We show that cosmic shear measurement can be made accurate to the second order in shear in the presence of a PSF and photon noise using an extension of the method of Zhang (2008). The sign of the second order correction is opposite to what is conventionally assumed. Neglecting the second order corrections can lead to a few percent uncertainties on cosmic shears, and becomes more important for cluster lensing mass reconstructions. Our shear measurement method is well defined mathematically. It does not require assumptions on the morphologies of galaxies and the point spread function. Contaminations to the shear signals from the background photon noise can be removed also in a well defined way. Using a large ensemble (10^7) of mock galaxies of unrestricted morphologies, we demonstrate that the shear recovery accuracy in this method reaches at least sub-percent levels even in the presence of large and correlated background noise. The recovery accuracy of shear-shear correlations are also tested under general conditions.
A long standing problem in weak lensing is about how to construct a cosmic shear estimator from galaxy images. Ideally, for each shear component, one wants a single quantity from each galaxy image, whose ensemble average is equal to the true shear value. We prove that such ideal shear estimators do not exist in the presence of the point spread function. Alternatively, from each galaxy image, one can construct two quantities for each shear component, and use the ratio of the ensemble averages of the two quantities to recover the shear in an unbiased way. We show that the later is achievable using the shear measurement method of Zhang (2008). We also demonstrate that with the new but less ideal shear estimator, weak lensing statistics such as n-point correlations should be carried out in a slightly different way, but with little additional cost.
We present a comparison of the SCUBA Half Degree Extragalactic Survey (SHADES) at 450, 850 and 1100 microns with deep guaranteed time 15 microns AKARI FU-HYU survey data and Spitzer guaranteed time data at 3.6-24 microns in the Lockman Hole East. The AKARI data was analysed using bespoke software based in part on the drizzling and minimum-variance matched filtering developed for SHADES, and was cross calibrated against Infrared Space Observatory (ISO) fluxes. Our stacking analyses find AKARI 15um galaxies with >~200 microJy contribute >10% of the 450 micron background, but only <4% of the 1100 micron background, suggesting that different populations contribute at mm-wavelengths. We confirm our earlier result that the ultra-deep 450 micron SCUBA-2 Cosmology Survey will be dominated by populations already detected by AKARI and Spitzer mid-infrared surveys. The superb mid-infrared wavelength coverage afforded by combining Spitzer and AKARI photometry is an excellent diagnostic of AGN contributions, and we find that (23-52)% of submm-selected galaxies have AGN bolometric fractions f_AGN>0.3.
We present Spitzer observations for a sample of close major-merger galaxy pairs (KPAIR sample) selected from 2MASS/SDSS-DR3 cross-matches. The goals are to study the star formation activity in these galaxies and to set a local bench mark for the cosmic evolution of close major mergers. The Spitzer KPAIR sample (27 pairs, 54 galaxies) includes all spectroscopically confirmed S+S and S+E pairs in a parent sample that is complete for primaries brighter than K=12.5 mag, projected separations of 5< s < 20 kpc/h, and mass ratios<2.5. The Spitzer data consist of images in 7 bands (3.6, 4.5, 5.8, 8, 24, 70, 160 um). Compared to single spiral galaxies in a control sample, only spiral galaxies in S+S pairs show significantly enhanced specific star formation rate (sSFR=SFR/M), whereas spiral galaxies in S+E pairs do not. Furthermore, the SFR enhancement of spiral galaxies in S+S pairs is highly mass-dependent. Only those with $\rm M \gsim 10^{10.5} M_\sun$ show significant enhancement. Relatively low mass ($\rm M \sim 10^{10} M_\sun$) spirals in S+S pairs have about the same SFR/M compared to their counterparts in the control sample. There is evidence for a correlation between the global star formation activities (but not the nuclear activities) of the component galaxies in massive S+S major-merger pairs (the "Holmberg effect"). There is no significant difference in the SFR/M between the primaries and the secondaries, nor between spirals of SEP<1 and those of SEP.1. The contribution of KPAIR galaxies to the cosmic SFR density in the local universe is only 1.7%.
Using an AKARI multi-wavelength mid-infrared (IR) survey, we identify luminous starburst galaxies at z> 0.5 based on the PAH luminosity, and investigate the nature of these PAH-selected starbursts. An extragalactic survey with AKARI towards the north ecliptic pole (NEP), the NEP-Deep survey, is unique in terms of a comprehensive wavelength coverage from 2 to 24um using all 9 photometric bands of the InfraRed Camera (IRC). This survey allows us to photometrically identify galaxies whose mid-IR emission is clearly dominated by PAHs. We propose a single colour selection method to identify such galaxies, using two mid-IR flux ratios at 11-to-7um and 15-to-9um (PAH-to-continuum flux ratio in the rest-frame), which are useful to identify starburst galaxies at z~0.5 and 1, respectively. We perform a fitting of the spectral energy distributions (SEDs) from optical to mid-IR wavelengths, using an evolutionary starburst model with a proper treatment of radiative transfer (SBURT), in order to investigate their nature. The SBURT model reproduces observed optical-to-mid-IR SEDs of more than a half of PAH-selected galaxies. Based on the 8um luminosity, we find ultra luminous infrared galaxies (ULIRGs) among PAH-selected galaxies. Their PAH luminosity is higher than local ULIRGs with a similar luminosity, and the PAH-to-total IR luminosity ratio is consistent with that of less luminous starburst galaxies. They are a unique galaxy population at high redshifts and we call these PAH-selected ULIRGs "PAH-luminous" galaxies. Although they are not as massive as submillimetre galaxies at z~2, they have the stellar mass of >3x10^{10} Msun and therefore moderately massive.
We present the first results from the new Bolshoi N-body cosmological LCDM simulation that uses cosmological parameters favored by current observations. The Bolshoi simulation was done in a volume 250Mpc on a side using 8billion particles with mass and force resolution adequate to follow subhalos down to a completeness limit of Vcirc=50km/ s circular velocity. Using excellent statistics of halos and subhalos (10M at every moment and 50M over the whole history) we present accurate approximations for statistics such as the halo mass function, the concentrations for distinct halos and subhalos, abundance of halos as function of their circular velocity, the abundance and the spatial distribution of subhalos. We find that at high redshifts the concentration falls to a minimum of about 3.8 and then rises slightly for higher values of halo mass. We find that while the Sheth-Tormen approximation for the mass function of halos found by spherical overdensity is accurate at low redshifts, it over-predicts the abundance of halos by nearly an order of magnitude by z=10. We find that the number of subhalos scales with the circular velocity of the host halo as Vhost**0.5, and that subhalos have nearly the same radial distribution as dark matter particles at radii 0.3-2 times the host halo virial radius. The subhalo velocity function n(>V) behaves as V**(-3). We give normalization of this relation for different masses and redshifts. Finally, we use an abundance-matching procedure to assign r-band luminosities to dark matter halos as a function of halo Vcirc, and find that the luminosity-velocity relation is in remarkably good agreement with the observed Tully-Fisher relation for galaxies in the range 50-200km/s.
We report the detection and measurement of the absolute brightness and spatial fluctuations of the cosmic infrared background (CIB) with the AKARI satellite. We have carried out observations at 65, 90, 140 and 160 um as a cosmological survey in AKARI Deep Field South (ADF-S), which is one of the lowest cirrus regions with contiguous area on the sky. After removing bright galaxies and subtracting zodiacal and Galactic foregrounds from the measured sky brightness, we have successfully measured the CIB brightness and its fluctuations across a wide range of angular scales from arcminutes to degrees. The measured CIB brightness is consistent with previous results reported from COBE data but significantly higher than the lower limits at 70 and 160 um obtained with the Spitzer satellite from the stacking analysis of 24-um selected sources. The discrepancy with the Spitzer result is possibly due to a new galaxy population at high redshift obscured by hot dust. From power spectrum analysis at 90 um, three components are identified: shot noise due to individual galaxies; Galactic cirrus emission dominating at the largest angular scales of a few degrees; and an additional component at an intermediate angular scale of 10-30 arcminutes, possibly due to galaxy clustering. The spectral shape of the clustering component at 90 um is very similar to that at longer wavelengths as observed by Spitzer and BLAST. Moreover, the color of the fluctuations indicates that the clustering component is as red as Ultra-luminous infrared galaxies (ULIRGs) at high redshift, These galaxies are not likely to be the majority of the CIB emission at 90 um, but responsible for the clustering component. Our results provide new constraints on the evolution and clustering properties of distant infrared galaxies.
We examine the possibility of achieving quintessential inflation, where the same field serves as both inflaton and quintessence, in the context of a five-dimensional braneworld. Braneworld cosmology provides an appropriate environment as it permits inflation with much steeper potentials than the conventional scenario, which is favourable to a late-time quintessence. We explore a wide space of models, together with contemporary observational data, to determine in which contexts such a picture is possible. We find that such a scenario, although attractive, is in fact impossible to achieve for the potentials studied due to the restrictiveness of current data.
We obtain exact expressions for the effect of primordial non-Gaussianity on the matter density perturbation up to second order in a LambdaCDM cosmology, fully accounting for the general relativistic corrections arising on scales comparable with the Hubble radius. We present our results both in the Poisson gauge and in the comoving and synchronous gauge, which are relevant for comparison to different cosmological observables.
We combine data from the MGC, SDSS and UKIDSS LAS surveys to produce ugrizYJHK luminosity functions and densities from within a common, low redshift volume (z<0.1, ~71,000 h_1^-3 Mpc^3 for L* systems) with 100 per cent spectroscopic completeness. In the optical the fitted Schechter functions are comparable in shape to those previously reported values but with higher normalisations (typically 0, 30, 20, 15, 5 per cent higher phi*-values in u, g, r, i, z respectively over those reported by the SDSS team). We attribute these to differences in the redshift ranges probed, incompleteness, and adopted normalisation methods. In the NIR we find significantly different Schechter function parameters (mainly in the M* values) to those previously reported and attribute this to the improvement in the quality of the imaging data over previous studies. This is the first homogeneous measurement of the extragalactic luminosity density which fully samples both the optical and near-IR regimes. Unlike previous compilations that have noted a discontinuity between the optical and near-IR regimes our homogeneous dataset shows a smooth cosmic spectral energy distribution (CSED). After correcting for dust attenuation we compare our CSED to the expected values based on recent constraints on the cosmic star-formation history and the initial mass function.
Using the relativistic Hartree-Fock approximation, we calculate the rates of atomic ionization by absorption of pseudoscalar particles in the mass range from 10 to $\sim$ 50 keV. We present numerical results for atoms relevant for the direct dark matter searches (e.g. Ar, Ge, I and Xe), as well as the analytical formula which fits numerical calculations with few per cent accuracy and may be used for multi-electron atoms, molecules and condensed matter systems.
We use observational data from Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), and Cosmic Microwave Background (CMB), along with requirements of Big Bang Nucleosynthesis (BBN), to constrain the running parameter $\lambda$ of Ho\v{r}ava-Lifshitz gravity, which determines the flow between the Ultra-Violet and the Infra-Red. We consider both the detailed and non-detailed balance versions of the gravitational sector, and we include the matter and radiation sectors. Allowing for variation of all the parameters of the theory, we construct the likelihood contours and we conclude that in $1\sigma$ confidence $\lambda$ is restricted to $|\lambda-1|\lesssim0.02$, while its best fit value is $|\lambda_{b.f}-1|\approx0.0006$. Although this observational analysis restricts the running parameter $\lambda$ very close to its IR value 1, it does not enlighten the discussion about the theory's possible conceptual and theoretical problems.
(Abridged) We complete the census of nuclear X-ray activity in 100 early type Virgo galaxies observed by the Chandra X-ray Telescope as part of the AMUSE-Virgo survey, down to a (3sigma) limiting luminosity of 3.7E+38 erg/s over 0.5-7 keV. The stellar mass distribution of the targeted sample, which is mostly composed of formally `inactive' galaxies, peaks below 1E+10 M_Sun, a regime where the very existence of nuclear super-massive black holes (SMBHs) is debated. Out of 100 objects, 32 show a nuclear X-ray source, including 6 hybrid nuclei which also host a massive nuclear cluster as visible from archival HST images. After carefully accounting for contamination from nuclear low-mass X-ray binaries based on the shape and normalization of their X-ray luminosity function, we conclude that between 24-34% of the galaxies in our sample host a X-ray active SMBH (at the 95% C.L.). This sets a firm lower limit to the black hole occupation fraction in nearby bulges within a cluster environment. At face value, the active fraction -down to our luminosity limit- is found to increase with host stellar mass. However, taking into account selection effects, we find that the average Eddington-scaled X-ray luminosity scales with black hole mass as M_BH^(0.62^{+0.13}_{-0.12}), with an intrinsic scatter of 0.46^({+0.08}_{-0.06}) dex. This finding can be interpreted as observational evidence for `down-sizing' of black hole accretion in local early types, that is, low mass black holes shine relatively closer to their Eddington limit than higher mass objects. As a consequence, the fraction of active galaxies, defined as those above a fixed X-ray Eddington ratio, decreases with increasing black hole mass.
The last decade has seen enormous progress in understanding the structure of the Milky Way and neighboring galaxies via the production of large-scale digital surveys of the sky like 2MASS and SDSS, as well as specialized, counterpart imaging surveys of other Local Group systems. Apart from providing snaphots of galaxy structure, these "cartographic" surveys lend insights into the formation and evolution of galaxies when supplemented with additional data (e.g., spectroscopy, astrometry) and when referenced to theoretical models and simulations of galaxy evolution. These increasingly sophisticated simulations are making ever more specific predictions about the detailed chemistry and dynamics of stellar populations in galaxies. To fully exploit, test and constrain these theoretical ventures demands similar commitments of observational effort as has been plied into the previous imaging surveys to fill out other dimensions of parameter space with statistically significant intensity. Fortunately the future of large-scale stellar population studies is bright with a number of grand projects on the horizon that collectively will contribute a breathtaking volume of information on individual stars in Local Group galaxies.
We examine the constraints on final state radiation from Weakly
Interacting Massive Particle (WIMP) dark matter candidates annihilating into
various standard model final states, as imposed by the measurement of the
isotropic diffuse gamma-ray background by the Large Area Telescope aboard the
Fermi Gamma-Ray Space Telescope. The expected isotropic diffuse signal from
dark matter annihilation has contributions from the local Milky Way (MW) as
well as from extragalactic dark matter. While the signal from the MW is very
insensitive to the adopted dark matter profile of the halos, the signal from
extragalactic halos is sensitive to the low mass cut-off of the halo mass
function. We adopt a conservative model for the low halo mass survival cut-off,
and only consider the primary final state radiation. This provides robust
constraints which reach the thermal production cross-section for low mass WIMPs
annihilating into hadronic modes. We also reanalyze limits from HESS
observations of the Galactic Ridge region using a conservative model for the
dark matter halo profile. When combined with the HESS constraint, the isotropic
diffuse spectrum rules out all interpretations of the PAMELA positron excess
based on dark matter annihilation into two lepton final states. Annihilation
into four leptons through new intermediate states, although constrained by the
data, is not excluded.
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Previous studies of the low surface brightness host of the blue compact galaxy (BCG) Haro 11 have suggested an abnormally red color of V-K=4.2+-0.8 for the host galaxy. This color is inconsistent with any normal stellar population over a wide range of stellar metallicities (Z=0.001-0.02). Similar though less extreme host colors have been measured for other BCGs and may be reconciled with population synthesis models, provided that the stellar metallicity of the host is higher than that of the ionized gas in the central starburst. We present the deepest V and K band observations to date of Haro 11 and derive a new V-K color for the host galaxy. Our new data suggest a far less extreme colour of V-K=2.3+-0.2, which is perfectly consistent with the expectations for an old host galaxy with the same metallicty as that derived from nebular emission lines in the star-forming center.
We confirm the existence of two compact elliptical (cE) galaxies in the central region of the Antlia cluster through MAGELLAN-MIKE and GEMINI-GMOS spectra. Only about a dozen galaxies of this rare type are known today up to a distance of 100 Mpc. With this finding, Antlia becomes the nearest galaxy cluster harbouring more than one cE galaxy among its galaxy population. One of these galaxies shows evidence of interaction with one of the giant ellipticals that dominate the central region of the cluster.
We report the discovery of a gravitationally lensed quasar identified serendipitously in the Sloan Digital Sky Survey (SDSS). The object, SDSS J094604.90+183541.8, was initially targeted for spectroscopy as a luminous red galaxy, but the SDSS spectrum has the features of both a z=0.388 galaxy and a z=4.8 quasar. We have obtained additional imaging that resolves the system into two quasar images separated by 3.06 arcsec and a bright galaxy that is strongly blended with one of the quasar images. We confirm spectroscopically that the two quasar images represent a single lensed source at z=4.8 with a total magnification of 3.2, and we derive a model for the lensing galaxy. This is the highest redshift lensed quasar currently known. We examine the issues surrounding the selection of such an unusual object from existing data and briefly discuss implications for lensed quasar surveys.
We present a high resolution (down to 0.18"), multi-transition imaging study of the molecular gas in the z = 4.05 submillimeter galaxy GN20. GN20 is one of the most luminous starburst galaxy known at z > 4, and is a member of a rich proto-cluster of galaxies at z = 4.05 in GOODS-North. We have observed the CO 1-0 and 2-1 emission with the VLA, the CO 6-5 emission with the PdBI Interferometer, and the 5-4 emission with CARMA. The H_2 mass derived from the CO 1-0 emission is 1.3 \times 10^{11} (\alpha/0.8) Mo. High resolution imaging of CO 2-1 shows emission distributed over a large area, appearing as partial ring, or disk, of ~ 10kpc diameter. The integrated CO excitation is higher than found in the inner disk of the Milky Way, but lower than that seen in high redshift quasar host galaxies and low redshift starburst nuclei. The VLA CO 2-1 image at 0.2" resolution shows resolved, clumpy structure, with a few brighter clumps with intrinsic sizes ~ 2 kpc. The velocity field determined from the CO 6-5 emission is consistent with a rotating disk with a rotation velocity of ~ 570 km s^{-1} (using an inclination angle of 45^o), from which we derive a dynamical mass of 3 \times 10^{11} \msun within about 4 kpc radius. The star formation distribution, as derived from imaging of the radio synchrotron and dust continuum, is on a similar scale as the molecular gas distribution. The molecular gas and star formation are offset by ~ 1" from the HST I-band emission, implying that the regions of most intense star formation are highly dust-obscured on a scale of ~ 10 kpc. The large spatial extent and ordered rotation of this object suggests that this is not a major merger, but rather a clumpy disk accreting gas rapidly in minor mergers or smoothly from the proto-intracluster medium. ABSTRACT TRUNCATED
The local void model has lately attracted considerable attention since it can explain the present apparent accelerated expansion of the universe without introducing dark energy. However, in order to justify this model as an alternative cosmological model to the standard $\Lambda$CDM model (FLRW universe plus dark energy), one has to test the model by various observations, such as CMB temperature anisotropy, other than the distance-redshift relation of SNIa. For this purpose, we derive some analytic formulae that can be used to rigorously compare consequences of this model with observations of CMB anisotropy and to place constraints on the position of observers in the void model.
The Palatini $f(R)$ gravity, is able to probably explain the late time cosmic acceleration without the need for dark energy, is studied. In this paper, we investigate a number of $f(R)$ gravity theories in Palatini formalism by means of statefinder diagnosis. We consider two types of $f(R)$ theories: (i) $f(R)=R+\alpha R^{m}-\beta R^{-n}$ and (ii) $f(R)=R+\alpha ln R+\beta$. We find that the evolutionary trajectories in the $s-r$ and $q-r$ planes for various types of the Palatini $f(R)$ theories reveal different evolutionary properties of the universe. Additionally, we use the observational $H(z)$ data to constrain models of $f(R)$ gravity.
Advanced observational facilities allow to trace back the chemical evolution of the Universe, on the one hand, from local objects of different ages and, secondly, by direct observations of redshifted objects. The chemical enrichment serves as one of the cornerstones of cosmological evolution. In order to understand this chemical evolution in morphologically different astrophysical objects models are constructed based on analytical descriptions or numerical methods. For the comparison of their chemical issues, as there are element abundances, gradients, and ratios, with observations not only the present-day values are used but also their temporal evolution from the first era of metal enrichment. Here we will provide some insight into basics of chemical evolution models, highlight advancements, and discuss a few applications.
Cosmological backreaction suggests a link between structure formation and the expansion history of the Universe. In order to quantitatively examine this connection we dynamically investigate a volume partition of the Universe into over- and underdense regions. This allows to trace structure formation using the volume fraction of the overdense regions $\lambda_{\CM}$ as its characterizing parameter. Employing results from cosmological perturbation theory and extrapolating the leading mode into the nonlinear regime, we construct a three-parameter model for the effective cosmic expansion history, involving $\lambda_{\CM_{0}}$, the matter density $\Omega_{m}^{\CD_{0}}$ and the Hubble rate $H_{\CD_{0}}$ of today's Universe. Taking standard values for $\Omega_{m}^{\CD_{0}}$ and $H_{\CD_{0}}$ as well as a reasonable value for $\lambda_{\CM_{0}}$, that we derive from $N$--body simulations, we determine the corresponding amounts of backreaction and spatial curvature. We find that the obtained values that are sufficient to generate today's structure also lead to a $\Lambda$CDM-like behavior of the scale factor, parametrized by the same parameters $\Omega_{m}^{\CD_{0}}$ and $H_{\CD_{0}}$, but without a cosmological constant. However, the temporal behavior of $\lambda_{\CM}$ does not faithfully reproduce the structure formation history. Surprisingly, however, the model matches with structure formation with the assumption of a low matter content, $\Omega_{m}^{\CD_{0}}\approx 3%$, a result that hints to a different interpretation of part of the backreaction effect as kinematical Dark Matter. (truncated)
We study the alignments of the low multipoles of CMB anisotropies with specific directions in the sky (i.e. the dipole, the north Ecliptic pole, the north Galactic pole and the north Super Galactic pole). Performing $10^5$ random extractions we have found that: 1) separately quadrupole and octupole are mildly orthogonal to the dipole but when they are considered together, in analogy to \cite{Copi2006}, we find an unlikely orthogonality at the level of 0.8% C.L.; 2) the multipole vectors associated to $\ell=4$ are unlikely aligned with the dipole at $99.1 %$ C.L.; 3) the multipole vectors associated to $\ell=5$ are mildly orthogonal to the dipole but when we consider only maps that show exactly the same correlation among the multipoles as in the observed WMAP 5yr ILC, these multipole vectors are unlikely orthogonal to the dipole at $99.7 %$ C.L..
Weak gravitational lensing of background galaxies is a unique, direct probe of the distribution of matter in clusters of galaxies. We review several important aspects of cluster weak gravitational lensing together with recent advances in weak lensing techniques for measuring cluster lensing profiles and constraining cluster structure parameters.
The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.
We study the internal circulation within the cocoon carved out by a relativistic jet emanating from an AGN, first developing model and then validating it using a series of numerical simulations. We notice that a significant increase of in this flow arises because gradients in the density and entropy develop near the hot spot, as a consequence of Crocco vorticity theorem. We find simple solutions for the streamlines, and we use them to predict the mass inflow rates towards the central regions. The 2D simulations we perform span a rather wide range of mechanical jet's input power and Black Hole masses, and we show that the predicted nuclear mass inflows are in good agreement with the theoretical model.
One of the most interesting explanations for the non-Gaussian Cold Spot detected in WMAP data by Vielva et al. 2004 is that it arises from the interaction of the CMB radiation with a cosmic texture (Cruz et al. 2007b). In this case, a lack of polarization is expected in the area of the spot as compared to the typical values associated to large fluctuations of a Gaussian and isotropic random field. In this work we characterize the polarization properties of the Cold Spot, under both hypotheses: a large Gaussian fluctuation and an anomalous feature. We propose as well a methodology to distinguish between them, and we discuss the discrimination power as a function of the instrumental noise level. In particular, we address the cases of current experiments, like WMAP and Planck, and others in development as QUIJOTE. We find that for an ideal experiment with a very high sensitivity in polarization, the Gaussian hypothesis could be rejected at a significance level lower than 0.8%. Whereas WMAP is quite far form providing us any useful information in this respect, we find that Planck will be able to provide a significance of around 7%; in addition, we show that the ground-based experiment QUIJOTE could provide a good significance of around 1%. If these numbers are used in combination with the already reported significance level of the Cold Spot from non-Gaussian analyses that only consider temperature studies, then the capability of QUIJOTE and Planck to reject the alternative hypothesis becomes 0.025% and 0.124%, respectively.
The active galaxy PKS 0208-512, detected at lower energies by COMPTEL, has been claimed to be a MeV blazar from EGRET. We report on the most recent INTEGRAL observations of the blazar PKS 0208-512, which are supplemented by Swift ToO observations. The high energy X-ray and gamma-ray emission of PKS 0208-512 during August - December 2008 has been studied using 682 ks of INTEGRAL guest observer time and ~ 56 ks of Swift/XRT observations. These data were collected during the decay of a gamma-ray flare observed by Fermi/LAT. At X-ray energies (0.2 - 10 keV) PKS 0208-512 is significantly detected by Swift/XRT, showing a power-law spectrum with a photon index of ~ 1.64. Its X-ray luminosity varied by roughly 30% during one month. At hard X-/soft gamma-ray energies PKS 0208-512 shows a marginally significant (~ 3.2 sigma) emission in the 0.5-1 MeV band when combining all INTEGRAL/SPI data. Non-detections at energies below and above this band by INTEGRAL/SPI may indicate intrinsic excess emission. If this possible excess is produced by the blazar, one possible explanation could be that its jet consists of an abundant electron-positron plasma, which may lead to the emission of an annihilation radiation feature. Assuming this scenario, we estimate physical parameters of the jet of PKS 0208-512.
Context: GRB afterglows are excellent probes of gas and dust in star-forming galaxies at all epochs. It has been posited that dust in the early Universe must be different from dust at lower z. To date two reports directly support this contention, one of which is based on the spectral shape of GRB 050904 at z = 6.295. Aims: We reinvestigate the afterglow to understand dust at high z. We address the claimed evidence for unusual (SN-origin) dust in its host galaxy by simultaneously examining the X-ray and optical/NIR spectrophotometric data. Methods: We derive the intrinsic SED of the afterglow at 0.47, 1.25 and 3.4 days, by re-reducing the Swift X-ray data, the 1.25 days FORS2 z-Gunn photometric data, the spectroscopic and z'-band photometric data at ~3 days from the Subaru telescope, as well as the critical UKIRT Z-band photometry at 0.47 days, upon which the claim of dust detection largely relies. Results: We find no evidence of dust extinction in the SED. We compute flux densities at lambda_rest = 1250 AA directly from the observed counts at all epochs. In the earliest epoch, 0.47 days, the Z-band suppression is found to be smaller (0.3 +- 0.2 mag) than previously reported and statistically insignificant (<1.5 sigma). Furthermore we find that the photometry of this band is unstable and difficult to calibrate. Conclusions: From the afterglow SED we demonstrate that there is no evidence for dust extinction -- the SED at all times can be reproduced without dust, and at 1.25 days in particular, significant extinction can be excluded, with A(3000 AA) < 0.27 mag at 95% confidence using the SN-type extinction curve. We conclude that there is no evidence of any extinction in the afterglow of GRB 050904 and that the presence of SN-origin dust in the host of GRB 050904 must be viewed skeptically. [abridged]
We present a new multi-phase sub-resolution model for star formation and feedback in SPH numerical simulations of galaxy formation. Our model, called MUPPI (MUlti-Phase Particle Integrator), describes each gas particle as a multi-phase system, with cold and hot gas phases, coexisting in pressure equilibrium, and a stellar component. Cooling of the hot tenuous gas phase feeds the cold gas phase. Stars are formed out of molecular gas with a given efficiency, which scales with the dynamical time of the cold phase. Our prescription for star formation is not based on imposing the Schmidt-Kennicutt relation, which is instead naturally produced by MUPPI. Energy from supernova explosions is deposited partly into the hot phase of the gas particles, and partly to that of neighboring particles. Mass and energy flows among the different phases of each particle are described by a set of ordinary differential equations which we explicitly integrate for each gas particle, instead of relying on equilibrium solutions. This system of equations also includes the response of the multi-phase structure to energy changes associated to the thermodynamics of the gas. We apply our model to two isolated disk galaxy simulations and two spherical cooling flows. MUPPI is able to reproduce the Schmidt-Kennicutt relation for disc galaxies. It also reproduces the basic properties of the inter-stellar medium in disc galaxies, the surface densities of cold and molecular gas, of stars and of star formation rate, the vertical velocity dispersion of cold clouds and the flows connected to the galactic fountains. Quite remarkably, MUPPI also provides efficient stellar feedback without the need to include a scheme of kinetic energy feedback. [abridged]
We have analyzed three XMM-Newton observations of the Seyfert 1 galaxy Mrk 509, with the goal to detect small variations in the ionized outflow properties. Such measurements are limited by the quality of the cross-calibration between RGS, the best instrument to characterize the spectrum, and EPIC-pn, the best instrument to characterize the variability. For all three observations we are able to improve the relative calibration of RGS and pn consistently to 4 %. In all observations we detect three different outflow components and, thanks to our accurate cross-calibration we are able to detect small differences in the ionization parameter and column density in the highest ionized component of the outflow. This constrains the location of this component of the outflow to within 0.5 pc of the central source. Our method for modeling the relative effective area is not restricted to just this source and can in principle be extended to other types of sources as well.
We use the microlensing variability observed for eleven gravitationally lensed quasars to show that the accretion disk size at a rest-frame wavelength of 2500 Angstroms is related to the black hole mass by log(R_{2500}/cm)=(15.78\pm0.12) + (0.80\pm0.17)\log(M_BH/10^9M_sun). This scaling is consistent with the expectation from thin disk theory (R ~ M_BH^{2/3}), but when interpreted in terms of the standard thin disk model (T ~ R^{-3/4}), it implies that black holes radiate with very low efficiency, log(eta) = -1.77\pm0.29 + log(L/L_E) where eta=L/(Mdot*c^2). Only by making the maximum reasonable shifts in the average inclination, Eddington factors and black hole masses can we raise the efficiency estimate to be marginally consistent with typical efficiency estimates (eta ~ 10%). With one exception, these sizes are larger by a factor of ~4 than the size needed to produce the observed 0.8 micron quasar flux by thermal radiation from a thin disk with the same T ~ R^{-3/4} temperature profile. While scattering a significant fraction of the disk emission on large scales or including a large fraction of contaminating line emission can reduce the size discrepancy, resolving it also appears to require that accretion disks have flatter temperature/surface brightness profiles.
In the coming years a new insight into galaxy formation and the thermal history of the Universe is expected to come from the detection of the highly redshifted cosmological 21 cm line. The cosmological 21 cm line signal is buried under Galactic and extragalactic foregrounds which are likely to be a few orders of magnitude brighter. Strategies and techniques for effective subtraction of these foreground sources require a detailed knowledge of their structure in both intensity and polarization on the relevant angular scales of 1-30 arcmin. We present results from observations conducted with the Westerbork telescope in the 140-160 MHz range with 2 arcmin resolution in two fields located at intermediate Galactic latitude, centred around the bright quasar 3C196 and the North Celestial Pole. They were observed with the purpose of characterizing the foreground properties in sky areas where actual observations of the cosmological 21 cm line could be carried out. The polarization data were analysed through the rotation measure synthesis technique. We have computed total intensity and polarization angular power spectra. Total intensity maps were carefully calibrated, reaching a high dynamic range, 150000:1 in the case of the 3C196 field. [abridged]
In Horava's theory of gravity, Lorentz symmetry is broken in exchange for renormalizability, but the original theory has been argued to be plagued with problems associated with a new scalar mode stemming from the very breaking of Lorentz symmetry. Recently, Blas, Pujolas, and Sibiryakov have proposed a healthy extension of Ho\v{r}ava gravity, in which the behavior of the scalar mode is improved. In this paper, we study scalar modes of cosmological perturbations in extended Horava gravity. The evolution of metric and density perturbations is addressed analytically and numerically. It is shown that for vanishing non-adiabatic pressure of matter the large scale evolution of cosmological perturbations converges to that described by a single constant, $\zeta$, which is an analog of a curvature perturbation on the uniform-density slicing commonly used in usual gravitational theories. The subsequent evolution is thus determined completely by the value of $\zeta$.
Black hole spins affect the efficiency of the "classical" accretion processes, hence the radiative output from quasars. Spins also determine how much energy is extractable from the hole itself. Recently it became clear that massive black hole spins also affect the retention of black holes in galaxies, be cause of the impulsive "gravitational recoil", up to thousands km/s, due to anisotropic emission of gravitational waves at merger. I discuss here the evolution of massive black hole spins along the cosmic history, due to the combination of mergers and accretion events. I describe recent simulations of accreting black holes in merger remnants, and discuss the implication for the spins of black holes in quasars.
It is widely accepted that the prompt transient signal in the 10 keV - 10 GeV band from gamma-ray bursts (GRBs) arises from multiple shocks internal to the ultra-relativistic expansion. The detailed understanding of the dissipation and accompanying acceleration at these shocks is a currently topical subject. This paper explores the relationship between GRB prompt emission spectra and the electron (or ion) acceleration properties at the relativistic shocks that pertain to GRB models. The focus is on the array of possible high-energy power-law indices in accelerated populations, highlighting how spectra above 1 MeV can probe the field obliquity in GRB internal shocks, and the character of hydromagnetic turbulence in their environs. It is emphasized that diffusive shock acceleration theory generates no canonical spectrum at relativistic MHD discontinuities. This diversity is commensurate with the significant range of spectral indices discerned in prompt burst emission. Such system diagnostics are now being enhanced by the broadband spectral coverage of bursts by the Fermi Gamma-Ray Space Telescope; while the Gamma-Ray Burst Monitor (GBM) provides key diagnostics on the lower energy portions of the particle population, the focus here is on constraints in the non-thermal, power-law regime of the particle distribution that are provided by the Large Area Telescope (LAT).
Detecting gamma rays and neutrinos is crucial for studying the unknown nature of the ultrahigh-energy cosmic-ray (UHECR) sources. Recent results from the Pierre Auger Observatory favor a heavy nuclear composition for the UHECRs. Under the requirement that heavy nuclei survive in these sources, using gamma-ray bursts as an example, we predict a diagnostic gamma-ray signal, unique to nuclei -- the emission of de-excitation gamma rays following photodisintegration. These gamma rays, boosted from MeV to TeV-PeV energies, would be detectable by gamma-ray telescopes such as VERITAS, HESS, and MAGIC, and especially the next-generation CTA and AGIS.
Plasmas have a natural tendency to develop pressure anisotropies with respect to the local direction of the magnetic field. These anisotropies trigger plasma instabilities at scales just above the ion Larmor radius with growth rates of a fraction of the ion cyclotron frequency - much faster than either the global dynamics or local turbulence. The instabilities can dramatically modify the macroscopic dynamics of the plasma. Nonlinear evolution of these instabilities is expected to drive pressure anisotropies towards marginal stability values, controlled by the plasma beta. This nonlinear evolution is worked out in an ab initio kinetic calculation for the simplest analytically tractable example - the parallel firehose instability in a high-beta plasma. A closed nonlinear equation for the firehose turbulence is derived and solved. In the nonlinear regime, the instability leads to secular (~t) growth of magnetic fluctuations. The fluctuations develop a k^{-3} spectrum, extending from scales somewhat larger than rho_i to the maximum scale that grows secularly with time (~t^{1/2}); the relative pressure anisotropy tends to the marginal value -2/beta. When a parallel ion heat flux is present, the firehose mutates into the new gyrothermal instability (GTI), which continues to be unstable up to pressure anisotropies that can be positive and are limited by the magnitude of the heat flux. Its nonlinear evolution also involves secular growth of the magnetic energy, but the fluctuation spectrum is eventually dominated by modes around a maximal scale ~ rho_i l_T/lambda_mfp, where l_T is the scale of the parallel temperature variation. Implications for momentum and heat transport are speculated about. This study is motivated the dynamics of the intracluster medium, but its relevance to solar wind and accretion flows is also discussed.
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We explore the rate and impact of galaxy mergers on the massive galaxy population using the amplitude of the two-point correlation function on small scales for M > 5e10 M_sun galaxies from the COSMOS and COMBO-17 surveys. Using a pair fraction derived from the Sloan Digital Sky Survey as a low-redshift benchmark, the large survey area at intermediate redshifts allows us to determine the evolution of the close pair fraction with unprecedented accuracy for a mass-selected sample: we find that the fraction of galaxies more massive than 5e10M_sun in pairs separated by less than 30 kpc in 3D space evolves as F(z) = (0.0130+/-0.0019)x(1+z)^1.21+/-0.25 between z = 0 and z = 1.2. Assuming a merger time scale of 0.5 Gyrs, the inferred merger rate is such that galaxies with mass in excess of 1e11 M_sun have undergone, on average, 0.5 (0.7) mergers involving progenitor galaxies both more massive than 5e10 M_sun since z = 0.6 (1.2). We also study the number density evolution of massive red sequence galaxies using published luminosity functions and constraints on the M/L evolution from the fundamental plane. Moreover, we demonstrate that the measured merger rate of massive galaxies is sufficient to explain this observed number density evolution in massive red sequence galaxies since z = 1.
Traditionally, inflationary models are analyzed in terms of parameters such as the scalar spectral index ns and the tensor to scalar ratio r, while dark energy models are studied in terms of the equation of state parameter w. Motivated by the fact that both deal with periods of accelerated expansion, we study the evolution of w during inflation, in order to derive observational constraints on its value during an earlier epoch likely dominated by a dynamic form of dark energy. We find that the cosmic microwave background and large-scale structure data is consistent with w_inflation=-1 and provides an upper limit of 1+w <~ 0.02. Nonetheless, an exact de Sitter expansion with a constant w=-1 is disfavored since this would result in ns=1.
We present constraints on the cosmic growth history with recent cosmological data, allowing for deviations from Lambda CDM as might arise if cosmic acceleration is due to modifications to GR or inhomogeneous dark energy. We combine measures of the cosmic expansion history, from Type 1a supernovae, baryon acoustic oscillations and the CMB, with constraints on the growth of structure from recent galaxy, CMB and weak lensing surveys along with ISW-galaxy cross-correlations. Deviations from Lambda CDM are parameterized by phenomenological modifications to the Poisson equation and the relationship between the two Newtonian potentials. We find modifications that are present at the time the CMB is formed are tightly constrained through their impact on the well-measured CMB acoustic peaks. By contrast, constraints on late-time modifications to the growth history, as might arise if modifications are related to the onset of cosmic acceleration, are far weaker, but remain consistent with Lambda CDM at the 95% confidence level. For these late-time modifications we find that differences in the evolution on large and small scales could provide an interesting signature by which to search for modified growth histories with future wide angular coverage, large scale structure surveys.
We present a population study of several types of galaxies within the protocluster surrounding the radio galaxy MRC0316-257 at z~3.1. In addition to the known population of Ly_alpha emitters (LAEs) and [OIII] emitters, we use colour selection techniques to identify protocluster candidates that are Lyman break galaxies (LBG) and Balmer break galaxies (BBGs). The radio galaxy field contains an excess of LBG candidates, with a surface density 1.6\pm0.3 times larger than found for comparable blank fields. This surface overdensity corresponds to an LBG volume overdensity of ~8\pm4. The BBG photometric redshift distribution peaks at the protocluster's redshift, but we detect no significant surface overdensity of BBG. This is not surprising because a volume overdensity similar to the LBGs would have resulted in a surface density of ~1.2 that found in the blank field. This could not have been detected in our sample. Masses and star formation rates of the candidate protocluster galaxies are determined using SED fitting. These properties are not significantly different from those of field galaxies. The galaxies with the highest masses and star formation rates are located near the radio galaxy, indicating that the protocluster environment influences galaxy evolution at z~3. We conclude that the protocluster around MRC0316-257 is still in the early stages of formation.
We compare the substructure evolution in pure dark matter (DM) halos with those in the presence of baryons (PDM and BDM). The prime halos have been analyzed by Romano-Diaz et al (2009). Models have been evolved from identical initial conditions using Constrained Realizations, including star formation and feedback. A comprehensive catalog of subhalos has been compiled and properties of subhalos analyzed in the mass range of 10^8 Mo - 10^11 Mo. We find that subhalo mass functions are consistent with a single power law, M_sbh^{-alpha}, but detect a nonnegligible shift between these functions, alpha -0.86 for the PDM, and -0.98 for the BDM. Overall, alpha const. with intrinsic scatter of +-15%. Second, we find that the radial mass distribution of subhalos can be approximated by a power law, R^{-gamma} with a steepening around the radius of a maximal circular velocity, Rvmax, in the prime halos. Gamma -1.5 for the PDM and -1 for the BDM, inside Rvmax, and is steeper outside. We detect little spatial bias between the subhalo populations and the DM of the main halos. The subhalo population exhibits much less triaxiality with baryons, in tandem with the prime halo. Finally, we find that, counter-intuitively, the BDM population is depleted at a faster rate than the PDM one within the central 30kpc of the prime. Although the baryons provide a substantial glue to the subhalos, the main halos exhibit the same trend. This assures a more efficient tidal disruption of the BDM subhalos. This effect can be reversed for a more efficient feedback from stellar evolution and supermassive black holes, which will expel baryons from the center and decrease the concentration of the prime halo.
Thanks to the INTEGRAL satellite the way of looking at the hard X-ray sky above 20 keV has changed substantially. Through the unique imaging and spectroscopy capabilities of the IBIS instrument that has formed the basis of the INTEGRAL surveys, this satellite has improved the knowledge on hard X-ray sources in terms of sensitivity and positional accuracy. Many of the sources belonging to these surveys are however of unidentified nature, but the combined use of available information at longer wavelengths (mainly soft X-rays and radio) and above all optical spectroscopy on the putative counterparts of these hard X-ray objects can reveal their exact nature. Since 2004 our group identified more than 100 it INTEGRAL sources, reducing drastically the percentage of unidentified objects in the various IBIS surveys and allowing statistical studies on them. Here we present a summary of this identification work and an outlook of our preliminary results on identification of newly-discovered sources belonging to the 4th IBIS catalog.
We quantify the importance of the mechanical energy released by radio-galaxies inside galaxy groups. We use scaling relations to estimate the mechanical energy released by 16 radio-AGN located inside X-ray detected galaxy groups in the COSMOS field. By comparing this energy output to the host groups' gravitational binding energy, we find that radio galaxies produce sufficient energy to unbind a significant fraction of the intra-group medium. This unbinding effect is negligible in massive galaxy clusters with deeper potential wells. Our results correctly reproduce the breaking of self-similarity observed in the scaling relation between entropy and temperature for galaxy groups.
Using Chandra observations we have identified a sample of seven off-nuclear X-ray sources, in the redshift range z=0.072-0.283, located within optically bright galaxies in the COSMOS Survey. Using the multi-wavelength coverage available in the COSMOS field, we study the properties of the host galaxies of these ULXs. In detail, we derived their star formation rate from H_alpha measurements and their stellar masses using SED fitting techniques with the aim to compute the probability to have an off-nuclear source based on the host galaxy properties. We divide the host galaxies in different morphological classes using the available ACS/HST imaging. We find that our ULXs candidates are located in regions of the SFR versus M$_star$ plane where one or more off-nuclear detectable sources are expected. From a morphological analysis of the ACS imaging and the use of rest-frame colours, we find that our ULXs are hosted both in late and early type galaxies. Finally, we find that the fraction of galaxies hosting a ULX ranges from ~0.5% to ~0.2% going from L[0.5-2 keV]=3 x 10^39 erg s^-1 to L[0.5-2 keV]= 2 x 10^40 erg s^-1.
We use deep HST/ACS observations to calculate the star formation history (SFH) of the Cetus dwarf spheroidal (dSph) galaxy. Our photometry reaches below the oldest main sequence turn-offs, which allows us to estimate the age and duration of the main episode of star formation in Cetus. This is well approximated by a single episode that peaked roughly 12+/-0.5 Gyr ago and lasted no longer than about 1.9+/-0.5 Gyr (FWHM). Our solution also suggests that essentially no stars formed in Cetus during the past 8 Gyrs. This makes Cetus' SFH comparable to that of the oldest Milky Way dSphs. Given the current isolation of Cetus in the outer fringes of the Local Group, this implies that Cetus is a clear outlier in the morphology-Galactocentric distance relation that holds for the majority of Milky Way dwarf satellites. Our results also show that Cetus continued forming stars through z ~ 1, long after the Universe was reionized, and that there is no clear signature of the epoch of reionization in Cetus' SFH. We discuss briefly the implications of these results for dwarf galaxy evolution models. Finally, we present a comprehensive account of the data reduction and analysis strategy adopted for all galaxies targeted by the LCID (Local Cosmology from Isolated Dwarfs) project. We employ two different photometry codes (DAOPHOT/ALLFRAME and DOLPHOT), three different SFH reconstruction codes (IAC-pop/MinnIAC, MATCH, COLE), and two stellar evolution libraries (BaSTI and Padova/Girardi), allowing for a detailed assessment of the modeling and observational uncertainties.
We use a model developed by Xu et al. (2010) to compute the 21 cm line absorption signatures imprinted by star-forming dwarf galaxies (DGs) and starless minihalos (MHs). The method, based on a statistical comparison of the equivalent width (W_\nu) distribution and flux correlation function, allows us to derive a simple selection criteria for candidate DGs at very high (z >= 8) redshift. We find that ~ 18% of the total number of DGs along a line of sight to a target radio source (GRB or quasar) can be identified by the condition W_\nu < 0; these objects correspond to the high-mass tail of the DG distribution at high redshift, and are embedded in large HII regions. The criterion W_\nu > 0.37 kHz instead selects ~ 11% of MHs. Selected candidate DGs could later be re-observed in the near-IR by the JWST with high efficiency, thus providing a direct probe of the most likely reionization sources.
Photometric scaling relations are studied for S0 galaxies and compared with those for spirals. New 2D K_s-band multi-component decompositions are presented for 122 early-type disk galaxies. Combining with our previous decompositions, the final sample consists of 175 galaxies. As a comparison sample we use the Ohio State University Bright Spiral Galaxy Survey (OSUBSGS), for which similar decompositions have previously been made by us. Our main results are: (1) Important scaling relations are present, indicating that the formative processes of bulges and disks in S0s are coupled like has been previously found for spirals. (2) We obtain median r_{eff}/h_r = 0.20, 0.15 and 0.10 for S0, S0/a-Sa and Sab-Sc galaxies: these are smaller than predicted by simulation models in which bulges are formed by galaxy mergers. (3) The properties of bulges of S0s are different from the elliptical galaxies, which is manifested in the M_K(bulge) vs r_{eff} relation, in the photometric plane, and to some extent also in the Kormendy relation. The bulges of S0s are similar to bulges of spirals with M_K(bulge) < -20 mag. Some S0s have small bulges, but their properties are not compatible with the idea that they could evolve to dwarfs by galaxy harassment. (4) The relative bulge flux B/T for S0s covers the full range found in the Hubble sequence. (5) The values and relations of the parameters of the disks of the S0 galaxies in NIRS0S are similar to those obtained for spirals in the OSUBSGS. Overall, our results support the view that spiral galaxies with bulges brighter than -20 mag in the K-band can evolve directly into S0s, due to stripping of gas followed by truncated star formation.
We investigate cosmological scenarios with a non-minimal derivative coupling between the scalar field and the curvature, examining both the quintessence and the phantom cases in zero and constant potentials. In general, we find that the universe transits from one de Sitter solution to another, determined by the coupling parameter. Furthermore, according to the parameter choices and without the need for matter, we can obtain a Big Bang, an expanding universe with no beginning, a cosmological turnaround, an eternally contracting universe, a Big Crunch, a Big Rip avoidance and a cosmological bounce. This variety of behaviors reveals the capabilities of the present scenario.
Non-WIMP dark matter candidates include particles motivated by minimality, candidates motivated by experimental anomalies, and exotic possibilities motivated primarily by the desire of clever iconoclasts to highlight how truly ignorant we are about the nature of dark matter. In this review, I discuss candidates that are not WIMPs, but nevertheless share the same theoretical motivations as WIMPs and also naturally have the correct relic density. There are two classes: superWIMP dark matter, where the desired relic density is inherited through decays, and WIMPless dark matter, where the dark matter's mass and couplings scale together to maintain the desired thermal relic density.
We consider a $4+N$ dimensional Einstein gravity coupled to a non-linear sigma model. This theory admits a solution in which the $N$ extra dimensions contract exponentially while the ordinary space expand exponentially. Physically, the non-linear sigma fields induce the dynamical compactification of the extra dimensions, which in turn drives inflation. No inflatons are required.
The Galactic disk retains a vast amount of information about how it came to be, and how it evolved over cosmic time. However, we know very little about the secular processes associated with disk evolution. One major uncertainty is the extent to which stars migrate radially through the disk, thereby washing out signatures of their past (e.g. birth sites). Recent theoretical work finds that such "blurring" of the disk can be important if spiral arms are transient phenomena. Here we describe an experiment to determine the importance of diffusion from the Solar circle with cosmic time. Consider a star cluster that has been placed into a differentially rotating, stellar fluid. We show that all clusters up to ~10^4 solar masses, and a significant fraction of those up to ~10^5 solar masses, are expected to be chemically homogeneous, and that clusters of this size can be assigned a unique "chemical tag" by measuring the abundances of <~10 independent element groups, with better age and orbit determinations allowing fewer abundance measurements. The star cluster therefore acts like a "tracer dye", and the present-day distribution of its stars provides a strong constraint on the rate of radial diffusion or migration in the Galactic disk. Sellwood & Binney have argued for strong radial transport driven by transient spiral perturbations: in principle, we could measure the strength of this migration directly.
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The WMAP experiment has detected reionization at the $5.5 \sigma$ level and has reported a mean optical depth of $0.088 \pm 0.015$. A powerful probe of reionization is the large-angle $EE$ polarization power spectrum, which is now (since the first five years of data from WMAP) cosmic variance limited for $2\le l \le6$. Here we consider partial reionization caused by WIMP dark matter annihilation, and calculate the expected polarization power spectrum. We compare the dark matter models with a standard 2-step reionization theory, and examine whether the models may be distinguished using current, and future CMB observations. We consider dark matter annihilation at intermediate redshifts ($z<60$) due to halos, as well as annihilation at higher redshifts due to free particles. In order to study the effect of high redshift dark matter annihilation on CMB power spectra, it is essential to include the contribution of residual electrons (left over from recombination) to the ionization history. Dark matter halos at redshifts $z<60$ influence the low multipoles $l<20$ in the $EE$ power spectrum, while the annihilation of free particle dark matter at high redshifts $z>100$ mainly affects multipoles $l>10$.
We study the internal circulation within the cocoon carved out by the relativistic jet emanating from an AGN within the ISM of its host galaxy. Firstly, we develop a model for the origin of the internal flow, noticing that a significant increase of large scale velocity circulation within the cocoon arises as significant gradients in the density and entropy are created near the hot spot (a consequence of Crocco's vorticity generation theorem). We find simple and accurate approximate solutions for the large scale flow,showing that a backflow towards the few inner parsec region develops. We solve the appropriate fluid dynamic equations, and we use these solutions to predict the mass inflow rates towards the central regions. We then perform a series of 2D simulations of the propagation of jets using FLASH 2.5, in order to validate the predictions of our model. The backflows which arise supply the central AGN region with very low angular momentum gas, at average rates of the order of $0.1-0.8 \rm{M}_{\odot} \rm{yr.}^{-1}$, the exact value seen to be strongly dependent on the central ISM density (for fixed input jet power). The time scales of these inflows are apparently weakly dependent on the jet/ISM parameters, and are of the order of $3-5\times 10^{7} \rm{yrs}$. We then argue that these backflows could (at least partially) feed the AGN, and provide a self-regulatory mechanism of AGN activity, that is not directly controlled by, but instead controls, the star formation rate within the central circumnuclear disk.
We investigate the assembly of groups and clusters of galaxies using the Millennium dark matter simulation and the associated gas simulations and semi-analytic catalogues of galaxies. In particular, in order to find an observable quantity that could be used to identify early-formed groups, we study the development of the difference in magnitude between their brightest galaxies to assess the use of magnitude gaps as possible indicators. We select galaxy groups and clusters at redshift z=1 with dark matter halo mass M(R200) > 1E13/h Msun, and trace their properties until the present time (z=0). We consider only the systems with X-ray luminosity L_X> 0.25E42/h^2 erg/s at z=0. While it is true that a large magnitude gap between the two brightest galaxies of a particular group often indicates that a large fraction of its mass was assembled at an early epoch, it is not a necessary condition. More than 90% of fossil groups defined on the basis of their magnitude gaps (at any epoch between 0<z<1) cease to be fossils within 4 Gyr, mostly because other massive galaxies are assembled within their cores, even though most of the mass in their haloes might have been assembled at early times. We show that, compared to the conventional definition of fossil galaxy groups based on the magnitude gap Delta m(12)> 2 (in the R-band, within 0.5R200 of the centre of the group), an alternative criterion Delta m(14)>2.5 (within the same radius) finds 50% more early-formed systems, and those that on average retain their fossil phase longer. However, the conventional criterion performs marginally better at finding early-formed groups at the high-mass end of groups. Nevertheless, both criteria fail to identify a majority of the early-formed systems.
The first published Fermi large area telescope (Fermi-LAT) measurement of the isotropic diffuse gamma-ray emission is in good agreement with a single power law, and is not showing any signature of a dominant contribution from dark matter sources in the energy range from 20 to 100 GeV. We use the absolute size and spectral shape of this measured flux to derive cross section limits on three types of generic dark matter candidates: annihilating into quarks, charged leptons and monochromatic photons. Predicted gamma-ray fluxes from annihilating dark matter are strongly affected by the underlying distribution of dark matter, and by using different available results of matter structure formation we assess these uncertainties. We also quantify how the dark matter constraints depend on the assumed conventional backgrounds and on the Universe's transparency to high-energy gamma-rays. In reasonable background and dark matter structure scenarios (but not in all scenarios we consider) it is possible to exclude models proposed to explain the excess of electrons and positrons measured by the Fermi-LAT and PAMELA experiments. Derived limits also start to probe cross sections expected from thermally produced relics (e.g. in minimal supersymmetry models) annihilating predominantly into quarks. For the monochromatic gamma-ray signature, the current measurement constrains only dark matter scenarios with very strong signals.
We use a sample of close galaxy pairs selected from the SDSS DR4 to investigate in what environments galaxy mergers occur and how the results of these mergers depend on differences in local galaxy density. Lower density environments have fractionally more galaxy pairs with small projected separations (r_p) and relative velocities (Delta V), but even high density environments contain significant populations of pairs with parameters that should be conducive to interactions. The connection between environment and Delta V also implies that the velocity selection of a pairs sample affects (biases) the environment from which the pairs are selected. Metrics of asymmetry and colour are used to identify merger activity and triggered star formation. The location of star formation is inferred by distinguishing bulge and disk colours and calculating bulge fractions from the SDSS images. Considering only pairs with Delta V < 200 km/s, we show that the asymmetry of galaxies with a close companion increases at all local densities at r_p < 30 kpc. However, of these closely separated pairs, galaxies in the lowest density environments exhibit the most significant trends towards increased star formation rates and bluer bulge colours. We interpret these results as evidence that whilst interactions occur at all densities, triggered star formation is seen only in low-to-intermediate density environments. We argue that this is likely due to the typically higher gas fractions of galaxies in low density environments. Finally, by cross-correlating our sample of galaxy pairs with a cluster catalogue, we find evidence for galaxy-galaxy interactions only at the cluster periphery.
We determine the intrinsic axial ratio distribution of the 'gas' disks of extremely faint M_B < -14.5 dwarf irregular galaxies. We start with the measured (beam corrected) distribution of apparent axial ratios in the HI 21cm images of dwarf irregular galaxies observed as part of the Faint Irregular Galaxy GMRT Survey (FIGGS). Assuming that the disks can be approximated as oblate spheroids, the intrinsic axial ratio distribution can be obtained from the observed apparent axial ratio distribution. We use a variety of methods to do this, and our final results are based on using Lucy's deconvolution algorithm. This method is constrained to produce physically plausible distributions, and also has the added advantage of allowing for observational errors to be accounted for. While one might a priori expect that gas disks would be thin (because collisions between gas clouds would cause them to quickly settle down to a thin disk), we find that the HI disks of faint dwarf irregulars are quite thick, with mean axial ratio <q> ~ 0.6. While this is substantially larger than the typical value of ~ 0.2 for the 'stellar' disks of large spiral galaxies, it is consistent with the much larger ratio of velocity dispersion to rotational velocity (sigma/v_c) in dwarf galaxy HI disks as compared to that in spiral galaxies. Our findings have implications for studies of the mass distribution in and the Tully - Fisher relation for faint dwarf irregular galaxies, where it is often assumed that the gas is in a thin disk.
Using self-consistent chemodynamical simulations including star formation, supernova feedback, and chemical enrichment, I show the dependence of cosmic star formation and chemical enrichment histories on the initial mass function (IMF). The effects of Pop-III IMF can be only seen in the elemental abundance ratios at z>4 or [Fe/H]<-2. The preferable IMF has a flatter slope in the case of high star formation rate (SFR) and smaller upper-mass (~20Msun) in the case of low SFR, which is consistent with the observed elemental abundances of dwarf spheroidal galaxies. However, the [alpha/Fe] problem of elliptical galaxies may require other solutions.
(Abridged) Our aim is to investigate whether the presence of baryons can have any significant influence on the properties of the local Hubble flow which has proved to be "cold". We use two cosmological zoom simulations in the standard LCDM cosmology with the same set of initial conditions to study the formation of a local group-like system within a sphere of ~7 Mpc/h. The first one is a pure dark matter simulation (runDM) while a complete treatment of the physics of baryons is introduced in the second one (runB). We found that galaxies identified in runB and their corresponding dark matter haloes in runDM have very similar spatial distributions and dynamical properties on large scales. Then, when analyzing the velocity field and the deviation from a pure Hubble flow in both simulations, namely when computing the dispersion of peculiar velocities of galaxies ?*(R) and those of their corresponding dark matter haloes in runDM, we found no particular differences for distances R=1 to 8 Mpc from the local group mass center. The results indicate that the "true" ?*(R) values can be estimated from the pure dark matter simulation with a mean error of 3 km/s when dark matter haloes are selected with maximum circular velocities of Vc?30 km/s, corresponding to a population of dark matter haloes in runB that host galaxies. By investigating the properties of the Hubble flow at distances R~0.7 to 3 Mpc, we also found that the estimation of the total mass enclosed at the radius of the zero-velocity surface R0, using the spherical infall model adapted to LCDM, can be underestimated by at least 50%.
The high-redshift (z=4.72) blazar J1430+4204 produced an exceptional radio outburst in 2006. We analyzed 15-GHz radio interferometric images obtained with the Very Long Baseline Array (VLBA) before and after the outburst, to search for possible structural changes on milli-arcsecond angular scales and to determine physical parameters of the source.
We analyse the constraints obtained from new atomic clock data on the possible time variation of the fine structure `constant' and the electron-proton mass ratio and show how they are strengthened when the seasonal variation of Sun's gravitational field at the Earth's surface is taken into account.
We use numerical simulations to investigate, for the first time, the joint effect of feedback from supernovae (SNe) and active galactic nuclei (AGN) on the evolution of galaxy cluster X-ray scaling relations. Our simulations are drawn from the Millennium Gas Project and are some of the largest hydrodynamical N-body simulations ever carried out. Feedback is implemented using a hybrid scheme, where the energy input into intracluster gas by SNe and AGN is taken from a semi-analytic model of galaxy formation. This ensures that the source of feedback is a population of galaxies that closely resembles that found in the real universe. We show that our feedback model is capable of reproducing observed local X-ray scaling laws, at least for non-cool core clusters, but that almost identical results can be obtained with a simplistic preheating model. However, we demonstrate that the two models predict opposing evolutionary behaviour. We have examined whether the evolution predicted by our feedback model is compatible with observations of high-redshift clusters. Broadly speaking, we find that the data seems to favour the feedback model for z<0.5, and the preheating model at higher redshift. However, a statistically meaningful comparison with observations is impossible, because the large samples of high-redshift clusters currently available are prone to strong selection biases. As the observational picture becomes clearer in the near future, it should be possible to place tight constraints on the evolution of the scaling laws, providing us with an invaluable probe of the physical processes operating in galaxy clusters.
The relation is studied between occurrence of a regular chain of star complexes and superclouds in a spiral arm, and other properties of the latter. A regular string of star complexes is located in the north-western arm of M31; they have about the same size 0.6 kpc with spacing of 1.1 kpc. Within the same arm segment the regular magnetic field with the wavelength of 2.3 kpc was found by Beck et al. (1989). We noted that this wavelength is twice as large as the spacing between complexes and suggested that they were formed in result of magneto-gravitational instability developed along the arm. In this NW arm, star complexes are located inside the gas-dust lane, whilst in the south-western arm of M31 the gas-dust lane is upstream of the bright and uniform stellar arm. Earlier, evidence for the age gradient has been found in the SW arm. All these are signatures of a spiral shock, which may be associated with unusually large (for M31) pitch-angle of this SW arm segment. Such a shock may prevent the formation of the regular magnetic field, which might explain the absence of star complexes there. Anti-correlation between shock wave signatures and presence of star complexes is observed in spiral arms of a few other galaxies. Regular chains of star complexes and superclouds in spiral arms are rare, which may imply that a rather specific mechanism is involved in their formation, and the most probable one is the Parker-Jeans instability. The spiral pattern of our Galaxy is briefly discussed; it may be of M101 type in the outer parts. The regular bi-modal spacing of HI superclouds is found in Carina and Cygnus (Outer) arms, which may be an indirect evidence for the regular magnetic field along these arms.
The effect of density fluctuations upon light propagation is calculated perturbatively in a matter dominated irrotational universe. The starting point is the perturbed metric (second order in the perturbation strength), while the output is the Hubble diagram. Density fluctuations cause this diagram to broaden to a strip. Moreover, the shift of the diagram mimics accelerated expansion.
A version of the swisscheese model is investigated. Nonoverlapping spheres are cut from a flat Friedmann-Robertson-Walker (FRW) universe. The mass they contained before is compressed within each sphere to a smaller sphere with homogeneous density distribution. Hence the inner spheres form sections of some closed FRW model. Between the outer and inner spheres there is a vacuum, where, due to spherical symmetry, the Schwarzschield metric describes the gravitational field. Within the inner spheres the closed FRW metric is valid, while outside the cut spheres the flat FRW metric is relevant. The metric and its first derivatives are continuous across the bordering surfaces of the different regions. We discuss this model in detail, and show that such a model universe can have a realistic density distribution on large scales, and exhibits accelerating expansion for a limited period of time. Especially, we determine the Hubble diagram and discuss its properties.
We use ACS data from the HST Treasury survey of the Coma cluster (z~0.02) to
study the properties of barred galaxies in the Coma core, the densest
environment in the nearby Universe. This study provides a complementary data
point for studies of barred galaxies as a function of redshift and environment.
From ~470 cluster members brighter than M_I = -11 mag, we select a sample of
46 disk galaxies (S0--Im) based on visual classification. The sample is
dominated by S0s for which we find an optical bar fraction of 47+/-11% through
ellipse fitting and visual inspection. Among the bars in the core of the Coma
cluster, we do not find any very large (a_bar > 2 kpc) bars. Comparison to
other studies reveals that while the optical bar fraction for S0s shows only a
modest variation across low-to-intermediate density environments (field to
intermediate-density clusters), it can be higher by up to a factor of ~2 in the
very high-density environment of the rich Coma cluster core.
We consider the problem of characterisation of burst sources detected with the Laser Interferometer Space Antenna (LISA) using the multi-modal nested sampling algorithm, MultiNest. We use MultiNest as a tool to search for modelled bursts from cosmic string cusps, and compute the Bayesian evidence associated with the cosmic string model. As an alternative burst model, we consider sine-Gaussian burst signals, and show how the evidence ratio can be used to choose between these two alternatives. We present results from an application of MultiNest to the last round of the Mock LISA Data Challenge, in which we were able to successfully detect and characterise all three of the cosmic string burst sources present in the release data set. We also present results of independent trials and show that MultiNest can detect cosmic string signals with signal-to-noise ratio (SNR) as low as ~7 and sine-Gaussian signals with SNR as low as ~8. In both cases, we show that the threshold at which the sources become detectable coincides with the SNR at which the evidence ratio begins to favour the correct model over the alternative.
Recently, the so-called Elko spinor field has been proposed to be a candidate of dark energy. It is a non-standard spinor and has unusual properties. When the Elko spinor field is used in cosmology, its unusual properties could bring some interesting consequences. In the present work, we consider two different issues concerning the dark energy model described by the Elko spinor fields. Firstly, we discuss the cosmological coincidence problem in the spinor dark energy model by using the dynamical system method. Our results show that the coincidence problem should be taken to heart in the investigations of spinor dark energy models. Next, we turn to another issue. In this work, we propose a simple method to reconstruct spinor dark energy from the cosmological observations. We find that this reconstruction method works fairly well.
To accommodate the observed accelerated expansion of the universe, one popular idea is to invoke a driving term in the Friedmann-Lema\^{i}tre equation of dark energy which must then comprise 70% of the present cosmological energy density. We propose an alternative interpretation which takes into account the temperature intrinsic to the information holographically stored on the screen which is the surface of the universe. Dark energy is thereby obviated and the acceleration is due to an entropic force naturally arising from the information storage on a surface screen. We consider an additional quantitative approach based upon the entropy and surface terms usually neglected in General Relativity and show that this leads to the entropic accelerating universe.
Recent observations by H.E.S.S. and MAGIC strongly suggest that the Universe is more transparent to very-high-energy gamma rays than previously thought. We show that this fact can be reconciled with standard blazar emission models provided that photon oscillations into a very light Axion-Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect indeed explains the observed data and in particular the spectrum of blazar 3C279.
We present a family of robust tracer mass estimators to compute the enclosed mass of galaxy haloes from samples of discrete positional and kinematical data of tracers, such as halo stars, globular clusters and dwarf satellites. The data may be projected positions, distances, line of sight velocities or proper motions. Forms of the estimator tailored for the Milky Way galaxy and for M31 are given. Monte Carlo simulations are used to quantify the uncertainty as a function of sample size. For the Milky Way, the satellite sample consists of 26 galaxies with line-of-sight velocities. We find that the mass of the Milky Way within 300 kpc is ~ 0.9 x 10^12 solar masses assuming velocity isotropy. However, the mass estimate is sensitive to the anisotropy and could plausibly lie between 0.7 - 3.4 x 10^12 solar masses. Incorporating the proper motions of 6 Milky Way satellites into the dataset, we find ~ 1.4 x 10^12 solar masses. The range here if plausible anisotropies are used is still broader, from 1.2 - 2.7 x 10^12 solar masses. For M31, there are 23 satellite galaxies with measured line-of-sight velocities, but only M33 and IC 10 have proper motions. We use the line of sight velocities and distances of the satellite galaxies to estimate the mass of M31 within 300 kpc as ~ 1.4 x 10^12 solar masses assuming isotropy. There is only a modest dependence on anisotropy, with the mass varying between 1.3 -1.6 x 10^12 solar masses. Given the uncertainties, we conclude that the satellite data by themselves yield no reliable insights into which of the two galaxies is actually the more massive.
In this work we investigate the thermodynamic properties satisfied by an expanding universe filled with a monoatomic ideal gas. We show that the equations for the energy density, entropy density and chemical potential remain the same as the ideal gas.
Context. Analytical and numerical analysis of the SimpleX radiative transfer algorithm, which features transport on a Delaunay triangulation. Aims. Verify whether the SimpleX radiative transfer algorithm conforms to mathematical expectations, to develop error analysis and present improvements upon earlier versions of the code. Methods. Voronoi-Delaunay tessellation, classical Markov theory. Results. Quantitative description of the error properties of the SimpleX method. Numerical validation of the method and verification of the analytical results. Improvements in accuracy and speed of the method. Conclusions. It is possible to transport particles such as photons in a physically correct manner with the SimpleX algorithm. This requires the use of weighting schemes or the modification of the point process underlying the transport grid. We have explored and applied several possibilities.
Energetic positrons produced in annihilation or decay of dark matter particles in the Milky Way can serve as an important indirect signature of dark matter. Computing the positron flux expected in a given dark matter model involves solving transport equations, which account for interaction of positrons with matter and galactic magnetic fields. Existing calculations solve the equations inside the diffusion zone, where galactic magnetic fields confine positrons, and assume vanishing positron density on the boundaries of this zone. However, in many models, a substantial fraction of the dark matter halo lies outside the diffusion zone. Positrons produced there can then enter the diffusion zone and get trapped, potentially reaching the Earth and increasing the expected flux. We calculate this enhancement for a variety of models. We also evaluate the expected enhancement of the flux of energetic photons produced by the inverse Compton scattering of the extra positrons on starlight and cosmic microwave background. We find maximal flux enhancements of order 20% in both cases.
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The new high energy data coming mainly from the Fermi and Swift satellites and from the ground based Cerenkov telescopes are making possible to study not only the energetics of blazar jets, but also their connection to the associated accretion disks. Furthermore, the black hole mass of the most powerful objects can be constrained through IR-optical emission, originating in the accretion disks. For the first time, we can evaluate jet and accretion powers in units of the Eddington luminosity for a large number of blazars. Firsts results are intriguing. Blazar jets have powers comparable to, and often larger than the luminosity produced by their accretion disk. Blazar jets are produced at all accretion rates (in Eddington units), and their appearance depends if the accretion regime is radiatively efficient or not. The jet power is dominated by the bulk motion of matter, not by the Poynting flux, at least in the jet region where the bulk of the emission is produced, at ~1000 Schwarzschild radii. The mechanism at the origin of relativistic jets must be very efficient, possibly more than accretion, even if accretion must play a crucial role. Black hole masses for the most powerful jets at redshift ~3 exceed one billion solar masses, flagging the existence of a very large population of heavy black holes at these redshifts.
We report the detection of 21-cm and H2 absorption lines in the same DLA system (log N(HI)=21.36+-0.10) at zabs=3.17447 towards SDSSJ133724+315254 (z=3.174). We estimate the spin temperature of the gas to be, Ts~600 K, intermediate between the expected values for cold and warm neutral media. This suggests that the HI absorption originates from a mixture of different phases. The total molecular fraction is low, f=10^-7, and H2 rotational level populations are not in equilibrium. The average abundance of the alpha-elements is, [S/H]=-1.45. N and Fe are found underabundant with respect to alpha-elements by ~1.0 dex and ~0.5 dex respectively. Using photoionization models we conclude that the gas is located more than 270 kpc away from the QSO. While the position of 21-cm absorption line coincides with the H2 velocity profile, their centroid are shifted by 2.7+-1.0 km/s from each other. However, the position of the strongest metal absorption component matches the position of the 21-cm absorption line within 0.5 km/s. From this, we constrain the variation of the combination of fundamental constants x=alpha^2 Gp/mu, Delta x/x=-(1.7+-1.7)x10^-6. This system is unique as we can at the same time have an independent constrain on mu using H2 lines. However only Werner band absorption lines are seen and the range of sensitivity coefficients is too narrow to provide a stringent constraint: Delta mu/mu <= 4.0x10^-4. The VLT/UVES spectrum reveals another DLA at zabs=3.16768 with log N(HI)=20.41+-0.15 and low metallicity, [Si/H]=-2.68+-0.11. We derive log N(DI)/N(HI)=-(4.93+-0.15) in this system. This is a factor of two smaller than the value expected from the best fitted value of Omega_b from the WMAP 5 yr data. This confirms the presence of astration of deuterium even at very low metallicity. [abridged]
We have initiated a survey using the newly commissioned X-shooter spectrograph to target candidate relatively metal-rich damped Lyman-alpha absorbers (DLAs). Our rationale is that high-metallicity DLAs due to the luminosity-metallicity relation likely will have the most luminous galaxy counterparts. We have chosen DLAs where the strongest rest-frame optical lines ([OII], [OIII], Hbeta and Halpha) fall in the NIR atmospheric transmission bands. In this first paper resulting from the survey, we report on the discovery of the galaxy counterpart of the z_abs= 2.354 DLA towards the z=2.926 Q2222$-0946. This DLA is amongst the most metal-rich z>2 DLAs studied so far at comparable redshifts and there is evidence for substantial depletion of refractory elements onto dust grains. We measure metallicities from ZnII, SiII, NiII, MnII and FeII of -0.46+/-0.07, -0.51+/-0.06, -0.85+/-0.06, -1.23+/-0.06, and -0.99+/-0.06, respectively. The galaxy is detected in the Lyalpha, [OIII] 4960, 5008 and Halpha emission lines at an impact parameter of about 0.8 arcsec (6 kpc at z_abs = 2.354). Based on the Halpha line, we infer a star-formation rate of 10 M_sun yr^-1. Compared to the recently determined Halpha luminosity function for z=2.2 galaxies the DLA-galaxy counterpart is a dwarf galaxy (L~0.1L*). The emission-line ratios are 3.6 (Lyalpha/Halpha) and 1.6 ([OIII]/Halpha). In particular, the Lyalpha line shows clear evidence for resonant scattering effects, namely an asymmetric, redshifted (relative to the systemic redshift) component and a much weaker blueshifted component. The properties of the galaxy counterpart of this DLA confirms the prediction that metal-rich DLAs are associated with the most luminous of the DLA-galaxy counterparts.
Aims: We reanalyse optical spectra of the z=6.7 gamma-ray burst GRB 080913,
adding hitherto unpublished spectra, in order to reassess the measurement of
the neutral fraction of the IGM at high redshifts.
Methods: In the data reduction we take particular care to minimise systematic
errors in the sky subtraction, which are evident in the published spectrum, and
compromise the analysis. The final combined spectrum has a higher S/N than the
previously published spectrum by a factor of 1.3.
Results: We find a single significant absorption line redward of the Ly-alpha
continuum break, which we identify with the SII+SiII blend (rest wavelength of
0.1260 micron), at z=6.733. The sharp spectral break at Ly-alpha implies a
comparatively low total column density of neutral hydrogen along the line of
sight, log [N(HI)/cm^-2] < 20. We model the absorption with a host-galaxy DLA,
surrounded by an ionised region of unknown size r, within the IGM of neutral
fraction, x(HI). Despite knowing the source redshift, and the improved S/N of
the spectrum, when fitting only over wavelengths redward of Ly-alpha, no useful
constraints on x(HI) can be obtained. We consider the possibility of including
the ionised region, blueward of Ly-alpha, in constraining the fit. For the
optimistic assumption that the ionised region is transparent, tau_{GP} << 1, we
find that the region is of small size r < 2 proper Mpc, and we obtain an upper
limit to the neutral fraction of the IGM at z=6.7 of x(HI) < 0.73.
In this Letter, the Evans and Koratkar Atlas of Hubble Space Telescope Faint Object Spectrograph Spectra of Active Galactic Nuclei and Quasars is used to study the redward asymmetry in CIV broad emission lines (BELs). It is concluded that there is a highly significant correlation between the spectral index from 10 GHz to 1350 $\AA$ and the amount of excess luminosity in the red wing of the CIV BEL ($>99.9999%$ significance level for the full sample and the radio loud subsample independently, but no correlation is found for the radio quiet subsample). This is interpreted as a correlation between radio core dominance and the strength of the CIV redward asymmetry. The data implies that within the quasar environment there is BEL gas with moderately blueshifted emission associated with the purely radio quiet quasar phenomenon (the accretion disk) and the radio jet emission mechanism is associated with a redward BEL component that is most prominent for lines of sight along the jet axis. Thus, radio quiet quasars have CIV BELs that tend to show blueshifted excess and radio loud quasars show either a red or blue excess with the tendency for a dominant red excess increasing as the line of sight approaches the jet axis.
Recent mass estimates of the well-studied massive galaxy cluster Abell 1689 based on X-ray observations and gravitational lensing, assuming spherical symmetry, still give discrepant results. Contamination by a cool component, substructure and triaxial gas distribution have been proposed to resolve this discrepancy. However, none of these solutions is fully consistent with all observations and predictions of cold dark matter models. A common assumption in all proposed solutions is strict hydrostatic equilibrium, i.e., the gas pressure is provided entirely by thermal pressure, P_he = P_th. We derive P_th and P_he from X-ray and gravitational lensing observations of A1689 in order to test the validity of this assumption. Assuming spherical symmetry, we find that P_th/P_he ~0.6 within the core region of this cluster. We analyze a sample of massive clusters of galaxies drawn from high resolution cosmological simulations, and find, in accord with previous studies, that there is a significant contribution from non-thermal pressure in the core region of relaxed clusters. Our results suggest an alternate explanation for the mass discrepancy in the core region of A1689: the assumption of strict hydrostatic equilibrium is not valid, i.e., P_th not = P_he, in this region.
We report on several features present in the energy spectrum from an ultra low-noise germanium detector operated at 2,100 m.w.e. By implementing a new technique able to reject surface events, a number of cosmogenic peaks can be observed for the first time. We discuss several possible causes for an irreducible excess of bulk-like events below 3 keVee, including a dark matter candidate common to the DAMA/LIBRA annual modulation effect, the hint of a signal in CDMS, and phenomenological predictions. Improved constraints are placed on a cosmological origin for the DAMA/LIBRA effect.
SDSS J120602.09+514229.5 is a gravitational lens system formed by a group of galaxies at redshift z=0.422 lensing a bright background galaxy at redshift z=2.001. The main peculiarity of this system is the presence of a luminous satellite near the Einstein radius, that slightly deforms the giant arc. This makes SDSS J120602.09+514229.5 the ideal system to test our grid-based Bayesian lens modelling method, designed to detect galactic satellites independently from their mass-to-light ratio, and to measure the mass of this dwarf galaxy despite its high redshift. Thanks to the pixelized source and potential reconstruction technique of Vegetti and Koopmans 2009a we are able to detect the luminous satellite as a local positive surface density correction to the overall smooth potential. Assuming a truncated Pseudo-Jaffe density profile, the satellite has a mass M=(2.75+-0.04)10^10 M_sun inside its tidal radius of r_t=0.68". We determine for the satellite a luminosity of L_B=(1.6+-0.8)10^9 L_sun, leading to a total mass-to-light ratio within the tidal radius of (M/L)_B=(17.2+-8.5) M_sun/L_sun. The central galaxy has a sub-isothermal density profile as in general is expected for group members. From the SDSS spectrum we derive for the central galaxy a velocity dispersion of sigma_kinem=380+-60 km/s within the SDSS aperture of diameter 3". The logarithmic density slope of gamma=1.7+0.25-0.30 (68% CL), derived from this measurement, is consistent within 1-sigma with the density slope of the dominant lens galaxy gamma~1.6, determined from the lens model. This paper shows how powerful pixelized lensing techniques are in detecting and constraining the properties of dwarf satellites at high redshift.
We build a mechanism of gravitational symmetry breaking (GSB) of a global U(1) symmetry based on the relaxation of the equivalence principle due to the mass variation of pseudo Nambu-Goldstone dark matter (DM) particles. This GSB process is described by the modified cosmological convergence mechanism of the Abnormally Weighting Energy (AWE) Hypothesis previously introduced by the authors. Several remarkable constraints from the Hubble diagram of far-away supernovae are derived, notably on the explicit and gravitational symmetry breaking energy scales of the model. We then briefly present some consequences on neutrino masses when this mechanism is applied to the particular case of the breaking of lepton number symmetry.
We generalize tensor-scalar theories of gravitation by the introduction of an abnormally weighting type of energy. This theory of tensor-scalar anomalous gravity is based on a relaxation of the weak equivalence principle that is now restricted to ordinary visible matter only. As a consequence, the convergence mechanism toward general relativity is modified and produces naturally cosmic acceleration as an inescapable gravitational feedback induced by the mass-variation of some invisible sector. The cosmological implications of this new theoretical framework are studied. This glimpses at an enticing new symmetry between the visible and invisible sectors, namely that the scalar charges of visible and invisible matter are exactly opposite.
We present parameter estimation forecasts for future 3D cosmic shear surveys for a class of Unified Dark Matter (UDM) models, where a single scalar field mimics both Dark Matter (DM) and Dark Energy (DE). These models have the advantage that they can describe the dynamics of the Universe with a single matter component providing an explanation for structure formation and cosmic acceleration. A crucial feature of the class of UDM models we use in this work is characterized by a parameter, c_inf (c=1), that is the value of the sound speed at late times and on which structure formation depends. We demonstrate that the properties of the DM-like behaviour of the scalar field can be estimated with very high precision with large-scale, fully 3D weak lensing surveys. We found that 3D weak lensing significantly constrains c_inf, and we find minimal errors 0.00013, for the fiducial value c_inf=0.0005, and 0.0004, for c_inf=0.05. Moreover, we compute the Bayesian evidence for UDM models over the LCDM model as a function of c_inf. For this purpose, we can consider the LCDM model as a UDM model with c_inf=0. We find an interesting maximum in the Bayes factor. This is due to the peculiar dynamics of UDM models. In fact, as the value of c_inf increases, its signature in the shear signal becomes more and more evident, until the sound speed is so high that the DM-like component of the scalar field can not cluster any more, thus the shear signal starts to be damped and the evidence decreases. Moreover, the expected evidence clearly shows that the survey data would unquestionably favour UDM models over the LCDM model, if c_inf>0.0001.
We test the asymmetry of the Cosmic Microwave Background anisotropy jointly in temperature and polarization. We study the hemispherical asymmetry, previously found only in the temperature field, with respect to the axis identified by Hansen et al. (2009). To this extent, we make use of the low resolution WMAP 5 year temperature and polarization Nside=16 maps and the optimal angular power spectrum estimator BolPol (Gruppuso et al. 2009). We consider two simple estimators for the power asymmetry and we compare our findings with Monte Carlo simulations which take into account the full noise covariance matrix. We confirm an excess of power in temperature angular power spectrum in the Southern hemisphere at a significant level, between 3 'sigma' and 4 'sigma' depending on the exact range of multipoles considered. We find a milder excess of power in the gradient (curl) component EE (BB) of polarized angular spectra in the Northern (Southern) hemisphere: this asymmetry is less significant level than the temperature one, and is evident only at low multipoles where the signal-to-noise ratio is larger. We do not find any significant hemispherical asymmetry in the cross-correlation power spectra, i.e. TE up to l=32, TB, EB for any multipoles. We also show that the Cold Spot found by Vielva et al. (2004) in the Southern Galactic hemisphere does not alter the significance of the hemispherical asymmetries on multipoles which can be probed by maps at resolution Nside=16. Although the origin of the hemispherical asymmetry in temperature remains unclear, the study of the polarization patter could add useful information on its explanation. We therefore forecast by Monte Carlo the Planck capabilities in probing polarization asymmetries.
We consider the non-supersymmetric models of chaotic (driven by a quadratic potential) and hybrid inflation, taking into account the minimal possible radiative corrections to the inflationary potential. We show that two simple coupling functions $f(\sg)$ (with a parameter $\cR$ involved) between the inflaton field $\sg$ and the Ricci scalar curvature ensure, for sub-Planckian values of the inflaton field, observationally acceptable values for the spectral index, $\ns$, and sufficient reheating after inflation. In the case of chaotic inflation we take $625<\cR<2.1\cdot10^7$ resulting to $\ns=0.955$ and tensor-to-scalar ratio $r=0.2$. In the case of hybrid inflation, the selected $f(\sg)$ assists us to obtain hilltop-type inflation. For values of the relevant mass parameter, m, less than $10^6 \TeV$ and the observationally central value of $\ns$, we find $\cR=(0.015-0.078)$ with the relevant coupling constants $\lambda=\kappa$ and the symmetry breaking scale, $M$, confined in the ranges $(2\cdot 10^{-7}-0.001)$ and $(1-16.8)\cdot10^{17} \GeV$, respectively.
The optical spectra of objects classified as QSOs in the SDSS DR6 are analyzed with the aim of determining the value of the fine structure constant in the past and then check for possible changes in the constant over cosmological timescales. The analysis is done by measuring the position of the fine structure lines of the [OIII] doublet (4959 and 5008) in QSO nebular emission. From the sample of QSOs at redshifts z < 0.8 a subsample was selected on the basis of the amplitude and width of the [OIII] lines. Two different method were used to determine the position of the lines of the [OIII] doublet, both giving similar results. Using a clean sample containing 1568 of such spectra, a value of Delta alpha /alpha=(+2.4 +-2.5) x 10^{-5} (in the range of redshifts 0-0.8) was determined. The use of a larger number of spectra allows a factor ~5 improvement on previous constraints based on the same method. On the whole, we find no evidence of changes in alpha on such cosmological timescales. The mean variation compatible with our results is 1/ <t> Delta alpha/alpha=(+0.7 +- 0.7) x 10^{-14} yr^{-1}. The analysis was extended to the [NeIII] and [SII] doublets, although their usefulness is limited due to the fact that all these doublets in QSOs tend to be fainter than [OIII], and that some of them are affected by systematics.
It has been argued that the effect of cosmological structure formation on the average expansion rate is negligible, because the linear approximation to the metric remains applicable in the regime of non-linear density perturbations. We discuss why the arguments based on the linear theory are not valid. We emphasise the difference between Newtonian gravity and the weak field, small velocity limit of general relativity in the cosmological setting.
A new scenario for the early era of the Universe is proposed. It corresponds to a smooth transition between a de Sitter-like phase and a radiation dominated era. We calculate the production of gravitons in this model.
(Abridged) We present results of Suzaku observations of the intracluster medium (ICM) in Abell 1689, combined with complementary analysis of the SDSS data and weak and strong lensing analysis of Subaru/Suprime-Cam and HST/ACS observations. Faint X-ray emission from the ICM around the virial radius is detected at 4.0 sigma significance. We find anisotropic gas temperature and entropy distributions in cluster outskirts correlated with large-scale structure of galaxies. The high temperature and entropy region in the northeastern (NE) outskirts is connected to an overdense filamentary structure. The outskirt regions in contact with low density void environments have low gas temperatures and entropies, deviating from hydrostatic equilibrium. These results suggest that thermalization of the ICM occurs faster along the filamentary structures than the void regions. A joint X-ray and lensing analysis shows that the hydrostatic mass is $\sim60-90%$ of spherical lensing one but comparable to a triaxial halo mass within errors in $0.6r_{2500} \simlt r \simlt 0.8r_{500}$, and that it is significantly biased as low as $\simlt60%$ within $0.4r_{2500}$, irrespective of mass models. The thermal gas pressure within $r_{500}$ is, at most, $\sim50$--60% of the total pressure to balance fully the gravity of the spherical lensing mass, and $\sim30$--40% around the virial radius. Although these constitute lower limits when one considers the possible halo triaxiality, these small relative contributions of thermal pressure would require additional sources of pressure, such as bulk and/or turbulent motions.
The epochs of origin of the first stars and galaxies, and subsequent growth of the first supermassive black holes, are among the most fundamental questions. Observations of the highest redshift Gamma-Ray Bursts (GRBs) will be the most compelling in situ probe of the history of initial star formation and consequent epoch of reionization if their prompt and precise detection can be followed immediately by sensitive near-IR imaging and spectroscopy. Blazars are the persistent analogs of GRBs and for the same reason (beaming) can be observed at highest redshifts where they might best trace the high accretion rate-driven jets and growth of supermassive black holes in galaxies. The proposed EXIST mission can uniquely probe these questions, and many others, given its unparalled combination of sensitivity and spatial-spectral-temporal coverage and resolution. Here we provide a brief summary of the mission design, key science objectives, mission plan and readiness for EXIST, as proposed to Astro2010.
The addition of Wide Field Camera 3 (WFC3) on the {\em Hubble Space Telescope} ({\em HST}) has led to a dramatic increase in our ability to study the $z>6$ Universe. The increase in the near-infrared (NIR) sensitivity of WFC3 over previous instruments has enabled us to reach apparent magnitudes approaching 29 (AB). This allows us to probe the rest-frame ultraviolet (UV) continuum, redshifted into the NIR at $z>6$. Taking advantage of the large optical depths at this redshift, resulting in the Lyman-$\alpha$ break, we use a combination of WFC3 imaging and pre-existing Advanced Camera for Surveys (ACS) imaging to search for $z\approx 7$ over 4 fields. Our analysis reveals 29 new $z\approx 7$ star forming galaxy candidates in addition to 16 pre-existing candidates already discovered in these fields. The improved statistics from our doubling of the robust sample of $z$-drop candidates confirms the previously observed evolution of the bright end of the luminosity function.
Tidal tails composed of stars should be unstable to the Jeans instability and this can cause them to look like beads on a string. The Jeans wavelength and tail diameter determine the wavelength and growth rate of the fastest growing unstable mode. Consequently the distance along the tail to the first clump and spacing between clumps can be used to estimate the mass density in the tail and its longitudinal velocity dispersion. Clumps in the tidal tails of the globular cluster Palomar 5 could be due to Jeans instability. We find that their spacing is consistent with the fastest growing mode if the velocity dispersion in the tail is similar to that in the cluster itself. While all tidal tails should exhibit gravitational instability, we find that clusters or galaxies with low concentration parameters are most likely to exhibit short wavelength rapidly growing Jeans modes in their tidal tails.
We reinterpret the proof of the Riemannian Penrose inequality by H. Bray. The modified argument turns out to have a nice feature so that the flow of Riemannian metrics appearing Bray's proof gives a Lorentzian metric of a spacetime. We also discuss a possible extension of our approach to charged black holes.
We investigate the Q-ball formation in the thermal logarithmic potential by means of the lattice simulation, and reconfirm qualitatively the relation between Q-ball charge and the amplitude of the Affleck-Dine field at the onset of its oscillation. We find time dependence of some properties of the Q ball, such as its size and the field value at its center. Since the thermal logarithmic potential decreases as the temperature falls down, the gravity-mediation potential will affect the properties of the Q ball. Even in the case when the gravity-mediation potential alone does not allow Q-ball solution, we find the transformation from the thich-wall type of the Q ball to the thin-wall type, contrary to the immediate destruction of the Q balls when the gravity-mediation potential becomes dominant at the center of the Q ball, mentioned in the literature.
We explore the cosmic evolution of a scalar field when the kinetic term is coupled to the Einstein tensor. When the kinetic term is coupled to one Einstein tensor, we find that in the absence of other matter sources or in the presence of pressureless matter, the scalar would behave as the pressureless matter. This enables the scalar field to be the candidate of cold dark matter. By taking into account of a scalar potential in this case, we find the scalar field may play the role of both dark matter and dark energy. For sufficiently small exponential potential parameter $\zeta$, the equation of state of the scalar is $w\simeq -1$ in the total history of the Universe. We also find that the equation of state for the scalar can cross the phantom divide. But due to the kinetic energy is always positive, the scalar field is stable to classically perturbations. On the other hand, if the kinetic term is coupled to many more Einstein tensors, we find the equation of state is always approximately equals to -1 regardless whether the potential is flat or not. Thus the scalar may also be the candidate of inflaton field.
We conjecture that quantum entanglement of matter and vacuum in the universe tend to increase with time, like entropy, and there is an effective force called quantum entanglement force associated with this tendency. It is also suggested that gravity and dark energy are types of the quantum entanglement force, similar to Verlinde's entropic force. If the entanglement entropy of the universe saturates the Bekenstein bound, this gives holographic dark energy with the equation of state consistent with current observational data. This connection between quantum information and gravity gives some new insights on the origin of gravity, dark energy, the holographic principle and arrow of time.
In a recent article R. T. Cahill claims that the cosmological model based on his "new physics of a dynamical 3-space" resolves the CMB-BBN Lithium-7 and Helium-4 abundance anomalies. In this note it is shown that this conclusion is wrong, resulting from a misunderstanding. In fact, primordial nucleosynthesis in this non-standard cosmological model exacerbates the LIthium-7 problem and creates new problems for primordial Helium-4 and Deuterium.
The prospects for future blazar surveys by next-generation very high energy (VHE) gamma-ray telescopes, such as Advanced Gamma-ray Imaging System (AGIS) and Cherenkov Telescope Array (CTA), are investigated using the latest model of blazar luminosity function and its evolution which is in good agreement with the flux and redshift distribution of observed blazars as well as the extragalactic gamma-ray background. We extend and improve the template of spectral energy distributions (SEDs) based on the blazar SED sequence paradigm, to make it reliable also in the VHE bands (above 100 GeV) by comparing with the existing VHE blazar data. Assuming the planned CTA sensitivities, a blind survey using a total survey time of ~100 hrs could detect ~3 VHE blazars, with larger expected numbers for wider/shallower surveys. We also discuss following-up of Fermi blazars. Detectability of VHE blazars in the plane of Fermi flux and redshift is presented, which would be useful for future survey planning. Prospects and strategies are discussed to constrain the extragalactic background light (EBL) by using the absorption feature of brightest blazar spectra, as well as cut-offs in the redshift distribution. We will be able to get useful constraints on EBL by VHE blazars at different redshifts ranging 0.3-1 TeV corresponding to z=0.10-0.36.
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