obtained in less than 30 minutes on a single processor, corresponding to a speed up of a factor of $\sim 32$. We also demonstrate the capabilities of {\sc CosmoNet} by extending the CMB power spectra and matter transfer function training to a more generic 10 parameter cosmological model, including tensor modes, a varying equation of state of dark energy and massive neutrinos. {\sc CosmoNet} and interfaces to both {\sc CosmoMC} and {\sc Bayesys} are publically available at {\tt www.mrao.cam.ac.uk/software/cosmonet}.
A program that we call the QUEST Data Processing Software Pipeline has been written to process the large volumes of data produced by the QUEST camera on the Samuel Oschin Schmidt Telescope at the Palomar Observatory. The program carries out both aperture and PSF photometry, combines data from different repeated observations of the same portion of sky, and produces a Master Object Catalog. A rough calibration of the data is carried out. This program, as well as the calibration procedures and quality checks on the output are described.
We present results of Swift optical, UV and X-ray observations of the afterglow of GRB 050801. The source is visible over the full optical, UV and X-ray energy range of the Swift UVOT and XRT instruments.Both optical and X-ray lightcurves exhibit a broad plateau (\Delta t/t ~ 1) during the first few hundred seconds after the gamma-ray event. We investigate the multiwavelength spectral and timing properties of the afterglow, and we suggest that the behaviour at early times is compatible with an energy injection by a newly born magnetar with a period of a few tenths of a millisecond, which keeps the forward shock refreshed over this short interval by irradiation. Reverse shock emission is not observed. Its suppression might be due to GRB ejecta being permeated by high magnetic fields, as expected for outflows powered by a magnetar.Finally, the multiwavelength study allows a determination of the burst redshift, z=1.56.
Hydrogen atmosphere white dwarfs with metals, so-called DAZs, require external accretion of material to explain the presence of weak metal line absorption in their photospheres. The source of this material is currently uknown, but could come from the interstellar medium, unseen companions, or relic planetesimals from asteroid belt or Kuiper belt analogues. Mid-infrared photometry of these white dwarfs provide additional information to solve the mystery of this accretion and to look for evidence of planetary systems that have survived post main sequence evolution. We present {\em Spitzer} IRAC photometry of five DAZs and search for excesses due to unseen companions or circumstellar dust disks. Three of our targets show unexpected {\em deficits} in flux, which we tentatively attribute to absorption due to SiO and CO.
We discuss various ideas for the origin of the soft X-ray excess seen in AGN. There are clear advantages to models where this arises from atomic processes in partially ionised rather than where it is a true continuum component. However, current data cannot distinguish between models where this material is seen in reflection or absorption. While higher energy data may break the degeneracies, we also suggest that strong outflows are extremely likely to be present, lending more physical plausibility to an absorption origin. This more messy picture of NLS1's means that they are probably not good places to test GR, but they do give insight into the spectra expected from the first QSO's in the early Universe.
We present the results of 10 yr of complementary spectroscopic and photometric observations of the solar twin 18 Scorpii. We show that over the course of its ~7 year chromospheric activity cycle, 18 Sco's brightness varies in the same manner as the Sun's and with a likely brightness variation of 0.09%, similar to the 0.1% decadal variation in the total solar irradiance.
AXIS (An XMM-Newton International Survey) is a survey of 36 high Galactic latitude XMM-Newton observations covering 4.8 deg2 and containing 1433 serendipitous X-ray sources detected with 5-sigma significance. We have studied the X-ray source counts in four energy bands soft (0.5-2 keV), hard (2-10 keV), XID (0.5-4.5 keV) and ultra-hard (4.5-7.5 keV). We have combined this survey with shallower and deeper surveys. Our source counts results are compatible with most previous samples in the soft, XID, ultra-hard and hard bands. The fractions of the XRB resolved in the surveys used in this work are 87%, 85%, 60% and 25% in the soft, hard, XID and ultra-hard bands, respectively. Extrapolation of our source counts to zero flux are not enough to saturate the XRB intensity. Only galaxies and/or absorbed AGN may be able contribute the remaining unresolved XRB intensity. Our results are compatible, within the errors, with recent revisions of the XRB intensity in the soft and hard bands. The maximum fractional contribution to the XRB comes from fluxes within about a decade of the break in the source counts (~1e-14 cgs), reaching ~50% of the total in the soft and hard bands. Using only AXIS sources, we have studied the angular correlation in those bands using a novel robust technique. Angular clustering (widely distributed over the sky and not confined to a few deep fields) is detected at 99-99.9% significance in the soft and XID bands, with no detection in the hard and ultra-hard band (probably due to the smaller number of sources). We cannot confirm the detection of significantly stronger clustering in the hard-spectrum hard sources. Medium depth surveys such as AXIS are essential to determine the evolution of the X-ray emission in the Universe below 10 keV.
We discuss the origin of the soft X-ray excess seen in AGN. There are clear advantages to models where this arises from atomic processes in partially ionised material rather than where it is a true continuum component. However, current data cannot distinguish between models where this material is seen in reflection or absorption, even for the archetypal 'reflection dominated' AGN MCG-6-30-15. Instead, we give physical arguments on the ionisation structure of X-ray illuminated material which exclude a reflection origin if the disc is in hydrostatic equilibrium. The same physical processes strongly favour an absorption origin for the soft excess, giving a more messy picture of the accretion environment. This implies that these apparently 'reflection dominated' AGN are not good places to test GR, but they do give insight into the spectra expected from the first QSO's in the early Universe.
We have found that at least seven hydrogen-deficient carbon (HdC) and R Coronae Borealis (RCB) stars, have 16O/18O ratios close to and in some cases less than unity, values that are orders of magnitude lower than measured in other stars (the Solar value is 500). Greatly enhanced 18O is evident in every HdC and RCB we have measured that is cool enough to have detectable CO bands. The three HdC stars measured have 16O/18O < 1, lower values than any of the RCB stars. These discoveries are important clues in determining the evolutionary pathways of HdC and RCB stars, for which two models have been proposed: the double degenerate (white dwarf (WD) merger), and the final helium-shell flash (FF). No overproduction of 18O is expected in the FF scenario. We have quantitatively explored the idea that HdC and RCB stars originate in the mergers of CO- and He-WDs. The merger process is estimated to take only a few days, with accretion rates of 150 Msun/ yr producing temperatures at the base of the accreted envelope of 1.2 - 1.9 x 10^8 K. Analysis of a simplified one-zone calculation shows that nucleosynthesis in the dynamically accreting material may provide a suitable environment for a significant production of 18O, leading to very low values of 16O/18O, similar to those observed. We also find qualitative agreement with observed values of 12C/13C and with the CNO elemental ratios. H-admixture during the accretion process from the small H-rich C/O WD envelope may play an important role in producing the observed abundances. Overall our analysis shows that WD mergers may very well be the progenitors of O18-rich RCB and HdC stars, and that more detailed simulations and modeling are justified.
We present an algorithm to photometrically calibrate wide field optical imaging surveys, that simultaneously solves for the calibration parameters and relative stellar fluxes using overlapping observations. The algorithm decouples the problem of "relative" calibrations, from that of "absolute" calibrations; the absolute calibration is reduced to determining a few numbers for the entire survey. We pay special attention to the spatial structure of the calibration errors, allowing one to isolate particular error modes in downstream analyses. Applying this to the Sloan Digital Sky Survey imaging data, we achieve ~1% relative calibration errors across 8500 sq.deg. in griz; the errors are ~2% for the u band. These errors are dominated by unmodelled atmospheric variations at Apache Point Observatory.
Contaldi et al. [1] have suggested that an initial period of kinetic energy domination in single field inflation may explain the lack of CMB power at large angular scales. We note that in this situation it is natural that there also be a spatial gradient in the initial value of the inflaton field, and that this can provide a spatial asymmetry in the observed CMB power spectrum, manifest at low multipoles. We investigate the nature of this asymmetry and comment on its relation to possible anomalies at low multipoles.
We calculate the observable properties of the most massive high-redshift galaxies in the hierarchical formation scenario where stellar spheroid and supermassive black hole growth are fueled by gas-rich mergers. Combining high-resolution hydrodynamical simulations of the hierarchical formation of a z~6 quasar, stellar population synthesis models, template AGN spectra, prescriptions for interstellar and intergalactic absorption, and the response of modern telescopes, the photometric evolution of galaxies destined to host z~6 quasars are modeled at redshifts z~4-14. These massive galaxies, with enormous stellar masses of M_star ~10^11.5-10^12 M_sun. and star formation rates of SFR~10^3-10^4 M_sun yr^-1 at z>~7, satisfy a variety of photometric selection criteria based on Lyman-break techniques including V-band dropouts at z>~5, i-band dropouts at z>~6, and z-band dropouts at z>~7. The observability of the most massive high-redshift galaxies is assessed and compared with a wide range of existing and future photometric surveys including SDSS, GOODS/HUDF, NOAO WDFS, UKIDSS, the IRAC Shallow Survey, Pan-STARRS, LSST, and SNAP. Massive stellar spheroids descended from z~6 quasars will likely be detected at z~4 by existing surveys, but owing to their low number densities the discovery of quasar progenitor galaxies at z>7 will likely require future surveys of large portions of the sky (>~0.5%) at wavelengths lambda>1 micron. The detection of rare, star-bursting, massive galaxies at redshifts z>~6 would provide support for the hierarchical formation of the earliest quasars and characterize the primitive star-formation histories of the most luminous elliptical galaxies.
We model the luminosity-dependent projected two-point correlation function of DEEP2 (z~1) and SDSS (z~0) galaxies within the Halo Occupation Distribution (HOD) framework. At both epochs, there is a tight correlation between central galaxy luminosity and halo mass, with the slope and scatter decreasing for larger halo masses, and the fraction of satellite galaxies decreasing at higher luminosity. Central L* galaxies reside in halos a few times more massive at z~1 than at z~0. We find little evolution in the relation between mass scales of host halos for central galaxies and satellite galaxies above the same luminosity threshold. Combining these HOD results with theoretical predictions of the typical growth of halos, we establish an evolutionary connection between the galaxy populations at the two redshifts by linking z~0 central galaxies to z~1 central galaxies that reside in their progenitor halos, which enables us to study the evolution of galaxies as a function of halo mass. We find that the stellar mass growth of galaxies depends on halo mass. On average, the majority of the stellar mass in central galaxies residing in z~0 low mass halos (~5x10^11 Msun/h) and only a small fraction of the stellar mass in central galaxies of high mass halos (~10^13 Msun/h) result from star formation between z~1 and z~0. In addition, the mass scale of halos where the star formation efficiency reaches a maximum is found to shift toward lower mass with time. Future work that combines HOD modeling of the clustering of galaxies at different redshifts with the assembly history and dynamical evolution of dark matter halos can lead to an understanding of the stellar mass growth due to both mergers and star formation as a function of host halo mass and provide powerful tests of galaxy formation theories. (Abridged).
We briefly review the current status of the study of long-duration gamma-ray burst (GRB) host galaxies. GRB host galaxies are mainly interesting to study for two reasons: 1) they may help us understand where and when massive stars were formed throughout cosmic history, and 2) the properties of host galaxies and the localisation within the hosts where GRBs are formed may give essential clues to the precise nature of the progenitors. The main current problem is to understand to what degree GRBs are biased tracers of star formation. If GRBs are only formed by low-metallicity stars, then their host galaxies will not give a representative view of where stars are formed in the Universe (at least not a low redshifts). On the other hand, if there is no dependency on metallicity then the nature of the host galaxies leads to the perhaps surprising conclusion that most stars are formed in dwarf galaxies. In order to resolve this issue and to fully exploit the potential of GRBs as probes of star-forming galaxies throughout the observable universe it is mandatory that a complete sample of bursts with redshifts and host galaxy detections is built.
I review the use of type-Ia supernovae (SNe) for cosmological studies. After briefly recalling the main features of type-Ia SNe that lead to their use as cosmological probes, I briefly describe current and planned type-Ia SNe surveys, with special emphasis on their physics reach in the presence of systematic uncertainties, which will be dominant in nearly all cases.
The serendipitous detection of stellar occultations by Outer Solar System objects is a powerful method for ascertaining the small end ($r \lesssim 15$ km) of the size distribution of Kuiper Belt Objects and may potentially allow the exploration of objects as far out as the Oort Cloud. The design and implementation of an occultation survey is aided by a detailed understanding of how diffraction and observational parameters affect the detection of occultation events. In this study, stellar occultations are simulated, accounting for diffraction effects, finite source sizes, finite bandwidths, stellar spectra, sampling, and signal-to-noise. Finally, the possibility of detecting small Outer Solar System objects from the Kuiper Belt all the way out to the Oort Cloud is explored for three photometric systems: a proposed space telescope, Whipple (Kaplan et al. 2003), the Taiwanese-American Occultation Survey (Lehner et al. 2006), and the Multi Mirror Telescope (Bianco 2007).
We report the discovery of a galaxy cluster serendipitously detected as an extended X-ray source in an offset observation of the group NGC 5044. The cluster redshift, z=0.281, determined from the optical spectrum of the brightest cluster galaxy, agrees with that inferred from the X-ray spectrum using the Fe K alpha complex of the hot ICM (z=0.27 +/- 0.01). Based on the 50 ks XMM observation, we find that within a radius of 383 kpc the cluster has an unabsorbed X-ray flux, f_X (0.5-2 keV) = 3.34 (+0.08, -0.13) x 10^{-13} erg/cm^2/s, a bolometric X-ray luminosity, L_X = 2.21 (+0.34, -0.19) x 10^{44} erg/s, kT = 3.57 +/- 0.12 keV, and metallicity, 0.60 +/- 0.09 solar. The cluster obeys the scaling relations for L_X and T observed at intermediate redshift. The mass derived from an isothermal NFW model fit is, M_vir = 3.89 +/- 0.35 x 10^{14} solar masses, with a concentration parameter, c = 6.7 +/- 0.4, consistent with the range of values expected in the concordance cosmological model for relaxed clusters. The optical properties suggest this could be a ``fossil cluster''.
We show that the Abell Cluster A586 exhibits evidence of the interaction between dark matter and dark energy and argue that this interaction implies a violation of the Equivalence Principle. This violation is found in the context of two different models of dark energy-dark matter interaction. We also argue, based on the spherical symmetry of the Abell Cluster A586 that skewness is not the most general quantity to test the Equivalence Principle.
Ionization front instabilities have long been of interest for their suspected role in a variety of phenomena in the galaxy, from the formation of bright rims and 'elephant trunks' in nebulae to triggered star formation in molecular clouds. Numerical treatments of these instabilities have historically been limited in both dimensionality and input physics, leaving important questions about their true evolution unanswered. We present the first three-dimensional radiation hydrodynamical calculations of both R-type and D-type ionization front instabilities in galactic environments (i.e., solar metallicity gas). Consistent with linear stability analyses of planar D-type fronts, our models exhibit many short-wavelength perturbations growing at early times that later evolve into fewer large-wavelength structures. The simulations demonstrate that both self-consistent radiative transfer and three-dimensional flow introduce significant morphological differences to unstable modes when compared to earlier two-dimensional approximate models. We find that the amplitude of the instabilities in the nonlinear regime is primarily determined by the efficiency of cooling within the shocked neutral shell. Strong radiative cooling leads to long, extended structures with pronounced clumping while weaker cooling leads to saturated modes that devolve into turbulent flows. These results suggest that expanding H II regions may either promote or provide turbulent support against the formation of later generations of stars, with potential consequences for star formation rates in the galaxy today.
The magnetic cataclysmic variable AE Aquarii hosts a rapidly rotating white dwarf which is thought to expel most of the material streaming onto it. Observations of AE Aqr have been obtained in the wavelength range of 5 - 70 microns with the IRS, IRAC, and MIPS instruments on board the Spitzer Space Telescope. The spectral energy distribution reveals a significant excess above the K4V spectrum of the donor star with the flux increasing with wavelength above 12.5 microns. Superposed on the energy distribution are several hydrogen emission lines, identified as Pf alpha and Hu alpha, beta, gamma. The infrared spectrum above 12.5 microns can be interpreted as synchrotron emission from electrons accelerated to a power-law distribution dN=E^{-2.4}dE in expanding clouds with an initial evolution timescale in seconds. However, too many components must then be superposed to explain satisfactorily both the mid-infrared continuum and the observed radio variability. Thermal emission from cold circumbinary material can contribute, but it requires a disk temperature profile intermediate between that produced by local viscous dissipation in the disk and that characteristic of a passively irradiated disk. Future high-time resolution observations spanning the optical to radio regime could shed light on the acceleration process and the subsequent particle evolution.
We numerically examine centrifugally supported shock waves in 2D rotating accretion flows around a stellar-mass (10M_sun) and a supermassive (10^6M_sun) black holes over a wide range of input accretion rates of 10^7 >\dot M/\dot M_E>10^{-4}. The resultant 2D-shocks are unstable with time and the luminosities show quasi-periodic oscillations (QPOs) with modulations of a factor of 2-3 andwith periods of a tenth seconds to several hours, depending on the black hole masses. The shock oscillation model may explain the intermediate frequency QPOs with 1-10 Hz observed in the stellar-mass black hole candidates and also suggest the existence of QPOs with the period of hours in AGNs. When the accretion rate is low, the luminosity increases in proportion to the accretion rate. However, when the accretion rate exceeds greatly the Eddington critical rate \dot M_E, the luminosity is insensitive to the accretion rate and is kept constantly around 3 L_E. On the other hand, the mass-outflow rate increases in proportion to the accretion rate and it amounts to about a few percent of theinput mass-flow rate.
The possibility to construct a galactic disk embedded in a multidimensional space-time is investigated. Particularly we are interested in a disk that lives in a universe endowed with Universal Extra Dimensions. The simplest example is a six dimensional space-time disk constructed by solving the vacuum Einstein equations for an extension of the Weyl's metric. In particular, we study a disk constructed from Schwarzschild and Chazy-Curzon solutions with a simple extension for the extra dimensions. Two integral constants of motion from projection of extradimensional particle velocities are the free parameters of the model. We prevent the ad hoc adjustment of such parameters with observed rotation curves, preferring to investigate values where the disk becomes stable. The stability is achieved when the disk is Newtonian-like (where such parameters are null) or for a tiny range of values that astonishingly makes the circular geodesics fit with great precision the rotation curves of many spiral galaxies. The stability calculation is done using both the Rayleigh criterion and a perturbative approach. We compare such results to well succeeded astrophysical dark matter profiles and demonstrate that our predictions give the same gravitational lensing as does a dynamically successful dark halo model. Finally, we consider the possibility that our model could constrain a Kaluza-Klein dark matter particle to be tested at Large Hadron Collider (LHC).
We present two diagnostic methods based on ideas of Principal Component Analysis and demonstrate their efficiency for sophisticated processing of multicolour photometric observations of variable objects.
We propose new diagnostics for circumstellar interaction in Type IIP supernovae by the detection of high velocity (HV) absorption features in Halpha and He I 10830 A lines during the photospheric stage. To demonstrate the method, we compute the ionization and excitation of H and He in supernova ejecta taking into account time-dependent effects and X-ray irradiation. We find that the interaction with a typical red supergiant wind should result in the enhanced excitation of the outer layers of unshocked ejecta and the emergence of corresponding HV absorption, i.e. a depression in the blue absorption wing of Halpha and a pronounced absorption of He I 10830 A at a radial velocity of about -10,000 km/s. We identify HV absorption in Halpha and He I 10830 A lines of SN 1999em and in Halpha of SN 2004dj as being due to this effect. The derived mass loss rate is close to 10^{-6} Msun/yr for both supernovae, assuming a wind velocity 10 km/s. We argue that, in addition to the HV absorption formed in the unshocked ejecta, spectra of SN 2004dj and SN 1999em show a HV notch feature that is formed in the cool dense shell (CDS) modified by the Rayleigh-Taylor instability. The CDS results from both shock breakout and radiative cooling of gas that has passed through the reverse shock wave. The notch becomes dominant in the HV absorption during the late photospheric phase, ~60 d. The wind density deduced from the velocity of the CDS is consistent with the wind density found from the HV absorption produced by unshocked ejecta.
This contribution has the aim to review the basics of existing knowledge on asteroid interiors and to explore the possibility of advancing the field by introducing into it some ideas from solid state physics.
We first show that the new WMAP 3 year data confirm the detection by Myers et al. (2004) of an extended SZ signal centred on 606 Abell clusters. We also detect SZ decrements around APM and 2MASS groups at increased significance than previously. We then follow the approach of Lieu et al. (2006) and compare the stacked WMAP results for the decrement in 31 clusters with ROSAT X-ray profiles where Lieu et al. found on average less SZ decrement in the WMAP 1 year data than predicted. We confirm that in the 3 year data these same clusters also show less SZ decrement than the X-ray data predicts. We then analysed the WMAP results for the 38 X-ray clusters with OVRO/BIMA measured SZ decrements as presented by Bonamente et al. (2006). We again find that the average decrement is measured to be significantly less (5.5 sigma) than predicted by the Chandra X-ray data. These X-ray data cause us to re-interpret our previous result; rather than seeing too much SZ at large scales, we may actually be seeing too little SZ decrement near the cluster centre. One possible explanation is that there is contamination of the WMAP SZ signal by radio sources in the clusters but we argue that this appears implausible. We then consider the possibility that the SZ decrement has been lensed away by foreground galaxy groups. Such a model predicts that the SZ decrement should depend on cluster redshift. This effect is clearly detected in the ACO cluster sample and also by comparing the samples of Lieu et al. and Bonamente et al. But the mass power-spectrum would require a far higher amplitude than currently expected if lensing was to explain the SZ deficit in high redshift clusters.
Weak gravitational lensing is a promising probe of dark matter and dark energy requiring accurate measurement of the shapes of faint, distant galaxies. Such measures are hindered by the finite resolution and pixel scale of typical cameras. On the other hand, as imaging telescopes are practically limited to a fixed number of pixels and operational life-span, so the survey area increases with pixel size. We investigate the optimum choice of pixel scale in this trade-off for a space-based mission, using the full engineering model and survey strategy of the proposed SuperNova/Acceleration Probe as an example. Our methodology is to simulate realistic astronomical images of known shear and to evaluate the surface density of sources where the shear is accurately recovered using the Rhodes, Refregier, & Groth algorithm in the context of the derived dark matter power spectrum. In addition to considering single exposures, we also examine the benefits of sub-pixel dithering. Although some of our results depend upon the adopted shape measurement method, the relative trends, particularly those involving the surface density of resolved galaxies, are robust. Our approach provides a practical counterpart to studies which consider the effects of pixelation from analytic principles, which necessarily assume an idealized shape measurement method. We find that the statistical error on the mass power spectrum is minimized with a pixel scale equal to 75-80% of the FWHM of the point-spread function, and that dithering is marginally beneficial at larger pixel scales.
We present high-quality, Keck spectroscopic data for a sample of 20 globular clusters (GCs) in the massive E0 galaxy NGC1407. A subset of twenty line-strength indices of the Lick/IDS system have been measured for both the GC system and the central integrated star-light of the galaxy. Ages, metallicities and [alpha/Fe] ratios have been derived using several different approaches. The majority GCs in NGC1407 studied are old, follow a tight metallicity sequence reaching values slightly above solar, and exhibit mean [alpha/Fe] ratios of ~ 0.3 dex. In addition, three GCs are formally derived to be young (~ 4 Gyr), but we argue that they are actually old GCs hosting blue horizontal branches. We report, for the first time, evidence for the existence of two chemically-distinct subpopulations of metal-rich (MR) GCs. We find some MR GCs exhibit significantly larger [Mg/Fe] and [C/Fe] ratios. Different star formation time-scales are proposed to explain the correlation between Mg and C abundances. We also find striking CN overabundances over the entire GC metallicity range. Interestingly, the behavior of C and N in metal-poor (MP) GCs clearly deviates from the one in MR GCs. In particular, for MR GCs, N increases dramatically while C essentially saturates. This may be interpreted as a consequence of the increasing importance of the CNO cycle with increasing metallicity.
Our mid-infrared survey of 124 white dwarfs with the Spitzer Space Telescope
and the IRAC imager has revealed an infrared excess associated with the white
dwarf WD 2115-560 naturally explained by circumstellar dust. This object is the
fourth white dwarf observed to have circumstellar dust. All four are DAZ white
dwarfs, i.e. they have both photospheric Balmer lines and photospheric metal
lines.
We discuss these four objects as a class, which we abbreviate "DAZd", where
the "d" stands for "dust". Using an optically-thick, geometrically-thin disk
model analogous to Saturn's rings, we find that the inner disk edges are at
>~0.1 to 0.2 Ro and that the outer disk edges are ~0.3 to 0.6 Ro. This model
naturally explains the accretion rates and lifetimes of the detected WD disks
and the accretion rates inferred from photospheric metal abundances.
I discuss recent observations of asymmetries in Doppler shifts across T Tauri jets, and argue that the observed asymmetric velocity shifts and gradients do not indicate jet rotation. These observations, therefor, cannot be used as a support of a magnetized disk wind. The interaction of the jets with a twisted-tilted (wrapped) accretion disk (or the variable velocity precessing model) accounts better for the observations.
The Panoramic Survey Telescope And Rapid Response System (Pan-STARRS) under development at the University of Hawaii's Institute for Astronomy is creating the first fully automated end-to-end Moving Object Processing System (MOPS) in the world. It will be capable of identifying detections of moving objects in our solar system and linking those detections within and between nights, attributing those detections to known objects, calculating initial and differentially-corrected orbits for linked detections, precovering detections when they exist, and orbit identification. Here we describe new kd-tree and variable-tree algorithms that allow fast, efficient, scalable linking of intra and inter-night detections. Using a pseudo-realistic simulation of the Pan-STARRS survey strategy incorporating weather, astrometric accuracy and false detections we have achieved nearly 100% efficiency and accuracy for intra-night linking and nearly 100% efficiency for inter-night linking within a lunation. At realistic sky-plane densities for both real and false detections the intra-night linking of detections into `tracks' currently has an accuracy of 0.3%. Successful tests of the MOPS on real source detections from the Spacewatch asteroid survey indicate that the MOPS is capable of identifying asteroids in real data.
The mass radius relationship of white dwarfs, near the Chandrasekhar Limit, is derived for a toy model of uniform density, using the variational principle. A power law scaling, reminiscent of those found in 2nd order phase transitions, is obtained. The derived exponent is shown to explain the relationship obtained from relativistic simulations.
We map substructure in three strong lensing systems having particularly good image data: the galaxy lens MG J0414+053 and the clusters SDSS J1004+411 and ACO 1689. Our method is to first reconstruct the lens as a pixelated mass map and then substract off the symmetric part (in the galaxy case) or a projected NFW (for the cluster lenses). In all three systems we find extended irregular structures, or meso-structures, having of order 10% of the total mass. In J0414+053, the meso-structure suggests a tidal tail connecting the main lens with a nearby galaxy; however this interpretation is tentative. In the clusters the identification of meso-structure is more secure, especially in ACO 1689 where two independent sets of lensed images imply very similar meso-structure. In all three cases the meso-structures are correlated with galaxies but much more extended and massive than the stellar components of single galaxies. Such extended structures cannot plausibly persist in such high-density regions without being mixed; the crossing times are too short. The meso-structures therefore appear to be merging or otherwise dynamically evolving systems.
The 5.9 M_sun star HD 125823 is a striking helium variable with a period of 8.82 d, ranging in helium spectral type from He-strong B2 to He-weak B8. For the first time we present here observational evidence for the appearance of emission lines of Mn and Fe in a variable magnetic star.
We present the results of a high spectral resolution study of a few spectroscopic binaries with HgMn primary stars. We detect for the first time in the spectra of HgMn stars that for many elements the line profiles are variable over the rotation period. The strongest profile variations are found for the elements Pt, Hg, Sr, Y, Zr, Mn, Ga, He and Nd. The slight variability of He and Y is also confirmed from the study of high resolution spectra of another HgMn star, alpha And.
One of the major uncertainties in close binary evolution is the efficiency of
mass transfer beta: the fraction of transferred mass that is accreted by a
secondary star. We attempt to constrain the mass-transfer efficiency for
short-period massive binaries undergoing case A mass transfer.
We present a grid of about 20,000 detailed binary evolution tracks with
primary masses 3.5-35 Msun, orbital periods 1-5 days at a metallicity Z=0.004,
assuming both conservative and non-conservative mass transfer. We perform a
systematic comparison, using least-squares fitting, of the computed models with
a sample of 50 double-lined eclipsing binaries in the Small Magellanic Cloud,
for which fundamental stellar parameters have been determined. About 60% of the
systems are currently undergoing slow mass transfer.
In general we find good agreement between our models and the observed
detached systems. However, for many of the semi-detached systems the observed
temperature ratio is more extreme than our models predict. For the 17
semi-detached systems that we are able to match, we find a large spread in the
best fitting mass-transfer efficiency; no single value of beta can explain all
systems. We find a hint that initially wider systems tend to fit better to less
conservative models. We show the need for more accurate temperature
determinations and we find that determinations of surface abundances of
nitrogen and carbon can potentially constrain the mass-transfer efficiency
further.
SN 2000ft is detected in two independent Planetary Camera images (F547W and F814W) taken May 13, 2000, about two months before the predicted date of the explosion (July 19, 2000), based on the analysis of its radio light evolution by Alberdi and collaborators. The apparent optical magnitudes and red color of SN 2000ft indicate that it is observed through an extinction of at least A$_V$= 3.0 magnitudes. The extinction corrected lower limit to the absolute visual magnitude (M$_V$ $\leq -$ 18.0), identifies SN 2000ft as a luminous supernova in the optical, as other luminous radio supernovae before. SN 2000ft exploded in a region located at only 0.1 arcsec (i.e. 34 +/- 3 pc) west of a faint cluster (C24). No parent cluster is identified within the detection limits of the HST short exposures. The unambiguous detection of SN 2000ft in the visual shows that multi-epoch sub-arcsecond (FWHM less than 0.1 arcsec) optical imaging is also a valid tool that should be explored further to detect supernovae in the dusty (circum)nuclear regions of (U)LIRGs.
Nonlinear coupling between 3-minute oscillations and Alfven waves in the solar lower atmosphere is studied. 3-minute oscillations are considered as acoustic waves trapped in a chromospheric cavity and oscillating along transversally inhomogeneous vertical magnetic field. It is shown that under the action of the oscillations the temporal dynamics of Alfven waves is governed by Mathieu equation. Consequently, the harmonics of Alfven waves with twice period and wavelength of 3-minute oscillations grow exponentially in time near the layer where the sound and Alfven speeds equal. Thus the 3-minute oscillations are resonantly absorbed by pure Alfven waves near this resonant layer. The resonant Alfven waves may penetrate into the solar corona taking energy from the chromosphere. Therefore the layer c_s=v_A may play a role of energy channel for otherwise trapped acoustic oscillations.
We study the consequences of a homogeneous dark energy fluid having a
non-vanishing velocity with respect to the matter and radiation large-scale
rest frames. We consider homogeneous anisotropic cosmological models with four
fluids (baryons, radiation, dark matter and dark energy) whose velocities can
differ from each other. Performing a perturbative calculation up to second
order in the velocities, we obtain the contribution of the anisotropies
generated by the fluids motion to the CMB quadrupole and compare with
observations. We also consider the exact problem for arbitrary velocities and
solve the corresponding equations numerically for different dark energy models.
We find that models whose equation of state is initially stiffer than
radiation, as for instance some tracking models, are unstable against velocity
perturbations, thus spoiling the late-time predictions for the energy
densities. In the case of scaling models, the contributions to the quadrupole
can be non-negligible for a wide range of initial conditions. We also consider
fluids moving at the speed of light (null fluids) with positive energy and show
that, without assuming any particular equation of state, they generically act
as a cosmological constant at late times.
We find the parameter region for which the models considered could be
compatible with the measured (low) quadrupole.
We compare the luminosity functions for red galaxies lying on the restframe (U-V) color-magnitude sequence in a homogeneous sample of ten X-ray luminous clusters from the MACS survey at z~0.5 to a similarly selected X-ray cluster sample at z~0.1. We exploit deep Hubble Space Telescope ACS imaging in the F555W and F814W passbands of the central 1.2Mpc diameter regions of the distant clusters to measure precise colors for the galaxies in these regions and statistically correct for contamination by field galaxies using observations of blank fields. We apply an identical analysis to ground-based photometry of the z~0.1 sample. This comparison demonstrates that the number of faint, Mv~ -19, red galaxies relative to the bright population seen in the central regions of massive clusters has roughly doubled over the 4 Gyrs between z~0.5 and z~0.1. We quantify this difference by measuring the dwarf to giant ratio on the red sequence which increases by a factor of at least 2.2+/- 0.4 since z~0.5. This is consistent with the idea that many faint, blue star-forming galaxies in high density environments are transforming onto the red sequence in the last half of the Hubble time.
In this chapter we will argue that studying such multi-scale multi-science systems gives rise to inherently hybrid models containing many different algorithms best serviced by different types of computing environments (ranging from massively parallel computers, via large-scale special purpose machines to clusters of PC's) whose total integrated computing capacity can easily reach the PFlop/s scale. Such hybrid models, in combination with the by now inherently distributed nature of the data on which the models `feed' suggest a distributed computing model, where parts of the multi-scale multi-science model are executed on the most suitable computing environment, and/or where the computations are carried out close to the required data (i.e. bring the computations to the data instead of the other way around). We presents an estimate for the compute requirements to simulate the Galaxy as a typical example of a multi-scale multi-physics application, requiring distributed Petaflop/s computational power.
The precision reached by the recent CMB measurements gives new insights into
the shape of the primordial power spectra of the cosmological perturbations. In
the context of inflationary cosmology, this implies that the CMB data are now
sensitive to the form of the inflaton potential. Most of the current approaches
devoted to the derivation of the inflationary primordial power spectra, or to
the inflaton potential reconstruction problem, rely on approximate analytical
treatments that may break down for exotic models. In this article, we
numerically solve the inflationary evolution of both the background and all the
perturbed quantities to extract the primordial power spectra exactly. Such a
method solely relies on General Relativity and linear perturbation theory. More
than providing a tool to test analytical approximations, one may consider,
without complications, the treatment of non-standard inflationary models as
those involving several fields, eventually non-minimally coupled to gravity.
The usefulness of the exact numerical approach to deal with CMB data is
illustrated by analysing the WMAP third year data in the context of single
field models. For this purpose, we introduce a new inflationary related
parameter encoding the basic properties of the reheating era. This reheating
parameter has significant observable effects and provides a self-consistency
test of inflationary models. As a working example, the marginalised probability
distributions of the reheating and potential parameters associated with the
small field models are presented.
The stellar disks of many spiral galaxies are twice as large as generally thought. We use archival data from the Galaxy Evolution Explorer mission (GALEX) to quantify the statistical properties of young stellar clusters in the outer, extended disks of a sample of eleven nearby galaxies. We find an excess of sources between 1.25 and 2 optical radii, R(25), for five of the galaxies, which statistically implies that at least a quarter of such galaxies have this cluster population (90% confidence level), and no significant statistical excess in the sample as a whole beyond 2 optical radii, even though one galaxy (M 83) individually shows such an excess. Although the excess is typically most pronounced for blue (FUV -NUV < 1, NUV < 25) sources, there is also an excess of sources with redder colors. Although from galaxy to galaxy the number of sources varies significantly, on average, the galaxies with such sources have 75 +- 10 blue sources at radii between 1.25 and 2 R(25). In addition, the radial distribution is consistent with the extended dust emission observed in the far IR and with the properties of H-alpha sources, assuming a constant cluster formation rate over the last few hundred Myrs. All of these results suggest that the phenomenon of low-level star formation well outside the apparent optical edges of disks (R ~ R(25)) is common and long-lasting.
CONTEXT: Manganese is an iron-peak element and although the nucleosynthesis path that leads to its formation is fairly well understood, it remains unclear which objects, SN II and/or SN Ia, that contribute the majority of Mn to the interstellar medium. It also remains unclear to which extent the supernovae Mn yields depend on the metallicity of the progenitor star or not. AIMS: By using a well studied and well defined sample of 95 dwarf stars we aim at further constraining the formation site(s) of Mn. METHODS: We derive Mn abundances through spectral synthesis of four Mn I lines at 539.4, 549.2, 601.3, and 601.6 nm. Stellar parameters and data for oxygen are taken from Bensby et al. (2003, 2004, 2005). RESULTS: When comparing our Mn abundances with O abundances for the same stars we find that the abundance trends in the stars with kinematics of the thick disk can be explained by metallicity dependent yields from SN II. We go on and combine our data for dwarf stars in the disks with data for dwarf and giant stars in the metal-poor thick disk and halo from the literature. We find that dwarf and giant stars show the same trends, which indicates that neither non-LTE nor evolutionary effects are a major concern for Mn. Furthermore, the [Mn/O] vs [O/H] trend in the halo is flat. CONCLUSIONS: We conclude that the simplest interpretation of our data is that Mn is most likely produced in SN II and that the Mn yields for such SNae must be metallicity dependent. Contribution from SN Ia in the metal-rich thin disk can not, however, be excluded.
We present a study of the evolved stellar populations in the dwarf spheroidal galaxy Fornax based on JHK imaging photometry. The observations cover an 18.5x18.5 arcmin central area with a mosaic of NTT/SOFI images. Our data sample all the red giant branch for the whole area. Deeeper observations reaching the red clump of helium-burning stars have also been obtained for a 4.5 x 4.5 arcmin region. Near-infrared photometry led to measurements of the distance to Fornax based on the K-band location of the RGB tip and the red clump. Once corrected for the mean age of the stellar populations in the galaxy, the derived distance modulus is 20.74 corresponding to a distance of 141 Kpc, in good agreement with estimates from optical data. By taking age effects into account, we have derived a distribution function of global metallicity [M/H] from optical-infrared colors of individual stars. Our photometric Metallicity Distribution Function covers the range -2.0<[M/H]<-0.6, with a main peak at [M/H]~-0.9 and a long tail of metal-poor stars, and less metal-rich stars than derived by recent spectroscopy. If metallicities from CaII triplet lines are correct, this result confirms a scenario of enhanced metal enrichment in the last 1-4 Gyr.
We present results of synthetic spectro-polarimetric diagnostics of radiative MHD simulations of solar surface convection with magnetic fields. Stokes profiles of Zeeman-sensitive lines of neutral iron in the visible and infrared spectral ranges emerging from the simulated atmosphere have been calculated in order to study their relation to the relevant physical quantities and compare with observational results. We have analyzed the dependence of the Stokes-I line strength and width as well as of the Stokes-V signal and asymmetries on the magnetic field strength. Furthermore, we have evaluated the correspondence between the actual velocities in the simulation with values determined from the Stokes-I (Doppler shift of the centre of gravity) and Stokes-V profiles (zero-crossing shift). We confirm that the line weakening in strong magnetic fields results from a higher temperature (at equal optical depth) in the magnetic flux concentrations. We also confirm that considerable Stokes-V asymmetries originate in the peripheral parts of strong magnetic flux concentrations, where the line of sight cuts through the magnetopause of the expanding flux concentration into the surrounding convective donwflow.
We report the discovery of ROXA J081009.9+384757.0 = SDSS J081009.9+384757.0, a z=3.95 blazar with a highly unusual Spectral Energy Distribution (SED). This object was first noticed as a probable high $f_x/f_r$, high-luminosity blazar within the error region of a $\approx 10^{-12}$ erg/s cm$^2$ ROSAT source which, however, also included a much brighter late-type star. We describe the results of a recent Swift observation that establishes beyond doubt that the correct counterpart of the X-ray source is the flat spectrum radio quasar. With a luminosity well in excess of $10^{47}$ erg/s, ROXA J081009.9+384757.0 is therefore one of the most luminous blazars known. We consider various possibilities for the nature of the electromagnetic emission from this source. In particular, we show that the SED is consistent with that of a blazar with synchrotron power peaking in the hard X-ray band. If this is indeed the case, the combination of high-luminosity and synchrotron peak in the hard-X-ray band contradicts the claimed anti-correlation between luminosity and position of the synchrotron peak usually referred to as the "blazar sequence". An alternative possibility is that the X-rays are not due to synchrotron emission, in this case the very peculiar SED of ROXA J081009.9+384757.0 would make it the first example of a new class of radio loud AGN.
Modern cosmological observations clearly reveal that the universe contains a
hierarchy of clustering. However, recent surveys show a transition to
homogeneity on large scales. The exact scale at which this transition occurs is
still a topic of much debate. There has been much work done in trying to
characterise the galaxy distribution using multifractals. However, for a number
of years the size, depth and accuracy of galaxy surveys was regarded as
insufficient to give a definitive answer. One of the main problems which arises
in a multifractal analysis is how to deal with observational selection effects:
ie `masks' in the survey region and a geometric boundary to the survey itself.
In this thesis I will introduce a volume boundary correction which is rather
similar to the approach developed by Pan and Coles in 2001, but which improves
on their angular boundary correction in two important respects: firstly, our
volume correction `throws away' fewer galaxies close the boundary of a given
data set and secondly it is computationally more efficient.
After application of our volume correction, I will then show how the
underlying generalised dimensions of a given point set can be computed. I will
apply this procedure to calculate the generalised fractal dimensions of both
simulated fractal point sets and mock galaxy surveys which mimic the properties
of the recent IRAS PSCz catalogue.
(ABRIDGED) AIMS: We searched for X-ray signatures of AGN in a sample of star-forming, relatively early-type, nearby spiral galaxies. Such galaxies are likely to host nuclear super-massive black holes. METHODS: For 9 sources from the Ho et al. optical sample, we isolated 10 individual Chandra X-ray sources closest to the optical position and calculated X-ray luminosities, Lx, X-ray colours, and, using Halpha luminosities, star formation rates. For 4 sources with sufficient counts, we extracted X-ray spectra and fitted standard spectral models. We assessed the significance of adding an Fe K alpha emission Gaussian component to a fit by means of a calibration of the standard F-test. For the rest of the sources, we estimated values for the intrinsic hydrogen column density, NHint, and the power-law photon index, Gamma, which can reproduce the observed X-ray colours. RESULTS: For the nuclear sources in NGC 2782 and NGC 3310, a Fe K alpha emission-line component is included with high significance, whilst the power-law is flat (Gamma~0 and 1.3). Both sources have high star-formation rates, with the rate for NGC 2782 being the highest in our sample. CONCLUSIONS: Our detection of Fe K alpha emission coming from a central, isolated source points towards a hidden AGN in 2 out of 9 relatively early-type, star-forming spirals. The presence of an AGN is thus likely in massive, disturbed starbursts, although the X-ray contribution is small: For the central source, the bolometric luminosity is a fraction of a few x 10^-5 of the Eddington luminosity. The X-ray flux of the central source is between 10 and 20% of the total galactic flux in the 0.2-10 keV band.
The organization of this paper is as follows: the basic formulae used to derive the age of synchrotron sources and the equipartition parameters are presented in Section 2, while the observational properties of diffuse radio sources (Radio halos, Radio relics, and Mini-halos) are presented in Section 3. Then in Section 4 we give a general outline of the models of the relativistic particle origin and re-acceleration; while the current results on cluster magnetic fields are described in Section 5. Finally, Section 6 reports the properties of cluster radio emitting galaxies.
Black hole binaries with extreme ($\gtrsim 10^4:1$) or intermediate ($\sim 10^2-10^4:1$) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed Laser Interferometer Space Antenna. These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis, and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany.
These lecture notes present the computation of the full system of Boltzmann equations describing the evolution of the photon, baryon and cold dark matter fluids up to second order in perturbation theory, as recently studied in (Bartolo, Matarrese & Riotto 2006, 2007). These equations allow to follow the time evolution of the cosmic microwave background anisotropies at all angular scales from the early epoch, when the cosmological perturbations were generated, to the present, through the recombination era. The inclusion of second-order contributions is mandatory when one is interested in studying possible deviations from Gaussianity of cosmological perturbations, either of primordial (e.g. inflationary) origin or due to their subsequent evolution. Most of the emphasis in these lectures notes will be given to the derivation of the relevant equations for the study of cosmic microwave background anisotropies and to their analytical solutions.
The black hole candidate X-ray transient XTE J1817-330 was observed by the Swift satellite over 160 days of its 2006 outburst with the XRT and UVOT instruments. At the start of the observations, the XRT spectra show that the 0.6-10 keV emission is dominated by an optically thick, geometrically thin accretion disk with an inner disk temperature of $\sim0.8$ keV, indicating that the source was in a high/soft state during the initial outburst phase. We tracked the source through its decline into the low/hard state with the accretion disk cooling to $\sim 0.2 \mathrm{keV}$ and the inner disk radius consistent with the innermost stable circular orbit at all times. Furthermore, the X-ray luminosity roughly follows $L_X \propto T^4$ during the decline, consistent with a geometrically stable blackbody. These results are the strongest evidence yet obtained that accretion disks do not automatically recede after a state transition, down to accretion rates as low as $0.001 L_{Edd}$. Meanwhile, the near-UV flux does not track the X-ray disk flux, and is well in excess of what is predicted if the near-UV emission is from viscous dissipation in the outer disk. The strong correlation between the hard X-ray flux and the near-UV flux, which scale as $L_X^{0.5}$, indicate that reprocessed emission is most likely the dominate contribution to the near-UV flux. We discuss our results within the context of accretion disks and the overall accretion flow geometry in accreting black holes.
We present the results of numerical simulations of shock wave-driven jets in the solar atmosphere. The dependence of observable quantities like maximum velocity and deceleration on parameters such as the period and amplitude of initial disturbances and the inclination of the magnetic field is investigated. Our simulations show excellent agreement with observations, and shed new light on the correlation between velocity and deceleration and on the regional differences found in observations.
New xmm observations have been performed around periastron and apastron passages in September 2005, during an epoch of presumably enhanced O star wind. Also, 2005 Chandra observations on LS 5039 are revisited. Moreover, a compilation of the Halpha EW and rxte/ASM X-ray count rate obtained since the 1990s is carried out, being both quantities compared with each other and also with historical radio data. xmm observations show higher and harder emission around apastron than around periastron. No signatures of thermal emission or a reflection iron line indicating the presence of an accretion disk are found in the spectrum, and the hydrogen column density (N_H) is compatible with a constant and with the interstellar value in both observations. The hardness ratio and the count rate seem uncorrelated at periastron and may be correlated at apastron. We find that LS 5039 was bright and hard in 2005 chandra observations. The ASM count rate shows changes by a ~80% on year timescales, and the Halpha EW shows yearly variations of a ~10%. Both quantities may be anticorrelated rather than correlated, unlike it was previously thought. At radio frequencies, the emission varies by ~20%, presenting some similarities with the X-rays. ASM, Halpha EW and radio data may hint to variability at orbital timescales. The low value and constancy of the N_H could imply that the X-ray emitter is located at >~10^12 cm from the compact object. We suggest that the non-thermal emission in LS 5039 is related to the jet and the stellar wind in a complex way and produced outside the system. However, the present data do not rule out the pulsar scenario.
We present an optical spectroscopic survey of 24 micron and 1.4 GHz sources, detected in the Spitzer Extragalactic First Look Survey (FLS), using the multi-fiber spectrograph, Hydra, on the WIYN telescope. We have obtained spectra for 772 sources, with flux densities above 0.15 mJy in the infrared, and 0.09 mJy in the radio. The redshifts measured in this survey are mostly in the range 0 < z < 0.4, with a distribution peaking at z = 0.2. Detailed spectral analysis of our sources reveals that the majority are emission-line star-forming galaxies, with star formation rates in the range 0.2-200 Msun/yr. The rates estimated from the H-alpha line fluxes are found to be on average consistent with those derived from the 1.4 GHz luminosities. For these star-forming systems, we find that the 24 micron and 1.4 GHz flux densities follow an infrared-radio correlation, that can be characterized by a value of q24 = 0.83, with a 1-sigma scatter of 0.31. Our WIYN/Hydra database of spectra complements nicely those obtained by the Sloan Digital Sky Survey, in the region at lower redshift, as well as the MMT/Hectospec survey of Papovich et al. (2006), and brings the redshift completeness to 70% for sources brighter than 2 mJy at 24 micron. Applying the classical 1/Vmax method, we derive new 24 micron and 1.4 GHz luminosity functions, using all known redshifts in the FLS. We find evidence for evolution in both the 1.4 GHz and 24 micron luminosity functions in the redshift range 0 < z < 1. The redshift catalog and spectra presented in this paper are available at the Spitzer FLS website.
The X-ray transient, 4U 1730-22, has not been detected in outburst since 1972, when a single outburst was detected by the Uhuru satellite. This neutron star or black hole X-ray binary is presumably in quiescence now, and here, we report on X-ray and optical observations of the 4U 1730-22 field designed to identify the system's quiescent counterpart. Using Chandra, we have found a very likely counterpart. The candidate counterpart is close to the center of the Uhuru error region and has a thermal spectrum. The 0.3-8 keV spectrum is well-described by a neutron star atmosphere model with an effective temperature of 131+/-21 eV. For a neutron star with a 10 km radius, the implied source distance is 10(+12)(-4) kpc, and the X-ray luminosity is 1.9E33 ergs/s assuming a distance of 10 kpc. Accretion from a companion star is likely required to maintain the temperature of this neutron star, which would imply that it is an X-ray binary and therefore, almost certainly the 4U 1730-22 counterpart. We do not detect an optical source at the position of the Chandra source down to R > 22.1, and this is consistent with the system being a Low-Mass X-ray Binary at a distance greater than a few kpc. If our identification is correct, 4U 1730-22 is one of the 5 most luminous of the 20 neutron star transients that have quiescent X-ray luminosity measurements.
Aims: We investigated the possible relationship between the evolutionary
stage of post-AGB stars and planetary nebulae (PNe) and the presence of water
masers in their envelopes.
Methods: We have used NASA's 70-m antenna in Robledo de Chavela (Spain) to
search for the water maser transition at 22235.08 MHz, towards a sample of 105
sources with IRAS colour characteristic of post-AGB stars and PNe at
declination >-32 deg. 83% of the sources in the sample are post-AGB stars, 15%
PNe or PN candidates, while only 2% seem to be HII regions.
Results: We have detected five water masers, of which four are reported for
the first time: two in PNe (IRAS 17443-2949 and IRAS 18061-2505), a ``water
fountain'' in a post-AGB star (IRAS 16552-3050), and one in a source previously
catalogued as a PN, but whose classification is uncertain (IRAS 17580-3111).
Conclusions: The unexpected detections of water masers in two objects among
the small subset of PNe led us to suggest that the PNe harbouring water masers
are a special type of massive, rapidly evolving PNe.
We present the GALEX UV photometry of the elliptical galaxies in Abell clusters at moderate redshifts (z < 0.2) for the study of the look-back time evolution of the UV upturn phenomenon. The brightest elliptical galaxies (M_r < -22) in 12 remote clusters are compared with the nearby giant elliptical galaxies of comparable optical luminosity in the Fornax and Virgo clusters. The sample galaxies presented here appear to be quiescent without signs of massive star formation or strong nuclear activity, and show smooth, extended profiles in their UV images indicating that the far-UV (FUV) light is mostly produced by hot stars in the underlying old stellar population. Compared to their counterparts in nearby clusters, the FUV flux of cluster giant elliptical galaxies at moderate redshifts fades rapidly with ~ 2 Gyrs of look-back time, and the observed pace in FUV - V color evolution agrees reasonably well with the prediction from the population synthesis models where the dominant FUV source is hot horizontal-branch stars and their progeny. A similar amount of color spread (~ 1 mag) in FUV - V exists among the brightest cluster elliptical galaxies at z ~ 0.1, as observed among the nearby giant elliptical galaxies of comparable optical luminosity.
We use a sample of 116 galaxy clusters at 0.1< z<1.3 observed with Chandra ACIS-I to investigate the scaling relation between luminosity and Yx (the product of gas mass and temperature). We find that the scatter in the relation is dominated by the core regions of the cluster, and recover a tight Lx-Yx relation (11% scatter in Lx) if sufficiently large core regions (0.15R500) are excluded from the luminosity measurement. The intrinsic scatter is well described by a lognormal distribution. The best fit Lx-Yx relations are consistent for both relaxed and disturbed/merging clusters although the scatter is smaller for the former. We investigate the Lx-Yx relation in low-quality data (e.g. for clusters detected in survey data) by estimating Lx from count rates in the (0.5-2) keV band in an aperture defined without knowledge of the cluster temperatures, and find that the scatter increases slightly to 21%. Finally we use the tight correlation between Yx and mass to measure the Lx-M relation for the sample. We find that the scatter is much lower than previous estimates. This is due to the full removal of cluster cores and more robust mass estimates (from Yx rather than temperature alone). Furthermore, for high-redshift clusters (z>0.5) the scatter in the Lx-M relation remains low if cluster cores are not excluded, likely due to the absence of cool core clusters at z>0.5. These results suggest that cluster masses can be reliably estimated from simple luminosity measurements in low quality data where direct mass estimates, or measurements of Yx are not possible. This has important applications in the estimation of cosmological parameters from X-ray cluster surveys.
Stecker, Malkan and Scully, have shown how ongoing deep surveys of galaxy luminosity functions, spectral energy distributions and backwards evolution models of star formation rates can be used to calculate the past history of intergalactic photon densities for energies from 0.03 eV to the Lyman limit at 13.6 eV and for redshifts out to 6 (called here the intergalactic background light or IBL). From these calculations of the IBL at various redshifts, they predict the present and past optical depth of the universe to high energy gamma-rays owing to interactions with photons of the IBL and the 2.7 K CMB. We discuss here how this proceedure can be reversed by looking for sharp cutoffs in the spectra of extragalactic gamma-ray sources such as blazars at high redshifts in the multi-GeV energy range with GLAST. By determining the cutoff energies of sources with known redshifts, we can refine our determination of the IBL photon densities in the past, i.e., the "archeo-IBL", and therefore get a better measure of the past history of the total star formation rate. Conversely, observations of sharp high energy cutoffs in the gamma-ray spectra of sources at unknown redshifts can be used instead of spectral lines to give a measure of their redshifts.
This letter reports on the Galactic stellar structures that appear in the foreground of our Canada-France-Hawaii-Telecopse/MegaCam survey of the halo of the Andromeda galaxy. We recover the main sequence and main sequence turn-off of the Triangulum-Andromeda structure recently found by Majewski and collaborators at a heliocentric distance of ~20 kpc. The survey also reveals another less populated main sequence at fainter magnitudes that could correspond to a more distant stellar structure at ~28 kpc. Both main sequences are smoothly distributed over the ~54 deg^2 covered by the survey but they show a density increase toward the Galactic plane that does not appear to be due to contamination from the Galactic disk or stellar halo. The discovery of a stellar structure behind the Triangulum-Andromeda structure that itself appears behind the low-latitude stream that surrounds the Galactic disk gives further evidence that the inner halo of the Milky Way is of a spatially clumpy nature.
Pulsars spin-down due to magnetic torque reducing its radius and increasing the central energy density. Some pulsar which are born with central densities close to the critical value of quark deconfinement may undergo a phase transition and structural re-arrengement. This process may excite oscillation modes and emmit gravitational waves. We determine the rate of quark core formation in neutron stars using a realistic population synthesis code.
OH (1720 MHz) masers serve as indicators of SNR - molecular cloud interaction sites. These masers are collisionally excited in warm (50-100K) shocked gas with densities of order 1e5 cm^-3 when the OH column density is in the range 1e16-1e17 cm^-2. Here I present excitation calculations which show that when the OH column density exceeds 1e17 cm^-2 at similar densities and temperatures, the inversion of the 1720 MHz line switches off and instead the 6049 MHz transition in the first excited rotational state of OH becomes inverted. This line may serve as a complementary signal of warm, shocked gas when the OH column density is large.
[Abridged] We revisit the metal budget at z~2. In the first two papers of this series, we already showed that ~30% (to <60% if extrapolating the LF) of the metals are observed in all z~2.5 galaxies detected in current surveys. Here, we extend our analysis to the metals outside galaxies, i.e. in intergalactic medium (IGM), using observational data and analytical calculations. Our results for the two are strikingly similar: (1) Observationally, we find that, besides the small (5%) contribution of DLAs, the forest and sub-DLAs contribute subtantially to make <30--45% of the metal budget, but neither of these appear to be sufficient to close the metal budget. The forest accounts for 15--30% depending on the UV background, and sub-DLAs for >2% to <17% depending on the ionization fraction. Together, the `missing metals' problem is substantially eased. (2) We perform analytical calculations based on the effective yield--mass relation. At z=2, we find that the method predicts that 2$--50% of the metals have been ejected from galaxies into the IGM, consistent with the observations. The metal ejection is predominantly by L<1/3L_B^*(z=2) galaxies, which are responsible for 90% the metal enrichment, while the 50 percentile is at L~1/10L^*_B(z=2). As a consequence, if indeed 50% of the metals have been ejected from galaxies, 3--5 bursts of star formation are required per galaxy prior to z=2. The ratio between the mass of metals outside galaxies to those in stars has changed from z=2 to z=0: it was 1:2 or 1:1 and is now 1:8 or 1:9. This evolution implies that a significant fraction of the IGM metals will cool and fall back into galaxies.
Galaxy interactions are known to trigger starbursts. The young star clusters formed in mergers may be young globular clusters. The ages of these young star clusters yield the timing of interaction-triggered star formation and provide an important way to reconstruct the history of merging galaxies. Here we present the first results from our investigation into age and metallicity of twelve young clusters in the merging galaxy pair NGC 4676, using spectra from the multi-object Low Resolution Imaging Spectrometer (LRIS) on Keck. For ten clusters, comparison of the Balmer emission lines with model equivalent widths (EWs) yields ages less than 10 Myr. Two spectra display Balmer absorption lines typical of star clusters dominated by A-type stars, with estimated ages of about 170 Myr. These ages are comparable to the dynamical age of the tidal tails and are consistent with star formation triggered during the first passage of the pair. The locations of these two clusters in the tidal tails are generally consistent with predictions of shock-induced star formation models. One of these older objects appears unresolved on the image and is luminous enough to qualify as a young globular cluster. Using EWs of the diagnostic lines [OII] and [OIII], we obtain oxygen abundances in the range 7.3 < 12+log(O/H) < 9.0. These values show a nearly flat distribution along the northern tail, suggesting efficient gas mixing in the tail.
The main chromospheric activity indicator is the S index, which is esentially the ratio of the flux in the core of the Ca II H and K lines to the continuum nearby, and is well studied basically for stars from F to K. Another usual chromospheric proxy is the H\alpha line, which is beleived to be tightly correlated with the Ca II index. In this work we characterize both chromospheric activity indicators, one associated with the H and K Ca II lines and the other with H\alpha, for the whole range of late type stars, from F to M. We present periodical medium-resolution echelle observations covering the complete visual range, which were taken at the CASLEO Argentinean Observatory. These observations are distributed along 7 years. We use a total of 917 flux-calibrated spectra for 109 stars which range from F6 to M5. We statistically study these two indicators for stars of different activity levels and spectral types. We directly derive the conversion factor which translate the known S index to flux in the Ca II cores, and extend its calibration to a wider spectral range. We investigate the relation between the activity measurements in the calcium and hydrogen lines, and found that the usual correlation observed is basically the product of the dependence of each flux with stellar colour, and not the product of similar activity phenomena.
If one has to explain dark matter by something else than supersymmetry, one of the most minimal solutions is adding another doublet of Higgs bosons. It has recently been noted, that if an unbroken discrete symmetry forbids the coupling to fermions of the new doublet, then the particle playing the role of the usual Higgs particle may naturally be very heavy, of the order of 500 GeV, without violating electroweak precision bounds. The lightest of the new scalar particles is a natural dark matter candidate, and for a mass between 10 and 80 GeV it can give the correct cosmic abundance as measured by WMAP. It would not yet have shown up in direct detection experiments, and for high Higgs masses also the indirect rates would seem rather small, in particular since tree-level processes giving W and Z final states are kinematically forbidden. However, we show that the loop-induced monochromatic \gamma\gamma and Z\gamma final states would be exceptionally strong for this dark matter candidate. The energy range and rates for these line processes make them ideal to search for in the soon upcoming GLAST satellite experiment.
We present a Chandra X-ray observation of G12.82-0.02, a shell-like radio supernova remnant coincident with the TeV gamma-ray source HESS J1813-178. We resolve the X-ray emission from the co-located ASCA source into a compact object surrounded by structured diffuse emission that fills the interior of the radio shell. The morphology of the diffuse emission strongly resembles that of a pulsar wind nebula. The spectrum of the compact source is well-characterized by a power-law with index approx. 1.3, typical of young and energetic rotation-powered pulsars. For a distance of 4.5 kpc, consistent with the X-ray absorption, the 2-10 keV X-ray luminosity of the putative pulsar and nebula is L(PSR) = 3.2E33 erg/s and L(PWN) = 1.4E34 erg/s, respectively. Both the flux ratio of L(PWN)/L(PSR) = 4.3 and the total luminosity of this system imply a pulsar spin-down power greater then 1E37 erg/s, on a par with the top ten most energetic young pulsars in the Galaxy. We associate the putative pulsar with the radio remnant and the TeV source and discuss the origin of the gamma-ray emission.
Several late-type stars present activity cycles resembling the Solar one. This fact has been observed mostly in stars ranging from F to K, i.e., in stars with a radiative core and an outer convective layer. This work aims at studying whether an activity cycle can be detected in the dM5.5e star Proxima Centauri, which is supposed to be completely convective. We present periodical medium-resolution echelle observations covering the complete visual range, which were taken at the CASLEO Argentinean Observatory. These observations are distributed over 7 years. We discarded the spectra that present flare activity, and analyze the remaining activity levels using four different statistical techniques to look for a period of activity. We find strong evidence of a cyclic activity, with a period of around 442 days. We also estimate that the Ca II S index varies around 130% due to activity variations outside of flares.
Cosmological measurements suggest that our universe contains a dark energy component. In order to study the dark energy evolution, we constrain a parameterized dark energy equation of state $w(z)=w_0 + w_1 \frac{z}{1+z}$ using the recent observational datasets: 157 Gold type Ia supernovae and the newly released 182 Gold type Ia supernovae by maximum likelihood method. It is found that the best fit $w(z)$ crosses -1 in the past and the present best fit value of $w(0)<-1$ obtained from 157 Gold type Ia supernovae. The crossing of -1 is not realized and $w_0=-1$ is not ruled out in $1\sigma$ confidence level for the 182 Gold type Ia supernovae. We also find that the range of parameter $w_0$ is wide even in $1\sigma$ confidence level and the best fit $w(z)$ is sensitive to the prior of $\Omega_m$.
The halo structure at high Galactic latitudes near both north and south poles is studied using SDSS and SuperCOSMOS data. For the southern cape halo, the archive of SuperCOSMOS photographic photometry sky survey is used. The coincident source rate between SuperCOSMOS data in $B_J$ band from $16^m.5$ to $20^m.5$ and SDSS data is about 92%, as from a common sky area in the south. While that in $R_F$ band is about 85% from $16^m.5$ to $19^m.5$. Transformed to SuperCOSMOS system and downgraded to the limiting magnitudes of SuperCOSMOS, the star counts in the northern Galactic cape from SDSS show up to $16.7\pm6.1%$ asymmetric ratio (defined as relative fluctuations over the rotational symmetry structure) in $B_J$ band, and up to $13.5\pm6.9%$ asymmetric ratio in $R_F$ band. From SuperCOSMOS data of $B_J$ and $R_F$ bands, the structure of southern Galactic hemisphere does not show the same obvious asymmetric structures like northern sky does from both original and downgraded SDSS star counts. An axisymmetric halo model with n=2.8 and q=0.7 can fit the projected number density from SuperCOSMOS fairly well, with an average error of about 9.17%. By careful analysis of the difference of star counts between the downgraded SDSS northern halo data and SuperCOSMOS southern halo data, it is clear that the asymmetric halo structure revealed by star counts of SDSS survey data is only visible in the north. Assuming a overall symmetry in the halo, the overdensity in Virgo is very likely a foreign component in the Galactic halo.
A recent data analysis of the far-infrared (FIR) map of the Galaxy and the Magellanic Clouds has shown that there is a tight correlation between two FIR colours: the 60 um-100 um and 100 um-140 um colours. This FIR colour relation called ``main correlation'' can be interpreted as indicative of a sequence of various interstellar radiation fields with a common FIR optical property of grains. In this paper, we constrain the FIR optical properties of grains by comparing the calculated FIR colours with the observational main correlation. We show that neither of the ``standard'' grain species (i.e. astronomical silicate and graphite grains) reproduces the main correlation. However, if the emissivity index at ~ 100--200 um is changed to ~ 1--1.5 (not ~ 2 as the above two species), the main correlation can be successfully explained. Thus, we propose that the FIR emissivity index is ~ 1--1.5 for the dust in the Galaxy and the Magellanic Clouds at ~ 100--200 um. We also consider the origin of the minor correlation called ``sub-correlation'', which can be used to estimate the Galactic star formation rate.
Distant planetary nebulae (PNe) are used to measure distances through the PN luminosity function, as kinematic tracers in determining the mass distribution in elliptical galaxies, and most recently, for measuring the kinematics of the diffuse stellar population in galaxy clusters. This article reviews the photometric and spectroscopic survey techniques that have been used to detect PNe beyond the Local Group, out to the Coma cluster at 100 Mpc distance. Contaminations by other emission sources and ways to overcome them will be discussed as well as some science highlights and future perspectives.
We study the stability of compressible cylindrical differentially rotating flow in the presence of the magnetic field, and show that compressibility alters qualitatively the stability properties of flows. Apart from the well-known magnetorotational instability that can occur even in incompressible flow, there exist a new instability caused by compressibility. The necessary condition of the newly found instability can easily be satisfied in various flows in laboratory and astrophysical conditions and reads $B_{s} B_{\phi} \Omega' \neq 0$ where $B_{s}$ and $B_{\phi}$ are the radial and azimuthal magnetic fields, $\Omega' =d \Omega/ds$ with $s$ being the cylindrical radius. Contrary to the magnetorotational instability that occurs only if $\Omega$ decreases with $s$, the newly found instability operates at any sign of $\Omega'$. The considered instability can arise even in a very strong magnetic field that suppresses the magnetorotational instability.
The non-detection of a point source in SN1987A imposes an upper limit for the optical luminosity of L=2L_sun. This limits the size of a possible fallback disk around the stellar remnant. Assuming a steady-state thin disk with blackbody emission requires a disk smaller than 100,000 km if the accretion rate is at 30% of the Eddington rate (Graves et al. 2005). We have performed detailed non-LTE radiation transfer calculations to model the disk spectrum more realistically. It turns out that the observational limit on the disk extension becomes even tighter, namely 70,000 km.
We present preliminary results of the generalized Principal Component Analysis (PCA) of light curves of 82 magnetic chemically peculiar (further mCP) stars applied to 54 thousand individual photometric observations in the uvby and Hp colours taken from the "On-line database of photometric observations of mCP stars". We show that every of the observed light curves (LCs) can be, with a sufficient accuracy, represented using five parameters of a harmonic polynomial of the second order, and that the third order and higher harmonics reflect only a noise. We found that a prevailing majority of the uvbyHp LCs can be satisfactorily well represented by a linear combination of a constant term and one or two (extraordinarily three) basic, mutually orthonormal functions, which reduces the number of parameters necessarily needed to describe the set of the LCs of a particular star. While the shape of an individual LC depends on the individual distribution of photometric spots on the surface of a rotating star, its amplitude depends on the mechanism of variability. To describe the amplitude of LCs we introduce a robust quantity effective amplitude. Applying the PCA to the set of the effective amplitudes of the LCs in all five colours we revealed at least three different sources of the light variability showing different wavelength dependence. The amplitudes of all the LCs harbor a component monotonously decreasing with increasing wavelengths. The second component reaches a remarkable extremum in the $v$ colour, and is in antiphase relative to the remaining four colours. It occurs particularly at cooler mCP-s having a high degree of the chemical peculiarity. The third component reflects a remarkable diversity of the light curves in the $u$ colour found in a few mCP stars.
In this letter we report on the discovery of a z=2.83 Lyman-alpha Blob (LAB) found in our wide field narrow-band survey within the Spitzer First Look Survey region. The blob is extended over at least 95kpc and has a total Lyman-alpha luminosity of 2.1 x 10^44 erg/s. It is only the sixth LAB known of this scale (> 50 kpc), and is associated with an embedded continuum source in g', R, i', K & 4.5 um bands. The LAB's optical spectrum shows clumpy structures and tantalising hints of a sharp red cut-off and shear within the Ly-alpha emission line. Studies of the LAB's surface brightness profile and of the continuum counterpart's spectral energy distribution (SED) indicate that the profuse Lyman-alpha emission is consistent with being powered by cold gas accreting onto a massive dark matter halo.
P-stars are compact stars made of up and down quarks in $\beta$-equilibrium with electrons in a chromomagnetic condensate. P-stars are able to account for compact stars as well as stars with radius comparable with canonical neutron stars. We compare p-stars with different available observations. Our results indicate that p-stars are able to reproduce in a natural manner several observations from isolated and binary pulsars. On the other hand, we argue that the standard model based on neutron stars is getting in troubles leading to the need for a drastic revision of the standard paradigm.
Context: X-ray surveys carried out with the Einstein and ROSAT satellites have resulted in rather unexpected detections of X-ray emission from late B-type and early A-type stars. These stars possess neither winds like early-type stars nor convective envelopes as late-type stars, so that the origin and production mechanism of this X-emission is unclear. Aims: We investigate whether the presence of large magnetic fields is related to the observed X-ray emission. Methods: We carried out Chandra high-angular resolution observations of a sample of late B-type and A-type stars with measured magnetic fields in the range from 0.2 - 17 kG. Out of the selected 10 sample stars, 6 objects had been previously detected as X-ray sources, some of them, however, with high positional uncertainty and a low signal-to-noise ratio, while 4 of our sample stars do have large magnetic fields but no previous detections of X-ray emission. Results: Our Chandra data confirm all previous ROSAT detections with an extremely high significance, and the limits of the offsets between X-ray and optical positions are greatly improved. In particular, HD 215441, known as Babcock's star with the strongest magnetic field by far (17 kG) of our sample stars, a rather faint and somewhat marginal ROSAT source, can clearly be detected. However, none of the 4 ROSAT non-detections could be detected with the new Chandra observations. Conclusions: The pure existence of a magnetic field of kiloGauss strength on a late B-type or A-type star is therefore not necessarily a prerequisite for finding X-ray emission among these stars. Understanding the observed X-ray emission from Babcock's star is a challenge for observational and theoretical astrophysics.
We investigate numerically a traveling wave pattern observed in experimental magnetized Taylor-Couette flow at low magnetic Reynolds number. By accurately modeling viscous and magnetic boundaries in all directions, we reproduce the experimentally measured wave patterns and their amplitudes. Contrary to previous claims, the waves are shown to be transiently amplified disturbances launched by viscous boundary layers rather than globally unstable magnetorotational modes.
Cosmological simulations of structures and galaxies formations have played a fundamental role in the study of the origin, formation and evolution of the Universe. These studies improved enormously with the use of supercomputers and parallel systems and, recently, grid based systems and Linux clusters. Now we present the new version of the tree N-body parallel code FLY that runs on a PC Linux Cluster using the one side communication paradigm MPI-2 and we show the performances obtained. FLY is included in the Computer Physics Communication Program Library. This new version was developed using the Linux Cluster of CINECA, an IBM Cluster with 1024 Intel Xeon Pentium IV 3.0 Ghz. The results show that it is possible to run a 64 Million particle simulation in less than 15 minutes for each timestep, and the code scalability with the number of processors is achieved. This lead us to propose FLY as a code to run very large N-Body simulations with more than $10^{9}$ particles with the higher resolution of a pure tree code. The FLY new version will be available at this http URL and CPC Program Library.
The possibility of velocity shear-induced linear transformations of different magnetohydrodynamic waves in the solar wind is studied both analytically and numerically. A quantitative analysis of the wave transformation processes for all possible plasma-$\beta$ regimes is performed. By applying the obtained criteria for effective wave coupling to the solar wind parameters, we show that velocity shear-induced linear transformations of Alfv\'en waves into magneto-acoustic waves could effectively take place for the relatively low-frequency Alfv\'en waves in the energy containing interval. The obtained results are in a good qualitative agreement with the observed features of density perturbations in the solar wind.
New source classes are expected to appear in the GLAST/LAT Catalog. Here, the problems faced for their identification are summarized, and some key features of the most likely new populations of the $\gamma$-ray sky are mentioned.
With the advent of large dedicated Type Ia supernova (SN Ia) surveys, K-corrections of SNe Ia and their uncertainties have become especially important in the determination of cosmological parameters. While K-corrections are largely driven by SN Ia broad-band colors, it is shown here that the diversity in spectral features of SNe Ia can also be important. For an individual observation, the statistical errors from the inhomogeneity in spectral features range from 0.01 (where the observed and rest-frame filters are aligned) to 0.04 (where the observed and rest-frame filters are misaligned). To minimize the systematic errors caused by an assumed SN Ia spectral energy distribution (SED), we outline a prescription for deriving a mean spectral template time series which incorporates a large and heterogeneous sample of observed spectra. We then remove the effects of broad-band colors and measure the remaining uncertainties in the K-corrections associated with the diversity in spectral features. Finally, we present a template spectroscopic sequence near maximum light for further improvement on the K-correction estimate. A library of ~600 observed spectra of ~100 SNe Ia from heterogeneous sources is used for the analysis.
We analyze photometric data of the active RS CVn--type star HR 1099 for the years 1975--2006 with an inversion technique and reveal the nature of two activity cycles of 15--16 yr and 5.3$\pm$0.1 yr duration. The 16 yr cycle is related to variations of the total spot area and is coupled with the differential rotation, while the 5.3 yr cycle is caused by the symmetric redistribution of the spotted area between the opposite stellar hemispheres (flip-flop cycle). We recover long-lived active regions comprising two active longitudes that migrate in the orbital reference frame with a variable rate because of the differential rotation along with changes in the mean spot latitudes. The migration pattern is periodic with the 16 yr cycle. Combining the longitudinal migration of the active regions with a previously measured differential rotation law, we recover the first stellar butterfly diagram without an assumption about spot shapes. We find that mean latitudes of active regions at opposite longitudes change antisymmetrically in the course of the 16 yr cycle: while one active region migrates to the pole, the other approaches the equator. This suggests a precession of the global magnetic field with respect to the stellar rotational axis.
2D stellar kinematics of 48 representative E and S0 galaxies obtained with the SAURON IFS reveal that early-type galaxies appear in two broad flavours, depending on whether they exhibit clear large-scale rotation or not. We define a new parameter LambdaR which involves luminosity weighted averages over the full 2D kinematic field, as a proxy to quantify the observed projected stellar angular momentum per unit mass. We use it as a basis for a new kinematic classification: early-type galaxies are separated into slow and fast rotators (SRs, FRs), depending on whether they have LambdaR values within their effective radius Re below or above 0.1, respectively. SRs and FRs are shown to be physically distinct classes of galaxies, a result which cannot simply be the consequence of a biased viewing angle. FRs tend to be relatively low luminosity galaxies. SRs tend to be brighter and more massive galaxies, but are still spread over a wide range of absolute magnitude. 3 slow rotators of our sample, among the most massive ones, are consistent with zero rotation. Remarkably, all other SRs contain a large kpc-scale KDC. All FRs show well aligned photometric and kinemetric axes, and small velocity twists, in contrast with most SRs which exhibit significant misalignments and velocity twists. In a companion paper (Paper X), we also show that FRs and SRs are distinct classes in terms of their orbital distribution. We suggest that gas is a key ingredient in the formation and evolution of FRs, and that the slowest rotators are the extreme evolutionary end point reached deep in gravitational potential wells where dissipationless mergers had a major role in the evolution, and for which most of the baryonic angular momentum was expelled outwards. (abridged)
Early-type galaxies (ETGs) are known to possess a number of quite useful scaling relations among their photometric and/or kinematical quantities. We propose a unified picture reducing both the fundamental plane and the photometric plane to suitable projections of a single relation. Modelling the ETG as a two component system, made out of a luminous Sersic profile and a NFW dark halo, and applying the virial theorem, we are able to express the velocity dispersion \sigma_0 as a function of the effective intensity I_e, the effective radius R_e and the Sersic index n. In a log-log plot, this relation reduces to a hyperplane (i.e., a plane in a four dimensional configuration space) which we dubbed the {\it Sersic Virial Hyperplane} (SVH). The tilt of the SVH can be fully explained in terms of a power-law scaling of the stellar (rather than the global) mass-to-light ratio Upsilon with the total luminosity L_T, while the scatter is determined by those on the c-M_{vir} relation between the concentration c and the virial mass M_{vir} of the dark halo component. In order to test whether such the observed SVH is consistent with the theoretical assumptions, we perform a detailed simulation of a large ETGs sample reproducing the same photometric properties of a SDSS low redshift ETGs catalog. It turns out that the simulated SVH is fully consistent with the observed one thus validating our theoretical approach.
We analyse the orbital distribution of E and S0 galaxies using SAURON integral-field stellar kinematics. We construct the anisotropy (V/sigma,epsilon) diagram, for the 48 E/S0 galaxies from the SAURON survey. For a subsample of 24 galaxies consistent with axisymmetry, we use three-integral axisymmetric Schwarzschild dynamical models to recover the detailed orbital distribution and we find good agreement with the anisotropy derived from the (V/sigma,epsilon) diagram. In the companion Paper IX we show that the early-type galaxies can be subdivided into two classes of systems with or without a significant amount of specific stellar angular momentum. Here we show that the two classes have different distributions on the (V/sigma,epsilon) diagram. The slow rotators are more common among the most massive systems and are generally classified as E from photometry alone. Those in our sample tend to be fairly round (epsilon<0.3), but can have significant kinematical misalignments, indicating that as a class they are moderately triaxial, and span a range of anisotropies (delta<0.3). The fast rotators are generally fainter and are classified either E or S0. They can appear quite flattened (epsilon<0.7), do not show significant kinematical misalignments (unless barred or interacting), indicating they are nearly axisymmetric, and span an even larger range of anisotropies (delta<0.5). These results are confirmed when we extend our analysis to 18 additional E/S0 galaxies observed with SAURON. All these results support the idea that fast rotators are nearly oblate and contain disk-like components. The role of gas must have been important for their formation. The slow rotators are weakly triaxial. Current collisionless merger models seem unable to explain their detailed observed properties. (Abridged)
We present a general recipe for constructing N-body realizations of galaxies comprised of near-spherical and disc components. First, an exact spherical distribution function for the spheroids (halo & bulge) is determined, such that it is in equilibrium with the gravitational monopole of the disc components. Second, an N-body realisation of this model is adapted to the full disc potential by growing the latter adiabatically from its monopole. Finally, the disc is sampled with particles drawn from an appropriate distribution function, avoiding local-Maxwellian approximations. We performed test simulations and find that the halo and bulge radial density profile very closely match their target model, while they become slightly oblate due to the added disc gravity. Our findings suggest that vertical thickening of the initially thin disc is caused predominantly by spiral and bar instabilities, which also result in a radial re-distribution of matter, rather than scattering off interloping massive halo particles.
We present high spatial and high temporal resolution observations of the quiet Sun in H-alpha obtained with the Swedish 1-m Solar Telescope on La Palma. We observe that many mottles, jet-like features in the quiet Sun, display clear up- and downward motions along their main axis. In addition, many mottles show vigorous transverse displacements. Unique identification of the mottles throughout their lifetime is much harder than for their active region counterpart, dynamic fibrils. This is because many seem to lack a sharply defined edge at their top, and significant fading often occurs throughout their lifetime. For those mottles that can be reliably tracked, we find that the mottle tops often undergo parabolic paths. We find a linear correlation between the deceleration these mottles undergo and the maximum velocity they reach, similar to what was found earlier for dynamic fibrils. Combined with an analysis of oscillatory properties, we conclude that at least part of the quiet Sun mottles are driven by magnetoacoustic shocks. In addition, the mixed polarity environment and vigorous dynamics suggest that reconnection may play a significant role in the formation of some quiet Sun jets.
Steep gradients of temperature and density, called cold fronts, are observed by Chandra in a leading edge of subclusters moving through the intracluster medium (ICM). The presence of cold fronts indicates that thermal conduction across the front is suppressed by magnetic fields. We carried out three-dimensional magnetohydrodynamic (MHD) simulations including anisotropic thermal conduction of a subcluster moving through a magnetically turbulent ICM. We found that turbulent magnetic fields are stretched and amplified by shear flows along the interface between the subcluster and the ambient ICM. Since magnetic fields reduce the efficiency of thermal conduction across the front, the cold front survives at least 1 Gyr. We also found that a moving subcluster works as an amplifier of magnetic fields. Numerical results indicate that stretched turbulent magnetic fields accumulate behind the subcluster and are further amplified by vortex motions. The moving subcluster creates a long tail of ordered magnetic fields, in which the magnetic field strength attains plasma beta < 10.
The luminosity functions of galaxies and quasars provide invaluable information about galaxy and quasar formation. Estimating the luminosity function from magnitude limited samples is relatively straightforward, provided that the distances to the objects in the sample are known accurately; techniques for doing this have been available for about thirty years. However, distances are usually known accurately for only a small subset of the sample. This is true of the objects in the Sloan Digital Sky Survey, and will be increasingly true of the next generation of deep multi-color photometric surveys. Estimating the luminosity function when distances are only known approximately (e.g., photometric redshifts are available, but spectroscopic redshifts are not) is more difficult. I describe two algorithms which can handle this complication: one is a generalization of the V_max algorithm, and the other is a maximum likelihood approach. Because these methods account for uncertainties in the distance estimate, they impact a broader range of studies. For example, they are useful for studying the abundances of galaxies which are sufficiently nearby that the contribution of peculiar velocity to the spectroscopic redshift is not negligible, so only a noisy estimate of the true distance is available. In this respect, peculiar velocities and photometric redshift errors have similar effects. The methods developed here are also useful for estimating the stellar luminosity function in samples where accurate parallax distances are not available.
We report on observations of gamma-ray burst (GRB 061126) with an extremely bright (R ~ 12 mag at peak) early-time optical afterglow. The optical afterglow is already fading as a power-law 22 seconds after the trigger, with no detectable prompt contribution in our first exposure, which was coincident with a large prompt-emission pulse. The optical-IR photometric SED is an excellent fit to a power-law but exhibits a moderate red-to-blue evolution in the spectral index at about 500 sec. This color change is contemporaneous with a switch from a relatively fast decay to slower decay. The rapidly decaying early afterglow is broadly consistent with synchrotron emission from a reverse shock, but a bright forward shock component predicted by the intermediate- to late-time X-ray observations under the assumptions of standard afterglow models is not observed. Indeed, despite its remarkable early-time brightness this burst would qualify as a dark burst at later times on the basis of its nearly flat optical-to-X-ray spectral index. Our photometric SED provides no evidence of host extinction, requiring either large quantities of grey dust in the host system (at z=1.1588 +/- 0.0006, based upon our late-time Keck spectroscopy) or separate physical origins for the X-ray and optical afterglows. In either case, events like GRB 061126 may represent a significant fraction of observed dark bursts with faint or absent optical afterglows, suggesting a need for redress of the interpretations concerning the origin of these events, and possibly of afterglows in general.
Taking advantage of the exceptional capabilities of ACS on board of HST, we
derive Surface Brightness Fluctuation (SBF) measurements in the B and I bands
from images of six elliptical galaxies with $1500 \leq cz \leq 3500$. Given the
low S/N ratio of the SBF signal in the blue band images, the reliability of the
measurements is verified both with numerical simulations and experimental data
tests.
This paper presents the first published B- and I-band SBF measurements for
distant ($\geq$ 20 Mpc) galaxies, essential for the comparisons of the models
to observations of normal ellipticals. By comparing I-band data with our new
Simple Stellar Population (SSP) models we find an excellent agreement and we
confirm that I-band SBF magnitudes are mainly sensitive to the metallicity of
the dominant stellar component in the galaxy, and are not strongly affected by
the contribution of possible secondary stellar components. As a consequence
I-band fluctuations magnitudes are ideal for distance studies. On the other
hand, we show that standard SSP models do not reproduce the B-band SBF
magnitudes of red ($(B-I)_0 \gsim 2.1$) galaxies in our sample. We explore the
capability of two non--canonical models in properly reproducing the high
sensitivity of B SBF to the presence of even small fractions of bright, hot
stars (metal poor stars, hot evolved stars, etc.). The disagreement is solved
both by taking into account hot (Post--AGB) stars in SSP models and/or by
adopting Composite Stellar Population models. Finally, we suggest a limit value
of the S/N for the B-band SBF signal required to carry out a detailed study of
stellar population properties based on this technique.
Understanding how seed black holes grow into intermediate and supermassive black holes (IMBHs and SMBHs, respectively) has important implications for the duty-cycle of active galactic nuclei (AGN), galaxy evolution, and gravitational wave astronomy. Most studies of the cosmological growth and merger history of black holes have used semianalytic models and have concentrated on SMBH growth in luminous galaxies. Using high resolution cosmological N-body simulations, we track the assembly of black holes over a large range of final masses -- from seed black holes to SMBHs -- over widely varying dynamical histories. We used the dynamics of dark matter halos to track the evolution of seed black holes in three different gas accretion scenarios. We have found that growth of Sagittarius A* - size SMBH reaches its maximum mass M_{SMBH}~10^6Msun at z~6 through early gaseous accretion episodes, after which it stays at near constant mass. At the same redshift, the duty-cycle of the host AGN ends, hence redshift z=6 marks the transition from an AGN to a starburst galaxy which eventually becomes the Milky Way. By tracking black hole growth as a function of time and mass, we estimate that the IMBH merger rate reaches a maximum of R_{max}=55 yr^-1 at z=11. From IMBH merger rates we calculate N_{ULX}=7 per Milky Way type galaxy per redshift in redshift range 2<z<6.
Thomson scattering of CMBR temperature anisotropies will cause the spectrum of the CMBR to differ from blackbody even when one resolves all anisotropies. A formalism for computing the anisotropic and inhomogeneous spectral distortions of intensity and polarization is derived in terms of Lorentz invariant central moments of the temperature distribution. The formalism is non-perturbative, requiring neither small anisotropies nor small metric or matter inhomogeneities; but it does assume cold electrons. The low order moments are not coupled to the higher order moments allowing one to truncate the equations without any loss of accuracy. This formalism is applied to a standard Lambda-CDM cosmology after reionization where the temperature anisotropies are dominated by the Doppler effect for the bulk motion of the gas with respect to the CMBR frame. The resultant spectral distortion is parameterized by u ~ 3e-8, where in this case u is observationally degenerate with the Sunyaev-Zel'dovich (SZ) y parameter. In comparison the expected thermal SZ y-distortion from the hot IGM is expected to be >30 times larger. However at z >5 the effect described here would have been the dominant source of spectral distortions. The effect could be much larger in non-standard cosmologies.
Magnetorotational Instability (MRI), the instability causing turbulent
transport in accretion disks, is studied in the kinetic regime. Radiatively
Inefficient Accretion Flows (RIAFs), like the one around the supermassive black
hole in the center of our Galaxy, are believed to be collisionless. Kinetic MHD
formalism, based on the moments of the Vlasov equation, is used for linear
analysis and nonlinear simulations.
ZEUS MHD code is modified to include key kinetic MHD terms: anisotropic
pressure tensor and anisotropic thermal conduction. Simulations use the local
shearing box approximation. Pressure anisotropy is created, because of the
adiabatic invariance ($\mu=p_\perp/B$), as magnetic field is amplified by the
MRI. Larmor radius scale instabilities--mirror, ion-cyclotron, and
firehose--are excited at large pressure anisotropy. Pressure isotropization due
to pitch angle scattering by these instabilities is included as a subgrid
model. A key result of the kinetic MHD simulations is that the anisotropic
(viscous) stress can be as large as the Maxwell stress.
A new numerical method to simulate anisotropic thermal conduction with large
temperature gradients is suggested. Simple tests show that the centered
differencing of anisotropic thermal conduction can result in heat flowing from
lower to higher temperatures, giving rise to negative temperatures. Limiting of
transverse temperature gradients does not accentuate temperature extrema.
At present, restrictions on the spin-independent parameter space of WIMP dark matter searches have been limited to the results provided by relatively heavy nuclei experiments, based on the conventional wisdom that only such experiments can provide significant spin-independent limits. We examine this wisdom, showing that light nuclei experiments can in fact provide comparable limits given comparable exposures, and indicating the potential of light nuclei detectors to simultaneously and competitively contribute to the search for both spin-independent and -dependent WIMP dark matter.
The inner parts of many spiral galaxies are dominated by bars. These are strong non-axisymmetric features which significantly affect orbits of stars and dark matter particles. One of the main effects is the dynamical resonances between galactic material and the bar. We detect and characterize these resonances in N-body models of barred galaxies by measuring angular and radial frequencies of individual orbits. We found narrow peaks in the distribution of orbital frequencies with each peak corresponding to a specific resonance. We found five different resonances in the stellar disk and two in the dark matter. The corotation resonance and the inner and outer Lindblad resonances are the most populated. The spatial distributions of particles near resonances are wide. For example, the inner Lindblad resonance is not localized at a given radius. Particles near this resonance are mainly distributed along the bar and span a wide range of radii. On the other hand, particles near the corotation resonance are distributed in two broad areas around the two stable Lagrange points. The distribution resembles a wide ring at the corotation radius. Resonances capture disk and halo material in near-resonant orbits. Our analysis of orbits in both N-body simulations and in simple analytical models indicate that resonances tend to prevent the dynamical evolution of this trapped material. Only if the bar evolves as a whole, resonances drift through the phase space. In this case particles anchored near resonant orbits track the resonance shift and evolve. The criteria to ensure a correct resonant behavior discussed in Weinberg and Katz (2007) can be achieved with few millions particles because the regions of trapped orbits near resonances are large and evolving.
We present radial stellar population parameters for a subsample of 12 galaxies from the 36 isolated early-type galaxies of Reda et al. Using new long-slit spectra, central values and radial gradients for the stellar age, metallicity [Z/H] and alpha-element abundance [E/Fe] are measured. Similarly, the central stellar population parameters are derived for a further 5 isolated early-type galaxies using their Lick indices from the literature. On average, the seventeen isolated galaxies have mean central [Z/H]o and [E/Fe]o of 0.29+/-0.03 and 0.17+/-0.03 respectively and span a wide range of ages from 1.7 to 15 Gyrs. We find that isolated galaxies follow similar scaling relations between central stellar population parameters and galaxy velocity dispersion to their counterparts in high density environments. However, we note a tendency for isolated galaxies to have slightly younger ages, higher [Z/H] and lower [E/Fe]. Such properties are qualitatively consistent with the expectation of an extended star formation history for galaxies in lower density environments. Generally we measure constant age and [E/Fe] radial gradients. We find that the age gradients anti-correlate with the central galaxy age. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies metallicity gradients are shallower with increasing mass. Such behaviour is not predicted in dissipational collapse models but might be expected in multiple mergers. The metallicity gradients correlate with the central age and metallicity, as well as to the age gradients. In conclusion, our stellar population data for isolated galaxies are more compatible with an extended merger/accretion history than early dissipative collapse.
The most common way to simplify extensive Monte-Carlo simulations of air showers is the use of the thinning approximation. We study its effect on the physical parameters reconstructed from simulated showers. To this end, we created a library of showers simulated without thinning with energies from 10^17 eV to 10^18 eV, different zenith angles and primaries. This library is publicly available. Various physically interesting applications of the showers simulated without thinning are discussed. Observables reconstructed from these showers are compared to those obtained with the thinning approximation. The amount of artificial fluctuations introduced by thinning is estimated. A simple method, multisampling, is suggested which results in a controllable suppression of artificial fluctuations and at the same time requires less demanding computational resources as compared to the usual thinning.
A possible way of detecting the first structure formation in a non-standard BBN Universe during the dark age, due to the resonant scattering of the CMBR photons in the rotational lines of the primordial CH molecule is discussed. The calculations are made within the framework of the $\Lambda$CDM cosmology and plausible first structure physical conditions. The carbon chemistry during the pregalactic epoch is considered. The relative abundance of the CH molecule is found to be $10^{-14}$ whereas the adopted [C]/[H] ratio was taken to be $10^{-10}$. The optical depth, dT/T and integration times are estimated. The calculated optical depth turns out to have values that are rather high, that clearly argue in favor of this molecule as an excellent candidate in searches of first structure formation. Possible observations with the GMT and ALMA are discussed.
We present here the sensitive HCN(1-0) observations made with the VLA of two submillimeter galaxies and two QSOs at high-redshift. HCN emission is the signature of dense molecular gas found in GMC cores, the actual sites of massive star formation. We have made the first detection of HCN in a submillimeter galaxy, SMM J16359+6612. The HCN emission is seen with a signal to noise ratio of 4$\sigma$ and appears to be resolved as a double-source of $\approxlt 2''$ separation. Our new HCN observations, combined with previous HCN detections and upper limits, show that the FIR/HCN ratios in these high redshift sources lie systematically above the FIR/HCN correlation established for nearby galaxies by about a factor of 2. Even considering the scatter in the data and the presence of upper limits, this is an indication that the FIR/HCN ratios for the early Universe molecular emission line galaxies (EMGs) deviate from the correlation that fits Galactic giant molecular cloud cores, normal spirals, LIRGs, and ULIRGs. This indicates that the star formation rate per solar mass of dense molecular gas is higher in the high-$z$ objects than in local galaxies including normal spirals LIRGs and ULIRGs. The limited HCN detections at high-redshift show that the HCN/CO ratios for the high-$z$ objects are high and are comparable to those of the local ULIRGs rather than those of normal spirals. This indicates that EMGs have a high fraction of dense molecular gas compared to total molecular gas traced by CO emission.
Effects of a generalized dark energy fluid is investigated on cosmic density fluctuations such as cosmic microwave background. As a general dark energy fluid, we take into consideration the possibility of the anisotropic stress for dark energy, which has not been discussed much in the literature. We comprehensively study its effects on the evolution of density fluctuations along with that of non-adiabatic pressure fluctuation of dark energy, then give constraints on such a generalized dark energy from current observations. We show that, though we cannot find any stringent limits on the anisotropic stress or the non-adiabatic pressure fluctuation themselves, the constrains on the equation of state of dark energy can be affected in some cases by the nature of dark energy fluctuation characterized by these properties. This may have important implications to the strategy to study the nature of dark energy.
Active star formation (SF) is tightly related to the dense molecular gas in the giant molecular clouds' dense cores. Our HCN (measure of the dense molecular gas) survey in 65 galaxies (including 10 ultraluminous galaxies) reveals a tight linear correlation between HCN and IR (SF rate) luminosities, whereas the correlation between IR and CO (measure of the total molecular gas) luminosities is nonlinear. This suggests that the global SF rate depends more intimately upon the amount of dense molecular gas than the total molecular gas content. This linear relationship extends to both the dense cores in the Galaxy and the hyperluminous extreme starbursts at high-redshift. Therefore, the global SF law in dense gas appears to be linear all the way from dense cores to extreme starbursts, spanning over nine orders of magnitude in IR luminosity.
We present results of NTT spectroscopy of the one known planetary nebula (PN) in the dwarf spheroidal galaxy Fornax, a gas-deficient Local Group galaxy that stopped its star formation activity a few hundred million years ago. We detected the [OIII] 4363 line with a signal-to-noise ratio of ~22. For the first time we detected the weak [SII] 6717,6731 lines (I(6717+6731) ~0.01 I(Hbeta)), determined the electron number density (Ne(SII) = 750 cm^-3), and calculated O, N, Ne, Ar, S, Cl, Fe, He and C abundances. The abundance analysis presented here is based on the direct calculation of the electron temperature Te and yields an oxygen abundance of 12+log(O/H) = 8.28+/-0.02. The analysis of the O, Ne, Ar and S abundances shows that the original ISM oxygen abundance was 0.27+/-0.10 dex lower and that third-dredge-up self-pollution in oxygen took place. The blue spectrum shows weak Wolf-Rayet features, and the progenitor star is classified as a weak emission-line star. Four of the five PNe in dwarf spheroidal galaxies are now known to show WR wind features. Overall, the metallicity of the progenitor of the PN fits in well with stellar spectroscopic abundances derived in previous studies as well as with the stellar age-metallicity relation of Fornax.
A classification scheme for close pairs of galaxies is proposed. The scheme
is motivated by the fact that the majority of apparent close pairs are in fact
wide pairs in three-dimensional space. This is demonstrated by means of
numerical simulations of random samples of binary galaxies and the scrutiny of
the resulting projected and spatial separation distributions.
Observational strategies for classifying close pairs according to the scheme
are suggested. As a result, physical -- i.e., bound and spatially -- close
pairs are identified.
The largest uncertainty for cosmological studies using clusters of galaxies is introduced by our limited knowledge of the statistics of galaxy cluster structure, and of the scaling relations between observables and cluster mass. To improve on this situation we have started an XMM-Newton Large Programme for the in-depth study of a representative sample of 33 galaxy clusters, selected in the redshift range z=0.055 to 0.183 from the REFLEX Cluster Survey, having X-ray luminosities above 0.4 X 10^44 h_70^-2 erg s^-1 in the 0.1 - 2.4 keV band. This paper introduces the sample, compiles properties of the clusters, and provides detailed information on the sample selection function. We describe the selection of a nearby galaxy cluster sample that makes optimal use of the XMM-Newton field-of-view, and provides nearly homogeneous X-ray luminosity coverage for the full range from poor clusters to the most massive objects in the Universe. For the clusters in the sample, X-ray fluxes are derived and compared to the previously obtained fluxes from the ROSAT All-Sky Survey. We find that the fluxes and the flux errors have been reliably determined in the ROSAT All-Sky Survey analysis used for the REFLEX Survey. We use the sample selection function documented in detail in this paper to determine the X-ray luminosity function, and compare it with the luminosity function of the entire REFLEX sample. We also discuss morphological peculiarities of some of the sample members. The sample and some of the background data given in this introductory paper will be important for the application of these data in the detailed studies of cluster structure, to appear in forthcoming publications.
The collision of Deep Impact with comet 9P/Tempel 1 generated a bright cloud of dust which dissipated during several days after the impact. The brightness variations of this cloud and the changes of its position and shape are governed by the physical properties of the dust grains. We use a Monte Carlo model to describe the evolution of the post-impact dust plume. The results of our dynamical simulations are compared to the data obtained with FORS2, the FOcal Reducer and low dispersion Spectrograph for the VLT of the European Southern Observatory (ESO), to derive the particle size distribution and the total amount of material contained in the dust ejecta cloud.
We determine the spectrum of particles accelerated at shocks with arbitrary speed and arbitrary scattering properties for different choices of the equation of state of the downstream plasma. More specifically we consider the effect of energy exchange between the electron and proton thermal components downstream, and the effect of generation of a turbulent magnetic field in the downstream plasma. The slope of the spectrum turns out to be appreciably affected by all these phenomena, especially in the Newtonian and trans-relativistic regime, while in the ultra-relativistic limit the universal spectrum $s\approx 4.3$ seems to be a very solid prediction.
Based on general relativity, it can be argued that deviations from a uniform Hubble flow should be thought of as variations in the Universe's expansion velocity field, rather than being thought of as peculiar velocities with respect to a uniformly expanding space. The aim of this paper is to use the observed motions of galaxies to map out variations in the Universe's expansion, and more importantly, to investigate whether real variations in the Hubble expansion are detectable given the observational uncertainties. All-sky maps of the observed variation in the expansion are produced using measurements obtained along specific lines-of-sight and smearing them across the sky using a Gaussian profile. A map is produced for the final results of the HST Extragalactic Distance Scale Key Project for the Hubble constant, a comparison map is produced from a set of essentially independent data, and Monte Carlo techniques are used to analyse the statistical significance of the variation in the maps. A statistically significant difference in expansion rate of 9 km/s/Mpc is found to occur across the sky. Comparing maps of the sky at different distances appears to indicate two distinct sets of extrema with even stronger statistically significant variations. Within our supercluster, variations tend to occur near the supergalactic plane, and beyond our supercluster, variations tend to occur away from the supergalactic plane. Comparison with bulk flow studies shows some concordance, yet also suggests the bulk flow studies may suffer confusion, failing to discern the influence of multiple perturbations.
We present the results of a survey of CO emission in 43 of the 48 representative E/S0 galaxies observed in the optical with the SAURON integral-field spectrograph. The CO detection rate is 12/43 or 28%. This is lower than previous studies of early-types but can probably be attributed to different sample selection criteria. As expected, earlier type, more luminous and massive galaxies have a relatively lower molecular gas content. We find that CO-rich galaxies tend to have higher H\beta but lower Fe5015 and Mgb absorption indices than CO-poor galaxies. Those trends appear primarily driven by the age of the stars, an hypothesis supported by the fact that the galaxies with the strongest evidence of star formation are also the most CO-rich. In fact, the early-type galaxies from the current sample appear to extend the well-known correlations between FIR luminosity, dust mass and molecular mass of other galaxy types. The star formation interpretation is also consistent with the SAURON galaxies' radio continuum and FIR flux ratios, and their inferred star formation efficiencies are similar to those in spiral galaxies. It thus appears that we have identified the material fueling (residual) star formation in early-type galaxies, and have demonstrated that it is actively being transformed. Nevertheless, the lack of strong correlations between the CO content and most stellar parameters is compatible with the idea that, in a significant number of sample galaxies, the molecular gas has been accreted from the outside and has properties rather independent from the old, pre-existing stellar component.
This chapter discusses the dynamical properties of the Kuiper belt population. Then, it focuses on the characteristics of the Kuiper belt that cannot be explained by its evolution in the framework of the current solar system. We review models of primordial solar system evolution that have been proposed to reproduce the Kuiper belt features, outlining advantages and problems of each of them.
Here we report on the first detection of circular polarization in molecular lines formed in cool magnetic regions (starspots) and in chromospheric emission lines formed in hot plages on the surfaces of active stars. Our survey of G-K-M stars included young main-sequence dwarfs and RS CVn-type giants and subgiants. All stars were found to possess surface magnetic fields producing Stokes V LSD signals in atomic lines of 0.05 to 0.5%. Several stars clearly showed circular polarization in molecular lines of 0.1 to 1%. The molecular Stokes V signal is reminiscent of that observed in sunspots. Chromospheric magnetic fields were detected on most active targets in Stokes V profiles of emission lines with peak polarization up to 2%. The observed molecular circular polarization on M dwarfs indicates single-polarity magnetic fields covering at least 10% of the stellar disk. Smaller signals on K stars imply that their magnetic fields are apparently weaker, more entangled than on M dwarfs, or more diluted by the bright photosphere.
We report on the first ever detection of circular polarization in molecular lines forming in magnetic regions on the surfaces of active stars. The new observations were obtained with the high-resolution spectropolarimeter ESPaDOnS recently installed at the Canada-France-Hawaii Telescope. In July 2005 we carried out a survey of 17 G-K-M stars including active main-sequence dwarfs and RS CVn-type giants and subgiants. All stars were found to possess surface magnetic fields producing average atomic Stokes V signals of 0.05% to 0.5%. Three stars clearly showed circular polarization in molecular lines of 0.5% to 1%. The molecular Stokes V signal is reminiscent of that observed in sunspots.
Archival X-ray observations of EF Eridani obtained in a low state revealed distinct X-ray detections at a luminosity L_X ~ 2 10^{29} erg/s, three orders of magnitude below its high state value. The plasma temperature was found to be as low as $kT \loa 2$ keV, a factor 10 below the high state. The X-ray/UV/IR spectral energy distribution suggests faint residual accretion rather than coronal emission as being responsible for the low-state X-ray emission. EF Eri thus showed a clear transition from being shock-dominated in the high state to be cyclotron-dominated in the low state. From the optical/UV spectral energy distribution we re-determine the photospheric temperature of the white dwarf to \~10000K. Contrary to earlier claims, WD model atmospheres produce sufficient UV flux to reproduce the published GALEX flux and orbital modulation.
We have studied the photometric properties of four fields around the high-redshift quasar pairs QP1310+0007, QP1355-0032, QP0110-0219, and QP0114-3140 at z $\sim$ 1 with the aim of identifying large-scale structures- galaxy clusters or groups- around them. This sample was observed with GMOS in Gemini North and South telescopes in the $g'$, $r'$, $i'$, and $z'$ bands, and our photometry is complete to a limiting magnitude of $i' \sim 24$ mag (corresponding to $\sim M_{i'}^* + 2$ at the redshift of the pairs). Our analysis shows that QP1310+0007, QP1355-0032 and QP0110-0219 are probably in rich clusters environments. For the pair QP0114-3140, the evidence for existence of galaxy clustering at the pair redshift is less compeling. This work suggest that $z \sim 1$ quasar pairs are excellent tracers of high density environments and this same technique may be useful to find clusters at higher redshifts.
Renormalization Group (RG) techniques have been successfully employed in
quantum field theory and statistical physics. Here we apply RG methods to study
the non-linear stages of structure formation in the Universe. Exact equations
for the power spectrum, the bispectrum, and all higher order correlation
functions can be derived for any underlying cosmological model.
A remarkable feature of the RG flow is the emergence of an intrinsic UV
cutoff, due to dark matter velocity dispersion, which improves the convergence
of the equations at small scales. As a consequence, the method is able to
follow the non-linear evolution of the power-spectrum down to zero redshift and
to length-scales where perturbation theory fails.
Our predictions accurately fit the results of $N$-body simulations in
reproducing the ``Baryon Acoustic Oscillations'' features of the
power-spectrum, which will be accurately measured in future galaxy surveys and
will provide a probe to distinguish among different dark energy models.
We present new photometric and spectroscopic measurements for the unique, young, low-mass evolutionary track calibrator AB Dor C. While the new Ks photometry is similar to that previously published in Close et al. (2005) the spectral type is found to be earlier. Based on new H & K IFS spectra of AB Dor C (Thatte et al. 2007; paper 1) we adopt a spectral type of M5.5+/-1.0 for AB Dor C. This is considerably earlier than the M8+/-1 estimated in Close et al. (2005) and Nielsen et al. (2005) yet is consistent with the M6+/-1 independently derived by Luhman & Potter (2005). However, the spectrum presented in paper 1 and analyzed here is a significant improvement over any previous spectrum of AB Dor C. We also present new astrometry for the system which further supports a 0.090+/-0.005 Msun mass for the system. Once armed with an accurate spectrum and Ks flux we find L=0.0021+/-0.0005 Lsun and Teff=2925{+170}{-145}K for AB Dor C. These values are consistent with a ~75 Myr 0.090+/-0.005 Msun object like AB Dor C according to the DUSTY evolutionary tracks (Chabrier et al. 2000). Hence masses can be estimated from the HR diagram with the DUSTY tracks for young low-mass objects like AB Dor C. However, we cautiously note that underestimates of the mass from the tracks can occur if one lacks a proper (continuum preserved) spectra or is relying on NIR fluxes alone.
We present an extension of the spectral deconvolution method (Sparks & Ford
2002) to achieve very high contrast at small inner working radii. We apply the
method to the specific case of ground based adaptive optics fed integral field
spectroscopy (without a coronagraph). Utilising the wavelength dependence of
the Airy and speckle patterns, we make an accurate estimate of the PSF that can
be scaled and subtracted from the data cube. The residual noise in the
resulting spectra is very close to the photon noise from the starlight halo. We
utilise the technique to extract a very high SNR H & K band spectrum of AB Dor
C, the low mass companion to AB Dor A. By effectively eliminating all
contamination from AB Dor A, the extracted spectrum retains both continuum and
spectral features. The achieved 1 sigma contrast is 9 mag at 0.2", 11 mag at
0.5", in 20 mins exposure time, at an effective spectral bandwidth of 5.5 nm,
proving that the method is applicable even in low Strehl regimes.
The spectral deconvolution method clearly demonstrates the efficacy of image
slicer based IFUs in achieving very high contrast imaging spectroscopy at small
angular separations, validating their use as high contrast
spectrographs/imagers for extreme adaptive optics systems.
Quantum gravitational effects in loop quantum cosmology lead to a resolution of the initial singularity and have the potential to solve the horizon problem and generate a quasi scale-invariant spectrum of density fluctuations. We consider loop modifications to the behavior of the inverse scale factor below a critical scale in closed models and assume a purely thermal origin for the fluctuations. We show that the no-go results for scale invariance in classical thermal models can be evaded even if we just consider modifications to the background (zeroth order) gravitational dynamics. Since a complete and systematic treatment of the perturbed Einstein equations in loop cosmology is still lacking, we simply parameterize their expected modifications. These change quantitatively, but not qualitatively, our conclusions. We thus urge the community to more fully work out this complex aspect of loop cosmology, since the full picture would not only fix the free parameters of the theory, but also provide a model for a non-inflationary, thermal origin for the structures of the Universe.
We surveyed thirteen very low mass (VLM; M < 0.2 M_sun) objects in the Taurus star-forming region using near-infrared diffraction-limited imaging techniques on the W.M. Keck I 10 m telescope. Of these thirteen, five were found to be binary, with separations ranging from 0.04" to 0.6" and flux ratios from 1.4 to 3.7. In all cases, the companions are likely to be physically associated with the primaries (probability > 4-sigma). Using the theoretical models of Baraffe et al. (1998), we find that all five new companions, as well as one of the primaries, are likely brown dwarfs. The discovery of these systems therefore increases the total number of known, young VLM binaries by ~50%. These new systems, along with other young VLM binaries from the literature, have properties that differ significantly from older field VLM binaries in that the young systems have wider separations and lower mass ratios, supporting the idea that VLM binaries undergo significant dynamical evolution ~5 - 10 Myr after their formation. The range of separations of these binaries, four of which are over 30 AU, argues against the ejection scenario of brown dwarf formation. While several of the young, VLM binaries discovered in this study have lower binding energies than the previously suggested minimum for VLM binaries, the apparent minimum is still significantly higher than that found among higher mass binaries. We suggest that this discrepancy may be due to the small mass of a VLM binary relative to the average perturbing star, leading to more substantial changes in their binding energy over time.
We compute the phase lags between the radial velocity curves and the light curves $\Delta \Phi_1= \phi^{V_r}_1 - \phi^{mag}_1$ for classical Cepheid model sequences both in the linear and the nonlinear regimes. The nonlinear phase lags generally fall below the linear ones except for high period models where they lie above, and of course for low pulsation amplitudes where the two merge. The calculated phase lags show good agreement with the available observational data of normal amplitude Galactic Cepheids. The metallicity has but a moderate effect on the phase lag, while the mass-luminosity relation and the parameters of the turbulent convective model (time-dependent mixing length) mainly influence the modal selection and the period, which is then reflected in the period -- $\Delta \Phi_1$ diagram. We discuss the potential application of this observable as a discriminant for pulsation modes and as a test for ultra-low amplitudes (ULA) pulsation.
We investigate the time sequence for the appearance of jets and molecular tori in the transition of stars from the Asymptotic Giant Branch to the planetary nebula phase. Jets and tori are prominent features of this evolution, but their origins are uncertain. Using optical and millimeter line kinematics, we determine the ejection history in a sample of well-observed cases. We find that jets and tori develop nearly simultaneously. We also find evidence that jets typically appear slightly later than tori, with a lag time of a few hundred years. These characteristics provide strong evidence that jets and tori are physically related, and they set new constraints on theories of jet formation. The ejection of a discrete torus followed by jets on a short time scale favors the class of models in which a companion interacts with the central star. Models with long time scales, or with jets followed by a torus, are ruled out.
We present a cosmic shear analysis of the 100 square degree weak lensing
survey, combining data from the CFHTLS-Wide, RCS, VIRMOS-DESCART and GaBoDS
surveys. Spanning ~100 square degrees, with an average source redshift z~0.8,
this combined survey allows us to place tight joint constraints on the matter
density parameter Omega_m, and the amplitude of the matter power spectrum
sigma_8, finding sigma_8*(Omega_m/0.24)^0.59 = 0.84+/-0.07. Tables of the
measured shear correlation function and the calculated covariance matrix for
each survey are included.
The accuracy of our results are a marked improvement on previous work owing
to three important differences in our analysis; we correctly account for cosmic
variance errors by including a non-Gaussian contribution estimated from
numerical simulations; we correct the measured shear for a calibration bias as
estimated from simulated data; we model the redshift distribution, n(z), of
each survey from the largest deep photometric redshift catalogue currently
available from the CFHTLS-Deep. This catalogue is randomly sampled to reproduce
the magnitude distribution of each survey with the resulting survey dependent
n(z) parametrised using two different models. While our results are consistent
for the n(z) models tested, we find that our cosmological parameter constraints
depend weakly (at the 5% level) on the inclusion or exclusion of galaxies with
low confidence photometric redshift estimates (z>1.5). These high redshift
galaxies are relatively few in number but contribute a significant weak lensing
signal. It will therefore be important for future weak lensing surveys to
obtain near-infra-red data to reliably determine the number of high redshift
galaxies in cosmic shear analyses.
We perform a maximum likelihood analysis of the cluster abundance measured in the SDSS using the maxBCG cluster finding algorithm. Our analysis is aimed at constraining the power spectrum normalization $\sigma_8$, and assumes flat cosmologies with a scale invariant spectrum, massless neutrinos, and CMB and supernova priors Omega_m*h^2=0.128+/-0.01 and h=0.72+/-0.05 respectively. Following the method described in the companion paper Rozo et al. 2007, we derive \sigma_8=0.92+/-0.10$ (1-sigma) after marginalizing over all major systematic uncertainties. We place strong lower limits on the normalization, sigma_8>0.76 (95% CL) (>0.68 at 99% CL). We also find that our analysis favors relatively low values for the slope of the Halo Occupation Distribution (HOD), alpha=0.83+/-0.06. The uncertainties of these determinations will substantially improve upon completion of an ongoing campaign to estimate dynamical, weak lensing, and X-ray cluster masses in the SDSS maxBCG cluster sample.
Local (shearing box) simulations of the nonlinear evolution of the magnetorotational instability in a collisionless plasma show that angular momentum transport by pressure anisotropy ($p_\perp \ne p_\parallel$, where the directions are defined with respect to the local magnetic field) is comparable to that due to the Maxwell and Reynolds stresses. Pressure anisotropy, which is effectively a large-scale viscosity, arises because of adiabatic invariants related to $p_\perp$ and $p_\parallel$ in a fluctuating magnetic field. In a collisionless plasma, the magnitude of the pressure anisotropy, and thus the viscosity, is determined by kinetic instabilities at the cyclotron frequency. Our simulations show that $\sim 50$ % of the gravitational potential energy is directly converted into heat at large scales by the viscous stress (the remaining energy is lost to grid-scale numerical dissipation of kinetic and magnetic energy). We show that electrons receive a significant fraction ($\sim [T_e/T_i]^{1/2}$) of this dissipated energy. Employing this heating by an anisotropic viscous stress in one dimensional models of radiatively inefficient accretion flows, we find that the radiative efficiency of the flow is greater than 0.5% for $\dot{M} \gtrsim 10^{-4} \dot{M}_{Edd}$. Thus a low accretion rate, rather than just a low radiative efficiency, is necessary to explain the low luminosity of many accreting black holes. For Sgr A* in the Galactic Center, our predicted radiative efficiencies imply an accretion rate of $\approx 3 \times 10^{-8} M_\odot {\rm yr^{-1}}$ and an electron temperature of $\approx 3 \times 10^{10}$ K at $\approx 10$ Schwarzschild radii; the latter is consistent with the brightness temperature inferred from VLBI observations.
We report the results of a series of three-dimensional simulations of the gravitationally confined detonation mechanism for Type Ia supernovae. In this mechanism, ignition occurs at one or several off-center points, resulting in a burning bubble of hot ash that rises rapidly, breaks through the surface of the star, and collides at point opposite breakout on the stellar surface. We find that detonation conditions are robustly reached in our three-dimensional simulations for a range of initial conditions and resolutions. These conditions are achieved as a result of an inwardly-directed jet that is produced by the compression of unburnt surface material when the surface flow collides with itself at the opposite point on the stellar surface. A high-velocity outwardly-directed jet is also produced. We mention recent observations of type Ia supernovae whose properties are consistent with those expected from these 3D simulations of the GCD.
We introduce a framework for describing the halo selection function of optical cluster finders. We treat the problem as being separable into a term that describes the intrinsic galaxy content of a halo (the Halo Occupation Distribution, or HOD) and a term that captures the effects of projection and selection by the particular cluster finding algorithm. Using mock galaxy catalogs tuned to reproduce the luminosity dependent correlation function and the empirical color-density relation measured in the SDSS, we characterize the maxBCG algorithm applied by Koester et al. to the SDSS galaxy catalog. We define and calibrate measures of completeness and purity for this algorithm, and demonstrate successful recovery of the underlying cosmology and HOD when applied to the mock catalogs. We identify principal components -- combinations of cosmology and HOD parameters -- that are recovered by survey counts as a function of richness, and demonstrate that percent-level accuracies are possible in the first two components, if the selection function can be understood to ~15% accuracy.
The atmosphere of the Sun is highly structured and dynamic in nature. From
the photosphere and chromosphere into the transition region and the corona
plasma-$\beta$ changes from above to below one, i.e. while in the lower
atmosphere the energy density of the plasma dominates, in the upper atmosphere
the magnetic field plays the governing role -- one might speak of a ``magnetic
transition''. Therefore the dynamics of the overshooting convection in the
photosphere, the granulation, is shuffling the magnetic field around in the
photosphere. This leads not only to a (re-)structuring of the magnetic field in
the upper atmosphere, but induces also the dynamic reaction of the coronal
plasma e.g. due to reconnection events. Therefore the (complex) structure and
the interaction of various magnetic patches is crucial to understand the
structure, dynamics and heating of coronal plasma as well as its acceleration
into the solar wind.
The present article will emphasize the need for three-dimensional modeling
accounting for the complexity of the solar atmosphere to understand these
processes. Some advances on 3D modeling of the upper solar atmosphere in
magnetically closed as well as open regions will be presented together with
diagnostic tools to compare these models to observations. This highlights the
recent success of these models which in many respects closely match the
observations.
Dynamical considerations, presented herein via analytic scalings and numerical experiments, imply that Earth-mass planets accreting in regions that become habitable zones of M dwarf stars form within several million years. Temperatures in these regions during planetary accretion are higher than those encountered by the material that formed the Earth. Collision velocities during and after the prime accretionary epoch are larger than for Earth. These factors suggest that planets orbiting low mass main sequence stars are likely to be either too distant (and thus too cold) for carbon/water based life on their surfaces or have abundances of the required volatiles that are substantially less than on Earth.
In this study, we test the theoretically predicted mass-loss behaviour as a function of stellar effective temperature across the so-called `bi-stability' jump (BSJ). We gathered radio observations of 30 OB supergiants (O8-B3). We derived the radio mass-loss rates and wind efficiencies, and compared our results with Halpha mass-loss rates and predictions based on radiation-driven wind models. he wind efficiency shows the possible presence of a local maximum around an effective temperature of 21~000 K -- in qualitative agreement with predictions. We also find that the radio mass-loss rates show good agreement with empirical Halpha rates. However, the empirical mass-loss rates are larger than the predicted rates from radiation-driven wind theory for objects above the BSJ temperature, whilst they are smaller for the rest. A new wind momenta-luminosity relation for O8-B0 stars has been derived.
Helium that accretes onto a Carbon/Oxygen white dwarf in the double white dwarf AM Canum Venaticorum (AM CVn) binaries undergoes unstable thermonuclear flashes when the orbital period is in the 3.5-25 minute range. At the shortest orbital periods (and highest accretion rates, Mdot > 10^-7 Msol/yr), the flashes are weak and likely lead to classical novae outbursts. However, as the orbit widens and the accretion rate drops, the mass required for the unstable ignition increases, leading to progressively more violent flashes up to a final flash with Helium shell mass ~ 0.02-0.1 Msol. The high pressures of these last flashes allow the burning to produce the radioactive elements 48Cr, 52Fe, and 56Ni that power a faint (M_V in the range of -15 to -18) and rapidly evolving (few days) thermonuclear supernova. We estimate one such explosion every 1000 years in a 10^11 Msol elliptical galaxy (~ 25% of the Type Ia supernovae rate). These ``.Ia'' supernovae (one-tenth as bright for one-tenth the time as a Type Ia supernovae) are excellent targets for deep (e.g. V=24) searches with nightly cadences, potentially yielding an all-sky rate of 10,000 per year.
We present an analysis of a new, detached, double-lined eclipsing binary system with K7 Ve components, discovered as part of the University of New South Wales Extrasolar Planet Search. The object is significant in that only 6 other binary systems are known with comparable or lower mass. Such systems offer important tests of mass-radius theoretical models. Follow-up photometry and spectroscopy were obtained with the 40-inch and 2.3m telescopes at SSO respectively. An estimate of the radial velocity amplitude from spectral absorption features, combined with the orbital inclination (83.5 deg) estimated from lightcurve fitting, yielded a total mass of M=(1.041 +/- 0.06)M_sun and component masses of M_A=(0.529 +/- 0.035)M_sun and M_B=(0.512 +/- 0.035)M_sun. The radial velocity amplitude estimated from absorption features (167 +/- 3)kmps was found to be less than the estimate from the H_alpha emission lines (175 +/- 1.5)kmps. The lightcurve fit produced radii of R_A=(0.641 +/- 0.05)R_sun and R_B=(0.608 +/- 0.06)R_sun, and a temperature ratio of T_B/T_A=0.980 +/- 0.015. The apparent magnitude of the binary was estimated to be V=13.9 +/- 0.2. Combined with the spectral type, this gave the distance to the binary as 169 +/- 14 pc. The timing of the secondary eclipse gave a lower limit on the eccentricity of the binary system of 0.0025 +/- 0.0005. This is the most statistically significant non-zero eccentricity found for such a system, possibly suggesting the presence of a third companion.
We present the BVR broad band polarimetric observations of 51 stars belonging to the young open cluster IC 1805. Along with the photometric data from the literature we have modeled and subtracted the foreground dust contribution from the maximum polarization (P_{max}) and colour excess (E_{B-V}). The mean value of the P_max for intracluster medium and the foreground are found to be 5.008 +/-0.005 % and 4.865 +/-0.022 % respectively. Moreover, the mean value of the wavelength of maximum polarization (lambda_{max}) for intracluster medium is 0.541 +/- 0.003 micro m, which is quite similar as the general interstellar medium (ISM). The resulting intracluster dust component is found to have negligible polarization efficiency as compared to interstellar dust. Some of the observed stars in IC 1805 have shown the indication of intrinsic polarization in their measurements.
We present a detailed study of localised magnetohydrodynamical (MHD) instabilities occuring in two--dimensional magnetized accretion disks. We model axisymmetric MHD disk tori, and solve the equations governing a two--dimensional magnetized accretion disk equilibrium and linear wave modes about this equilibrium. We show the existence of novel MHD instabilities in these two--dimensional equilibria which do not occur in an accretion disk in the cylindrical limit. The disk equilibria are numerically computed by the FINESSE code. The stability of accretion disks is investigated analytically as well as numerically. We use the PHOENIX code to compute all the waves and instabilities accessible to the computed disk equilibrium. We concentrate on strongly magnetized disks and sub--Keplerian rotation in a large part of the disk. These disk equilibria show that the thermal pressure of the disk can only decrease outwards if there is a strong gravitational potential. Our theoretical stability analysis shows that convective continuum instabilities can only appear if the density contours coincide with the poloidal magnetic flux contours. Our numerical results confirm and complement this theoretical analysis. Furthermore, these results show that the influence of gravity can either be stabilizing or destabilizing on this new kind of MHD instability. In the likely case of a non--constant density, the height of the disk should exceed a threshold before this type of instability can play a role. This localised MHD instability provides an ideal, linear route to MHD turbulence in strongly magnetized accretion disk tori.
The mass composition of high energy cosmic rays above $10^{17}$ eV is a crucial issue to solve some open questions in astrophysics such as the acceleration and propagation mechanisms. Unfortunately, the standard procedures to identify the primary particle of a cosmic ray shower have low efficiency mainly due to large fluctuations and limited experimental observables. We present a statistical method for composition studies based on several measurable features of the longitudinal development of the CR shower such as $N_{max}$, $X_{max}$, asymmetry, skewness and kurtosis. Principal component analysis (PCA) was used to evaluate the relevance of each parameter in the representation of the overall shower features and a linear discriminant analysis (LDA) was used to combine the different parameters to maximize the discrimination between different particle showers. The new parameter from LDA provides a separation between primary gammas, proton and iron nuclei better than the procedures based on $X_{max}$ only. The method proposed here was successfully tested in the energy range from $10^{17}$ to $10^{20}$ eV even when limitations of shower track length were included in order to simulate the field of view of fluorescence telescopes.
The energy conditions give upper bounds on the luminosity distance. We apply these upper bounds to the 192 essence supernova Ia data to show that the Universe had experienced accelerated expansion. This conclusion is drawn directly from the distance modulus-reshift graph. In addition to be a very simple method, this method is also totally independent of any cosmological model. From the degeneracy of the distance modulus at low redshift, we argue that the choice of $w_0$ for probing the property of dark energy is misleading. One explicit example is used to support this argument.
To ascertain the nature of the brightest mid-infrared sources in the Large Magellanic Cloud (LMC), we have applied the Buchanan et al. (2006) 2MASS-MSX color classification system, which is based on the results of Spitzer Space Telescope spectroscopy, to a mid-infrared flux-limited sample of 254 LMC objects for which 2MASS and MSX photometry is available. We find 72 sources are most likely H II regions; 49 sources are identified as oxygen rich objects, where 42 of these are red supergiants and 7 are likely oxygen rich asymptotic giant branch (AGB) stars; 77 sources are identified as carbon-rich AGB stars; and 7 objects are found to be foreground Mira variables in the halo of the Milky Way. An additional 49 objects cannot be reliably classified based on their positions in 2MASS/MSX color-color and color-magnitude diagrams. The very large ratio of carbon-rich to oxygen-rich objects among the luminous and heavily dust-enshrouded AGB stars in our sample (~10:1) is consistent with the hypothesis that carbon stars form easily in lower metallicity environments. The number and color distributions of sources found for this sample can be extrapolated to other galaxies with metallicities similar to that of the LMC, so as to ascertain the nature of their populations of very luminous mid-infrared sources.
The star CD-42o11721 is a curious B[e] star sometimes pointed as an evolved B[e] supergiant and sometimes as a young HAeBe star, due to very uncertain or even unknown stellar parameters, especially the distance. In this paper, we present new data gained from high-resolution optical spectroscopy and a detailed description of IR data of this star. We present a qualitative study of the numerous emission lines in our optical spectra and the classification of their line profiles, which indicate a non-spherically symmetric circumstellar environment. The first real detection of numerous [Fe II] emission lines and of many other permitted and forbidden emission lines is reported. From our optical data, we derive an effective temperature of 14000 K, a radius of 17.3 +/- 0.6 Rsun, as well as a luminosity of 1.0 +/- 0.3 x 10^{4} Lsun. We advocate that CD-42o11721 might be a post-main sequence object, even though a pre-main sequence nature cannot be ruled out due to the uncertain distance. We further found that the SED in the optical and infrared can best be fitted with an outflowing disk-forming wind scenario rather than with a spherical symmetric envelope or with a flared disc, supporting our tentative classification as a B[e] supergiant.
We consider the effect on the propagation of light of inhomogeneities with sizes of order 10 Mpc or larger. The Universe is approximated through a variation of the Swiss-cheese model. The spherical inhomogeneities are void-like, with central underdensities surrounded by compensating overdense shells. We study the propagation of light in this background, assuming that the source and the observer occupy random positions, so that each beam travels through several inhomogeneities at random angles. The distribution of luminosity distances for sources with the same redshift is asymmetric, with a peak at a value larger than the average one. The width of the distribution and the location of the maximum increase with increasing redshift and length scale of the inhomogeneities. We compute the induced dispersion and bias on cosmological parameters derived from the supernova data. They are too small to explain the perceived acceleration without dark energy, even when the length scale of the inhomogeneities is comparable to the horizon distance. Moreover, the dispersion and bias induced by gravitational lensing at the scales of galaxies or clusters of galaxies are larger by at least an order of magnitude.
The phase-integral approximation devised by Fr\"oman and Fr\"oman, is used for computing cosmological perturbations in the power-law inflationary model. The phase-integral formulas for the scalar and tensor power spectra are explicitly obtained up to ninth-order of the phase-integral approximation. We show that, the phase-integral approximation exactly reproduces the shape of the power spectra for scalar and tensor perturbations as well as the spectral indices. We compare the accuracy of the phase-integral approximation with the results for the power spectrum obtained with the slow-roll and uniform approximation methods.
AIM:Learn more about the origin of shells and dust in early type galaxies. METHOD: V-I colours of shells and underlying galaxies are derived, using HST Advanced Camera for Surveys (ACS) data. A galaxy model is made locally in wedges and subtracted to determine shell profiles and colours. We applied Voronoi binning to our data to get smoothed colour maps of the galaxies. Comparison with N-body simulations from the literature gives more insight to the origin of the shell features. Shell positions and dust characteristics are inferred from model galaxy subtracted images. RESULT: The ACS images reveal shells well within the effective radius in some galaxies (at 1.7 kpc in the case of NGC 5982). In some cases, strong nuclear dust patches prevent detection of inner shells. Most shells have colours which are similar to the underlying galaxy. Some inner shells are redder than the galaxy. All six shell galaxies show out of dynamical equilibrium dust features, like lanes or patches, in their central regions. Our detection rate for dust in the shell ellipticals is greater than that found from HST archive data for a sample of normal early-type galaxies, at the 95% confidence level. CONCLUSIONS: The merger model describes better the shell distributions and morphologies than the interaction model. Red shell colours are most likely due to the presence of dust and/or older stellar populations. The high prevalence and out of dynamical equilibrium morphologies of the central dust features point towards external influences being responsible for visible dust features in early type shell galaxies. Inner shells are able to manifest themselves in relatively old shell systems.
I briefly review the theoretical models of radiatively efficient, geometrically thin and optically thick accretion disk spectra that currently exist for AGN. I then discuss three recent observational developments that have real potential to teach us about the physics of these flows. Finally, I present results on the most recent, thermodynamically consistent simulations of magnetorotational turbulence and discuss what these simulations are suggesting about the vertical structure of accretion disks.
The dynamical mass of clusters of galaxies, calculated in terms of modified Newtonian dynamics, is a factor of two or three times smaller than the Newtonian dynamical mass but remains significantly larger than the observed baryonic mass in the form of hot gas and stars in galaxies. Here I consider further the suggestion that the undetected matter might be in the form of cosmological neutrinos with mass on the order of 2 eV. If the neutrinos and baryons have comparable velocity dispersions and if the two components maintain their cosmological density ratio, then the electron density in the cores of clusters should be proportional to temperature to the 3/2 power, as appears to be true in non-cooling flow clusters. This is equivalent to the ``entropy floor'' proposed to explain the steepness of the observed luminosity-temperature relation, but here preheating of the medium is not required. Two fluid (neutrino-baryon) hydrostatic models of clusters, in the context of MOND, reproduce the observed luminosity-temperature relation of clusters. If the beta law is imposed the gas density distribution, then the self-consistent models predict the general form of the observed temperature profile in both cooling and non-cooling flow clusters.
We attempt to explain the non-thermal emission arising from galaxy clusters as a result of the re-acceleration of electrons by compressible turbulence induced by cluster mergers. In our model intracluster medium (ICM) is represented by a high beta plasma in which turbulent motions are driven at large scales. The corresponding injection velocities are higher than the Alfven velocity. As a result, the turbulence is approximately isotropic up to the scale at which the turbulent velocity gets comparable with the Alfven velocity. Under the hypothesis that turbulence in the ICM is highly super- Alfvenic the magnetic field is passively advected and the field lines are bended on scales smaller than that of the classical, unmagnetized, ion-ion mean free path. This affects ion diffusion and the strength of the effective viscosity. Under these conditions the bulk of turbulence in hot (5-10 keV temperature) galaxy clusters is likely to be dissipated at collisionless scales via resonant coupling with thermal and fast particles. We use collisionless physics to derive the amplitude of the different components of the energy of the compressible modes, and review and extend the treatment of plasma damping in the ICM. We calculate the acceleration of both protons and electrons taking into account both TTD acceleration and non-resonant acceleration by large scale compressions. We find that relativistic electrons can be re-accelerated in the ICM up to energies of several GeV provided that the rms velocity of the compressible turbulent-eddies is (V_L/c_s)^2~0.15-0.3. We find that under typical conditions ~ 2-5 % of the energy flux of the cascading of compressible motions injected at large scales goes into the acceleration of fast particles and that this may explain the observed non-thermal emission from merging galaxy clusters.
Dusty, starforming galaxies and active galactic nuclei that contribute to the integrated background intensity at far-infrared wavelengths trace the large-scale structure. Below the point source detection limit, correlations in the large-scale structure lead to clustered anisotropies in the unresolved component of the far-infrared background (FIRB). The angular power spectrum of the FIRB anisotropies can be measured in surveys with the Spectral and Photometric Imaging Receiver (SPIRE) on the upcoming Herschel observatory. To study statistical properties of these anisotropies, the confusion from foreground Galactic dust emission needs to be reduced even in the ``cleanest'' regions of the sky. The multi-frequency coverage of SPIRE allows foreground dust to be partly separated from extragalactic anisotropies. The separation improves for fields with sizes greater than about 500 deg.$^2$ when combined with Planck data, while an area of about 1000 degrees$^2$ provides maximal information on the anisotropy power spectrum. We discuss scientific studies that can be done with anisotropy measurements of the unresolved FIRB.
We present a new general relativistic hydrodynamics code specifically designed to study magneto-rotational, relativistic, stellar core collapse. The code is an extension of an existing (and thoroughly tested) hydrodynamics code, which has been applied in the recent past to study relativistic rotational core collapse. It is based on the conformally-flat approximation of Einstein's field equations and conservative formulations for the magneto-hydrodynamics equations. As a first step towards magneto-rotational core collapse simulations the code assumes a passive (test) magnetic field. The paper is focused on the description of the technical details of the numerical implementation, with emphasis on the magnetic field module. A number of code tests are presented and discussed, along with a representative core collapse simulation.