arXiv.org > astro-ph

Astrophysics

astro-ph new abstracts, Tue, 21 Feb 06 01:00:18 GMT
0602407 -- 0602441 received

astro-ph/0602407 [abs, ps, pdf, other] :

Title: The X-ray-to-Optical Properties of Optically-Selected Active Galaxies Over Wide Luminosity and Redshift Ranges
Authors: A. T. Steffen, I. Strateva, W. N. Brandt, D. M. Alexander, A. M. Koekemoer, B. D. Lehmer, D. P. Schneider, C. Vignali
Comments: 16 pages, 10 figures, emulateapj. Accepted for publication in AJ

We present partial-correlation analyses that examine the strengths of the relationships between L_UV, L_X, Alpha_OX, and redshift for optically-selected AGNs. We extend the work of Strateva et al. (2005), that analyzed optically-selected AGNs from the Sloan Digital Sky Survey (SDSS), by including 52 moderate-luminosity, optically-selected AGNs from the COMBO-17 survey with corresponding deep (~250 ks to 1 Ms) X-ray observations from the Extended Chandra Deep Field-South. The COMBO-17 survey extends ~3 magnitudes deeper than the SDSS and probes the moderate-luminosity AGNs that numerically dominate the AGN population in the Universe. We also include recently published observations of 19 high-redshift, optically-selected AGNs, and 46 luminous, low-redshift AGNs from the Bright Quasar Survey. The full sample used in our analysis consists of 333 AGNs, extending out to z~6, with 293 (88%) having X-ray detections. The sample spans five decades in UV luminosity and four decades in X-ray luminosity. We confirm that Alpha_OX is strongly anti-correlated with L_UV (13.6 sigma), the highest significance found for this relation to date, and find evidence suggesting that the slope of this relation may be dependent on L_UV. We find that no significant correlation exists between Alpha_OX and redshift (1.3 sigma), and constrain the maximum evolution of AGN UV-to-X-ray flux ratios to be less than 30% (1 sigma) out to z=5. Using our sample's high X-ray detection fraction, we also find a significant anti-correlation (3.0 sigma) between Alpha_OX and L_X . We make comparisons to earlier studies on this topic and discuss implications for X-ray vs. optical luminosity functions.

 

astro-ph/0602408 [abs, ps, pdf, other] :

Title: Star Clusters with Primordial Binaries: I. Dynamical Evolution of Isolated Models
Authors: D.C. Heggie, M. Trenti, P. Hut
Comments: 14 pages, 22 figures, MNRAS accepted

In order to interpret the results of complex realistic star cluster simulations, which rely on many simplifying approximations and assumptions, it is essential to study the behavior of even more idealized models, which can highlight the essential physical effects and are amenable to more exact methods. With this aim, we present the results of N-body calculations of the evolution of equal-mass models, starting with primordial binary fractions of 0 - 100 %, with values of N ranging from 256 to 16384. This allows us to extrapolate the main features of the evolution to systems comparable in particle number with globular clusters. In this range, we find that the steady-state `deuterium main sequence' is characterized by a ratio of the core radius to half-mass radius that follows qualitatively the analytical estimate by Vesperini & Chernoff (1994), although the N dependence is steeper than expected. Interestingly, for an initial binary fraction f greater than 10%, the binary heating in the core during the post collapse phase almost saturates (becoming nearly independent of f), and so little variation in the structural properties is observed. Thus, although we observe a significantly lower binary abundance in the core with respect to the Fokker-Planck simulations by Gao et al. (1991), this is of little dynamical consequence. At variance with the study of Gao et al. (1991), we see no sign of gravothermal oscillations before 150 halfmass relaxation times. At later times, however, oscillations become prominent. We demonstrate the gravothermal nature of these oscillations.

 

astro-ph/0602409 [abs, ps, pdf, other] :

Title: Star Clusters with Primordial Binaries: II. Dynamical Evolution of Models in a Tidal Field
Authors: M. Trenti, D.C. Heggie, P. Hut
Comments: 13 pages, 17 figures, MNRAS submitted

[abridged] We extend our analysis of the dynamical evolution of simple star cluster models, in order to provide comparison standards that will aid in interpreting the results of more complex realistic simulations. We augment our previous primordial-binary simulations by introducing a tidal field, and starting with King models of different central concentrations. We present the results of N-body calculations of the evolution of equal-mass models, starting with primordial binary fractions of 0 - 100 %, and N values from 512 to 16384. We also attempt to extrapolate some of our results to the larger number of particles that are necessary to model globular clusters. We characterize the steady-state `deuterium main sequence' phase in which primordial binaries are depleted in the core in the process of `gravitationally burning'. In this phase we find that the ratio of the core to half-mass radius, r_c/r_h, is similar to that measured for isolated systems. In addition to the generation of energy due to hardening and depletion of the primordial binary population, the overall evolution of the star clusters is driven by a competing process: the tidal disruption of the system. We find that the depletion of primordial binaries before tidal dissolution of the system is possible only if the initial number is below 0.05 N, in the case of a King model with W_0=7 and N=4096 (which is one of our longest living models). We compare our findings, obtained by means of direct N-body simulations but scaled, where possible, to larger N, with similar studies carried out by means of Monte Carlo methods.

 

astro-ph/0602410 [abs, ps, pdf, other] :

Title: Complex small-scale structure in the infrared extinction towards the Galactic Centre
Authors: Andrew J. Gosling, Katherine M. Blundell, Reba Bandyopadhyay
Comments: 5 pages, 3 figures, accepted for publication in ApJL

A high level of complex structure, or ``granularity'', has been observed in the distribution of infrared-obscuring material towards the Galactic Centre (GC), with a characteristic scale of 5arcsec - 15arcsec, corresponding to 0.2 - 0.6pc at a GC distance of 8.5kpc. This structure has been observed in ISAAC images which have a resolution of 0.6arcsec, significantly higher than that of previous studies of the GC.
We have discovered granularity throughout the GC survey region, which covers an area of 1.6deg x 0.8deg in longitude and latitude respectively (300pc x 120pc at 8.5kpc) centred on Sgr A*. This granularity is variable over the whole region, with some areas exhibiting highly structured extinction in one or more wavebands and other areas displaying no structure and a uniform stellar distribution in all wavebands. The granularity does not appear to correspond to longitude, latitude or radial distance from Sgr A*. We find that regions exhibiting high granularity are strongly associated with high stellar reddening.

 

astro-ph/0602411 [abs, ps, pdf, other] :

Title: Measurement of \Omega_m, \Omega_{\Lambda} from a blind analysis of Type Ia supernovae with CMAGIC: Using color information to verify the acceleration of the Universe
Authors: A. Conley, G. Goldhaber, L. Wang, G. Aldering, R. Amanullah, E. D. Commins, V. Fadeyev, G. Folatelli, G. Garavini, R. Gibbons, A. Goobar, D. E. Groom, I. Hook, D. A. Howell, A. G. Kim, R. A. Knop, M. Kowalski, N. Kuznetsova, C. Lidman, S. Nobili, P. E. Nugent, R. Pain, S. Perlmutter, E. Smith, A. L. Spadafora, V. Stanishev, M. Strovink, R. C. Thomas, W. M. Wood-Vasey (The Supernova Cosmology Project)
Comments: 53 pages, 15 figures, accepted for publication in The Astrophysical Journal. Also available at this http URL

We present measurements of \Omega_m and \Omega_{\Lambda} from a blind analysis of 21 high-redshift supernovae using a new technique (CMAGIC) for fitting the multi-color lightcurves of Type Ia supernovae, first introduced in Wang et al. (2003). CMAGIC takes advantage of the remarkably simple behavior of Type Ia supernovae on color-magnitude diagrams, and has several advantages over current techniques based on maximum magnitudes. Among these are a reduced sensitivity to host galaxy dust extinction, a shallower luminosity-width relation, and the relative simplicity of the fitting procedure. This allows us to provide a cross check of previous supernova cosmology results, despite the fact that current data sets were not observed in a manner optimized for CMAGIC. We describe the details of our novel blindness procedure, which is designed to prevent experimenter bias. The data are broadly consistent with the picture of an accelerating Universe, and agree with a flat Universe within 1.7\sigma, including systematics. We also compare the CMAGIC results directly with those of a maximum magnitude fit to the same SNe, finding that CMAGIC favors more acceleration at the 1.6\sigma level, including systematics and the correlation between the two measurements. A fit for w assuming a flat Universe yields a value which is consistent with a cosmological constant within 1.2\sigma.

 

astro-ph/0602412 [abs, ps, pdf, other] :

Title: The Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements
Authors: Misty C. Bentz (1), Bradley M. Peterson (1), Richard W. Pogge (1), Marianne Vestergaard (2), Christopher A. Onken (1,3) ((1) The Ohio State University, (2) Steward Observatory, (3) Herzberg Institute of Astrophysics)
Comments: 27 pages, 5 tables, 4 figures, accepted to ApJ; full resolution figures available at this http URL

We have obtained high resolution images of the central regions of 14 reverberation-mapped active galactic nuclei (AGN) using the Hubble Space Telescope Advanced Camera for Surveys High Resolution Camera to account for host-galaxy starlight contamination of measured AGN luminosities. We measure the host-galaxy starlight contribution to the continuum luminosity at 5100 A through the typical ground-based slit position and geometry used in the reverberation-mapping campaigns. We find that removing the starlight contribution results in a significant correction to the luminosity of each AGN, both for lower luminosity sources, as expected, but also for the higher luminosity sources such as the PG quasars. After accounting for the host galaxy starlight, we revisit the well-known broad-line region radius--luminosity relationship for nearby AGN. We find the power-law slope of the relationship for the H beta line to be 0.518 +/- 0.039, shallower than previously reported and consistent with the slope of 0.5 expected from the naive theoretical assumption that all AGN have, on average, the same ionizing spectrum and the same ionization parameter and gas density in the H beta line-emitting region.

 

astro-ph/0602413 [abs, ps, pdf, other] :

Title: Final Results from the BIMA CMB Anistropy Survey and Search for Signature of the SZ effect
Authors: K.S. Dawson, W.L. Holzapfel, J.E. Carlstrom, M. Joy, S.J. LaRoque
Comments: Submitted to ApJ, 17 pages

We report the final results of our study of the cosmic microwave background (CMB) with the BIMA array. Over 1000 hours of observation were dedicated to this project exploring CMB anisotropy on scales between 1' and 2' in eighteen 6.6' FWHM fields. In the analysis of the CMB power spectrum, the visibility data is divided into two bins corresponding to different angular scales. Modeling the observed excess power as a flat band of average multipole ell= 5237, we find Delta T_1^2=220_{-120}^{+140} muK^2 at 68% confidence and Delta T_1^2 >0 muK^2 with 94.7% confidence. In a second band with average multipole of ell = 8748, we find Delta T_2^2 consistent with zero, and an upper limit 880 muK^2 at 95% confidence. An extensive series of tests and supplemental observations with the VLA provide strong evidence against systematic errors or radio point sources being the source of the observed excess power. The dominant source of anisotropy on these scales is expected to arise from the Sunyaev-Zel'dovich (SZ) effect in a population of distant galaxy clusters. If the excess power is due to the SZ effect, we can place constraints on the normalization of the matter power spectrum sigma_8 = 1.03^{+0.20}_{-0.29} at 68% confidence. The distribution of pixel fluxes in the BIMA images are found to be consistent with simulated observations of the expected SZ background and rule out instrumental noise or radio sources as the source of the observed excess power with similar confidence to the detection of excess power. Follow-up optical observations to search for galaxy over-densities anti-correlated with flux in the BIMA images, as might be expected from the SZ effect, proved to be inconclusive.

 

astro-ph/0602414 [abs, ps, pdf, other] :

Title: Universe Evolution in a 5D Ricci-flat Cosmology
Authors: Chengwu Zhang, Hongya Liu, Lixin Xu, P. S. Wesson
Comments: 8 pages, 6 figures, ws-mpla.cls, to be published in MPLA

We use Wetterich's parameterization equation of state (EOS) of dark energy to a $5D$ Ricci-flat cosmological solution and we suppose the universe contains three major components: matter, radiation and dark energy. By using the relation between the scale factor and the redshift $z$, we show that the two arbitrary functions contained in the $5D$ solution could be solved out analytically in terms of the variable $z$. Thus the whole $5D$ solution could be constructed uniquely if the current values of the three density parameters $\Omega_{m0}$, ${\Omega_{r0}}$, $\Omega_{x0}$, the EOS $w_{0}$%, and the bending parameter $b$ contained in the EOS are all known. Furthermore, we find that all the evolutions of the mass density $\Omega_{m} $, the radiation density ${\Omega_{r}}$, the dark energy density $\Omega_{x}$, and the deceleration parameter $q$ depend on the bending parameter $b$ sensitively. Therefore it is deserved to study observational constraints on the bending parameter $b$.

 

astro-ph/0602415 [abs, ps, pdf, other] :

Title: Dark Energy from Brane-world Gravity
Authors: Roy Maartens (Portsmouth)
Comments: Based on a talk at the 3rd Aegean Summer School, Chios, September 2005

Recent observations provide strong evidence that the universe is accelerating. This confronts theory with a severe challenge. Explanations of the acceleration within the framework of general relativity are plagued by difficulties. General relativistic models require a "dark energy" field with effectively negative pressure. An alternative to dark energy is that gravity itself may behave differently from general relativity on the largest scales, in such a way as to produce acceleration. The alternative approach of modified gravity also faces severe difficulties, but does provide a new angle on the problem. This review considers an example of modified gravity, provided by brane-world models that self-accelerate at late times.

 

astro-ph/0602416 [abs, ps, pdf, other] :

Title: Metallicity Gradient And The Hybrid Formation Scenario For Early-Type Galaxies
Authors: Ricardo L. C. Ogando (1 and 2), Marcio A. G. Maia (2), Cristina Chiappini (3), Paulo S. S. Pellegrini (2), Ricardo P. Schiavon (4), Luiz N. da Costa (2) ((1) IF/UFRJ, (2) ON/MCT, (3) OAT/INAF, (4) DA/UVA)
Comments: 2 pages, 2 figures, contributed talk to the 11th Latin American Regional IAU meeting, Pucon, Chile 2005. To be published by Revista Mexicana de Astronomia y Astrofisica, Conference Series (rmaa.cls)

We present radial gradients of the Lick index Mg2 for 40 early-type galaxies. In plots of Grad(Mg2) versus mass indicators, such as log(sigma), the lower boundary of the points distribution may be populated by galaxies which predominantly formed by monolithic collapse. Galaxies showing flatter gradients at higher masses could represent objects which suffered important merging episodes. Thus, our results support a hybrid formation scenario. To remove possible age effects, we computed metallicity gradients (Grad[Z/H]) using Mg2 and Hbeta indices for an [alpha/Fe]=0.3 single stellar population model. The conclusions remain the same.

 

astro-ph/0602417 [abs, ps, pdf, other] :

Title: Corona of Magnetars
Authors: Andrei M. Beloborodov, Christopher Thompson
Comments: 72 pages, 14 figures, submitted to ApJ

We develop a theoretical model that explains the formation of hot coronae around strongly magnetized neutron stars -- magnetars. The starquakes of a magnetar shear its external magnetic field, which becomes non-potential and is threaded by an electric current. Once twisted, the magnetosphere cannot untwist immediately because of its self-induction. The induced electric field lifts particles from the stellar surface, accelerates them, and initiates avalanches of pair creation in the magnetosphere. The created plasma corona maintains the electric current demanded by curl(B) and regulates the self-induction e.m.f. by screening. This corona persists in dynamic equilibrium: it is continually lost to the stellar surface on the light-crossing time of 10^{-4} s and replenished with new particles. In essence, the twisted magnetosphere acts as an accelerator that converts the toroidal field energy to particle kinetic energy. Using a direct numerical experiment, we show that the corona self-organizes quickly (on a millisecond timescale) into a quasi-steady state, with voltage ~1 GeV along the magnetic lines. The heating rate of the corona is 10^{36}-10^{37} erg/s, in agreement with the observed persistent, high-energy output of magnetars. We deduce that a static twist that is suddenly implanted into the magnetosphere will decay on a timescale of 1-10 yrs. The particles accelerated in the corona impact the solid crust, knock out protons, and regulate the column density of the hydrostatic atmosphere of the star. The transition layer between the atmosphere and the corona is the likely source of the observed 100-keV emission from magnetars. The corona emits curvature radiation and can supply the observed IR-optical luminosity. (Abridged)

 

astro-ph/0602418 [abs, ps, pdf, other] :

Title: Expansion Parallax for the Compact Planetary Nebula M2-43
Authors: L. Guzman, Y. Gomez, L. F. Rodriguez
Comments: 5 pages, 2 figures

We present high quality radio continuum observations made with the Very Large Array (VLA) at 3.6 cm at two epochs toward the planetary nebula M2-43. The comparison of the two epochs, obtained with a time separation of 4.07 years, clearly shows the expansion of the planetary nebula with an angular rate of 0.61 +/- 0.09 mas/year.Assuming that the expansion velocity in the plane of the sky (determined from these measurements) and the expansion velocity in the line of sight (determined from optical spectroscopy available in the literature) are equal, we find a distance to the planetary nebula of 6.9 +/- 1.5 kpc. This is the largest distance for a planetary nebula measured up to now with this technique.

 

astro-ph/0602419 [abs, ps, pdf, other] :

Title: Stellar Explosions by Magnetic Towers
Authors: Dmitri A. Uzdensky, Andrew I. MacFadyen
Comments: 18 pages, 8 figures, submitted to ApJ

We propose a magnetic mechanism for the collimated explosion of a massive star relevant for GRBs, XRFs and asymmetric supernovae. We apply Lynden-Bell's magnetic tower scenario to the interior of a massive rotating star after the core has collapsed to form a black hole with an accretion disk or a millisecond magnetar acting as a central engine. We solve the force-free Grad-Shafranov equation to calculate the magnetic structure and growth of a tower embedded in a stellar environment. The pressure of the toroidal magnetic field, continuously generated by differential rotation of the central engine, drives a rapid expansion which becomes vertically collimated after lateral force balance with the surrounding gas pressure is reached. The collimation naturally occurs because hoop stress concentrates magnetic field toward the rotation axis and inhibits lateral expansion. This leads to the growth of a self-collimated magnetic tower. When embedded in a massive star, the supersonic expansion of the tower drives a strong bow shock behind which an over-pressured cocoon forms. The cocoon confines the tower by supplying collimating pressure and provides stabilization against disruption due to MHD instabilities. Because the tower consists of closed field lines starting and ending on the central engine, mixing of baryons from the cocoon into the tower is suppressed. The channel cleared by the growing tower is thus plausibly free of baryons and allows the escape of magnetic energy from the central engine through the star. While propagating down the stellar density gradient, the tower accelerates and becomes relativistic. During the expansion, fast collisionless reconnection becomes possible resulting in dissipation of magnetic energy which may be responsible for GRB prompt emission.

 

astro-ph/0602420 [abs, pdf] :

Title: The problem of what exists
Authors: P.C.W. Davies
Comments: 18 pages, one figure, conference paper

Popular multiverse models such as the one based on the string theory landscape require an underlying set of unexplained laws containing many specific features and highly restrictive prerequisites. I explore the consequences of relaxing some of these prerequisites with a view to discovering whether any of them might be justified anthropically. Examples considered include integer space dimensionality, the immutable, Platonic nature of the laws of physics and the no-go theorem for strong emergence. The problem of why some physical laws exist, but others which are seemingly possible do not, takes on a new complexion following this analysis, although it remains an unsolved problem in the absence of an additional criterion.

 

astro-ph/0602421 [abs, ps, pdf, other] :

Title: A Keck/DEIMOS Spectroscopy of Lyman Alpha Blobs at Redshift z=3.1
Authors: Yuichi Matsuda (1), Toru Yamada (2), Tomoki Hayashino (3), Ryosuke Yamauchi (3), Yuki Nakamura (3) ((1) Kyoto U., (2) NAO, Japan, (3) Tohoku U.)
Comments: Accepted for publication in ApJL

We present the results of an intermediate resolution (~2 angstrom) spectroscopy of a sample of 37 candidate Lyman alpha blobs and emitters at redshift z=3.1 using the DEIMOS spectrograph on the 10 m Keck telescope. The emission lines are detected for all the 37 objects and have variety in their line profiles. The Lyman alpha velocity widths (FWHM) of the 28 objects with higher quality spectra, measured by fitting a single Gaussian profile, are in the range of 150 - 1700 km/s and correlate with the Lyman alpha spatial extents. All the 12 Lyman alpha blobs (>16 arcsec^2) have large velocity widths of > 500 km/s. While there are several possible physical interpretations of the Lyman alpha velocity widths (motion of gravitationally-bound gas clouds, inflows, merging of clumps, or outflows from superwinds), the large velocity widths of the Lyman alpha blobs suggest that they are the sites of massive galaxy formation. If we assume gravitationally-bound gas clouds, the dynamical masses of the Lyman alpha blobs are estimated to be ~10^12 - 10^13 Msun. Even for the case of outflows, the outflow velocities are likely to be the same order of the rotation velocities as inferred from the observational evidence for local starburst galaxies.

 

astro-ph/0602422 [abs, ps, pdf, other] :

Title: The G-dwarf problem in the Galactic spheroid
Authors: R. Caimmi (Department of Astronomy, Padua University)
Comments: 43 pages, 13 figures

Using two alternative [O/H]-[Fe/H] dependences, the empirical oxygen abundance distribution (EGD) is deduced from two different samples of old objects, available in the literature. Under the assumption that each distribution (with the addition of a third one previously known) is typical for the corresponding subsystem, the EGD of the Galactic spheroid is determined weighting by mass. A fit to the data is provided by simple models of chemical evolution implying both homogeneous and inhomogeneous mixing, where star formation is inhibited during halo formation and enhanced during bulge formation, with respect to the disk. The theoretical oxygen abundance distribution (TGD) is first determined for the halo and the bulge separately, and then for the Galactic spheroid weighting by mass. Simple models implying inhomogeneous mixing are able to fit an empirical age-metallicity relation (EAMR) recently determined from a homogeneous sample of globular clusters, which shows a non monotonic trend characterized by large dispersion. Some possible implications related to the formation of Galactic spheroid and disk are also discussed.

 

astro-ph/0602423 [abs, ps, pdf, other] :

Title: Disentangling telluric lines in stellar spectra
Authors: Petr Hadrava
Comments: Accepted for publication in Astronomy and Astrophysics

The use of a method of spectra disentangling for telluric lines is explained in detail, with a particular emphasis on high-precision radial-velocity measurements for the search for extrasolar planets. New improvements to the method are introduced.

 

astro-ph/0602424 [abs, ps, pdf, other] :

Title: Catalog Extraction in SZ Cluster Surveys: a matched filter approach
Authors: J.-B. Melin, J. G. Bartlett, J. Delabrouille
Comments: 14 pages, 11 figures, submitted to A&A

We present a method based on matched multifrequency filters for extracting cluster catalogs from Sunyaev-Zel'dovich (SZ) surveys. We evaluate its performance in terms of completeness, contamination rate and photometric recovery for three representative types of SZ survey: a high resolution single frequency radio survey (AMI), a high resolution ground-based multiband survey (SPT), and the Planck all-sky survey. These surveys are not purely flux limited, and they loose completeness significantly before their point-source detection thresholds. Contamination remains relatively low at <5% (less than 30%) for a detection threshold set at S/N=5 (S/N=3). We identify photometric recovery as an important source of catalog uncertainty: dispersion in recovered flux from multiband surveys is larger than the intrinsic scatter in the Y-M relation predicted from hydrodynamical simulations, while photometry in the single frequency survey is seriously compromised by confusion with primary cosmic microwave background anisotropy. The latter effect implies that follow-up in other wavebands (e.g., 90 GHz, X-ray) of single frequency surveys will be required. Cluster morphology can cause a bias in the recovered Y-M relation, but has little effect on the scatter; the bias would be removed during calibration of the relation. Point source confusion only slightly decreases multiband survey completeness; single frequency survey completeness could be significantly reduced by radio point source confusion, but this remains highly uncertain because we do not know the radio counts at the relevant flux levels.

 

astro-ph/0602425 [abs, ps, pdf, other] :

Title: Light element synthesis in baryon isocurvature models
Authors: D. Lohiya, P. Kumar
Comments: 10 pages, 1 figure

The prejudice against baryon isocurvature models is primarily because of their inconsistency with early universe light element nucleosynthesis results. We propose that incipient low metallicity (Pop II) star forming regions can be expected to have environments conducive to Deuterium production by spallation, up to levels observed in the universe.

 

astro-ph/0602426 [abs, ps, pdf, other] :

Title: A CO, HCN and CI line survey of Ultra Luminous Infrared Galaxies
Authors: Padelis P. Papadopoulos
Comments: One page, IAU Symposium 231: "Astrochemistry Throughout the Universe: Recent Successes and Current Challenges"
Journal-ref: 2005IAUS..231..173P

Preliminary results from a sensitive survey of the CO J=1--0, 2--1,
3--2, 4--3, 6--5, HCN J=1--0, 3--2, 4--3, and CI J=1--0 lines of a sample of 30 Ultraluminous Infrared Galaxies (ULIRGs) are presented. These reveal a tandalizing picture of the physical conditions of the molecular gas in these extraordinary galaxies ($\rm L_{FIR}>10^{12} L_{\odot}$), with a diffuse phase dominating the low-J CO lines and a much denser and warmer phase dominating the CO 4-3 and 6-5 and all the HCN lines. The CI J=1--0 emission was found to be a robust tracer of their total molecular gas mass under a large range of physical conditions, a potent alternative to the much weaker emission from the $ ^{13}$CO isotopologue, and especially promising as an H$_2$ tracer for similar objects at high redshifts.

 

astro-ph/0602427 [abs, ps, pdf, other] :

Title: Weak gravitational lensing in brane-worlds
Authors: László Á. Gergely, Barbara Darázs
Comments: 7 pages, to appear in the Proceedings of the FIKUT4 Conference, Publications of the Astronomy Department of E\"{o}tv\"{o}s University, PADEU vol. 17 (2006)

We derive the deflection angle of light rays caused by a brane black hole with mass m and tidal charge q in the weak lensing approach, up to the second order in perturbation theory. We point out when the newly derived second order contributions become important.

 

astro-ph/0602428 [abs, ps, pdf, other] :

Title: On the cross-correlation of sub-mm sources and optically-selected galaxies
Authors: Chris Blake, Alexandra Pope, Douglas Scott, Bahram Mobasher
Comments: 10 pages, 10 figures, accepted for publication by MNRAS

Bright sub-mm galaxies are expected to arise in massive highly-biased haloes, and hence exhibit strong clustering. We argue that a valuable tool for measuring these clustering properties is the cross-correlation of sub-mm galaxies with faint optically-selected sources. We analyze populations of SCUBA-detected and optical galaxies in the GOODS-N survey area. Using optical/IR photometric-redshift information, we search for correlations induced by two separate effects: (1) cosmic magnification of background sub-mm sources by foreground dark matter haloes traced by optical galaxies at lower redshifts; and (2) galaxy clustering due to sub-mm and optical sources tracing the same population of haloes where their redshift distributions overlap. Regarding cosmic magnification, we find no detectable correlation. Our null result is consistent with a theoretical model for the cosmic magnification, and we show that a dramatic increase in the number of sub-mm sources will be required to measure the effect reliably. Regarding clustering, we find evidence at the 3.5-sigma level for a cross-correlation between sub-mm and optical galaxies analyzed in identical photometric redshift slices. The data hint that the sub-mm sources have an enhanced bias parameter compared to the optically-selected population (with a significance of 2-sigma). The next generation of deep sub-mm surveys can potentially perform an accurate measurement of each of these cross-correlations, adding a new set of diagnostics for understanding the development of massive structure in the Universe.

 

astro-ph/0602429 [abs, ps, pdf, other] :

Title: Dynamical stability of collisionless stellar systems and barotropic stars: the nonlinear Antonov first law
Authors: P.H. Chavanis

We complete previous investigations on the dynamical stability of barotropic stars and collisionless stellar systems. A barotropic star that minimizes the energy functional at fixed mass is a nonlinearly dynamically stable stationary solution of the Euler-Poisson system. Formally, this minimization problem is similar to a condition of ``canonical stability'' in thermodynamics. A stellar system that maximizes an H-function at fixed mass and energy is a nonlinearly dynamically stable stationary solution of the Vlasov-Poisson system. Formally, this maximization problem is similar to a condition of ``microcanonical stability'' in thermodynamics. Using a thermodynamical analogy, we provide a derivation and an interpretation of the nonlinear Antonov first law in terms of ``ensembles inequivalence'': a spherical stellar system with f=f(epsilon) and f'(epsilon)<0 is nonlinearly dynamically stable with respect to the Vlasov-Poisson system if the corresponding barotropic star with the same equilibrium density distribution is nonlinearly dynamically stable with respect to the Euler-Poisson system. This is similar to the fact that ``canonical stability implies microcanonical stability'' in thermodynamics. The converse is wrong in case of ``ensembles inequivalence'' which is generic for systems with long-range interactions like gravity. We show that criteria of nonlinear dynamical stability can be obtained very simply from purely graphical constructions by using the method of series of equilibria and the turning point argument of Poincare, as in thermodynamics.

 

astro-ph/0602430 [abs, ps, pdf, other] :

Title: Can sterile neutrinos be the dark matter?
Authors: Uros Seljak (ICTP, Princeton), Alexey Makarov (Princeton), Patrick McDonald (CITA), Hy Trac (Princeton)
Comments: 4 pages, 2 figures, submitted to PRL

We use the Ly-alpha forest power spectrum measured by the SDSS and high-resolution spectroscopy observations in combination with cosmic microwave background and galaxy clustering constraints to place limits on a sterile neutrino as a dark matter candidate in the warm dark matter (WDM) scenario. Such a neutrino would be created in the early universe through mixing with an active neutrino and would suppress structure on scales smaller than its free streaming scale. We ran a series of high-resolution hydrodynamic simulations with varying neutrino mass to describe the effect of a sterile neutrino on the Ly-alpha forest power spectrum. We find that the mass limit is m_s >14 keV at 95% c.l. (10keV at 99.9%), which is nearly an order of magnitude tighter constraint than previously published limits and is above the upper limit allowed by X-ray constraints, excluding this candidate as dark matter in this model. The corresponding limit for a neutrino that decoupled early while in thermal equilibrium is 2.5keV (95% c.l.).

 

astro-ph/0602431 [abs, ps, pdf, other] :

Title: On the Large-Scale Structure of the Universe as given by the Voronoi Diagrams
Authors: Zaninetti Lorenzo
Comments: 14 pages, 9 figures. Chinese Journal of Astronomy and Astrophysics to be published

The size distributions of 2D and 3D Voronoi cells and of cells of $V_p(2,3)$,--2D cut of 3D Voronoi diagram--are explored, with the single-parameter (re-scaled) gamma distribution playing a central role in the analytical fitting. Observational evidence for a cellular universe is briefly reviewed. A simulated $V_p(2,3)$ map with galaxies lying on the cell boundaries is constructed to compare, as regards general appearance, with the observed CFA map of galaxies and voids, the parameters of the simulation being so chosen as to reproduce the largest observed void size.

 

astro-ph/0602432 [abs, ps, pdf, other] :

Title: Large amplitude oscillation of a polar crown filament in the pre-eruption phase
Authors: Hiroaki Isobe (1,2), Durgesh Tripathi (2) ((1) Univ. Cambridge, (2) Univ. Tokyo)
Comments: 4 pages, 3 figures, accepted for publication in A&A

We report observation of a large-amplitude filament oscillation followed by an eruption. This is used to probe the pre-eruption condition and the trigger mechanism of solar eruptions. We used the EUV images from the Extreme-Ultraviolet Imaging Telescope on board SOHO satellite and the H-alpha images from the Flare Monitoring Telescope at Hida Observatory. The observed event is a polar crown filament that erupted on 15 Oct. 2002. The filament clearly exhibited oscillatory motion in the slow-rising, pre-eruption phase. The amplitude of the oscillation was larger than 20 km/s, and the motion was predominantly horizontal. The period was about 2 hours and seemed to increase during the oscillation, indicating weakening of restoring force. These results strongly indicate that, even in the slow-rise phase before the eruption, the filament retained equilibrium and behaved as an oscillator, and the equilibrium is stable to nonlinear perturbation. Moreover, the transition from such nonlinear stability to either instabilities or a loss of equilibrium that leads to the eruption occurred in the Alfven time scale. This suggests that the onset of the eruption was triggered by a fast magnetic reconnection that stabilized the pre-eruption magnetic configuration, rather than by the slow shearing motion at the photosphere.

 

astro-ph/0602433 [abs, pdf] :

Title: Issues in Gravitational Clustering and Cosmology
Authors: Jatush V. Sheth
Comments: PhD thesis, 194 pages

Observations reveal that on large scales the universe is spanned by a percolating network of superclusters interspersed with large and almost empty regions -- voids. This thesis reports the construction of a sophisticated computational algorithm called SURFGEN which is used to determine the morphological and topological properties of the cosmic web. SURFGEN generates a triangulated surface from a discrete data set representing (say) the distribution of galaxies in real (or redshift) space. SURFGEN then determines, for this surface, the Minkowski functionals which provide us with an excellent diagnostic of three dimensional shapes of clusters, superclusters and voids. SURFGEN is applied to N-body simulations, mock galaxy catalogues as well as to galaxies in the Las Campanas Redshift Survey. In all cases the morphology of individual superclusters and voids is determined and the connectedness of the cosmic web is assessed by means of the Genus curve and percolation analysis. We demonstrate that SURFGEN can successfully discriminate between rival models of structure formation and is a very useful tool with which to quantify the geometrical and topological properties of large scale structure.

 

astro-ph/0602434 [abs, ps, pdf, other] :

Title: Systematics in the X-ray Cluster Mass Estimators
Authors: E. Rasia, S. Ettori, L. Moscardini, P. Mazzotta, S. Borgani, K. Dolag, G. Tormen, L.M. Cheng, A. Diaferio
Comments: submitted to MNRAS, 12 pages

We examine the systematics affecting the X-ray mass estimators applied to a set of five simulated galaxy clusters. They have been processed through the X-ray Map Simulator, X-MAS, to provide Chandra-like long exposures that are analyzed to reconstruct the gas temperature, density, and mass profiles used as input. We find that at R_2500 the mass profile obtained via a direct application of the hydrostatic equilibrium equation is consistent within 1 sigma with the actual mass; although we notice this estimator shows high statistical errors due to high level of Chandra background. Instead, the poorness of the beta-model in describing the gas density profile makes the evaluated masses to be underestimated by \sim 40 per cent with respect to the true mass, both with an isothermal and a polytropic temperature profile. We also test ways to recover the mass by adopting an analytic mass model, such as those proposed by Navarro et al. (1997) and Rasia et al. (2004), and fitting the temperature profile expected from the hydrostatic equilibrium equation to the observed one. These methods and the one of the hydrostatic equilibrium equation provide a more robust mass estimation than the ones based on the beta-model. In the present work the main limitation for a precise mass reconstruction is to ascribe to the relatively high level of the background chosen to reproduce the Chandra one. After artificially reducing it by a factor of 100, we find that the estimated mass significantly underestimates the true mass profiles. This is manly due (i) to the neglected contribution of the gas bulk motions to the total energy budget and (ii) to the bias towards lower values of the X-ray temperature measurements because of the complex thermal structure of the emitting plasma.

 

astro-ph/0602435 [abs, pdf] :

Title: Discovery of Very High Energy gamma - ray emission from the extreme BL Lac object H2356-309 with H.E.S.S
Authors: S. Pita (APC), W. Benbow, L. Costamante, A. Djannati-Ataï (APC), D. Horns, M. Ouchrif (LPNHE), M. Tluczykont (LLR), the HESS Collaboration
Comments: To appear on proceeding of 29th International Cosmic Ray Conference (ICRC 2005)

The understanding of acceleration mechanisms in active galactic nuclei (AGN) jets and the measurement of the extragalactic-background-light (EBL) density are closely linked and require the detection of a large sample of very-high-energy (VHE) emitting extragalactic objects at varying redshifts. We report here on the discovery with the H.E.S.S. (High Energy Stereoscopic System) atmospheric-Cherenkov telescopes of the VHE Gamma-ray emission from H2356 - 309, an extreme BL Lac object located at a redshift of 0.165. The observations of this object, which was previously proposed as a southern-hemisphere VHE candidate source, were performed between June and December 2004. The total exposure is 38.9 hours live time, after data quality selection, which yields the detection of a signal at the level of 9.0$\sigma$ (standard deviations) .

 

astro-ph/0602436 [abs, pdf] :

Title: Multi-resolution analysis of the H.E.S.S. Galactic Survey Sources and Search for Counterparts in CO and HI data
Authors: A. Lemiere (APC), R. Terrier (APC), A. Djannati-Ataï (APC) (the HESS Collaboration)
Comments: To appear in proceeding 29th International Cosmic Ray Conference (ICRC 2005)

From May to July 2004, the central radian of the Galactic Plane was scanned by the H.E.S.S. (High Energy Stereoscopic System) telescopes at energies above 200 GeV. This survey was performed from -$3^o$ to +$3^o$ in latitude, for a total of 230 hours, revealing eight new VHE sources at a significance level greater than 6$\sigma$(standard deviations). We present a multi-resolution analysis of these sources based on a continuous wavelet transformation (CWT). Using CO and HI data, we investigate the possible associations of the potential counterparts proposed in [1], with sites of enhanced interstellar matter density.

 

astro-ph/0602437 [abs, ps, pdf, other] :

Title: A powerful hydrodynamic booster for relativistic jets
Authors: Miguel A. Aloy (1,2), Luciano Rezzolla (3,4,5) ((1) Max-Planck-Institut fuer Astrophysik; (2) Departamento de Astronomia y Astrofisica, Universidad de Valencia; (3) Max-Planck-Institut fuer Gravitationsphysik; (4) SISSA, International School for Advanced Studies and INFN; (5) Department of Physics and Astronomy, Louisiana State University)
Comments: 4 pages, 4 figures (1 in color). ApJ Letters accepted

Velocities close to the speed of light are a robust observational property of the jets observed in microquasars and AGNs, and are expected to be behind much of the phenomenology of GRBs. Yet, the mechanism boosting relativistic jets to such large Lorentz factors is still essentially unknown. Building on recent general-relativistic, multidimensional simulations of progenitors of short GRBs, we discuss a new effect in relativistic hydrodynamics which can act as an efficient booster in jets. This effect is purely hydrodynamical and occurs when large velocities tangential to a discontinuity are present in the flow, yielding Lorentz factors $\Gamma \sim 10^2-10^3$ or larger in flows with moderate initial Lorentz factors. Although without a Newtonian counterpart, this effect can be explained easily through the most elementary hydrodynamical flow: i.e., a relativistic Riemann problem.

 

astro-ph/0602438 [abs, ps, pdf, other] :

Title: Solar and stellar system tests of the cosmological constant
Authors: M. Sereno (Univ. Zurich), Ph. Jetzer (Univ. Zurich)
Comments: 4 pages; this is a preprint of an article accepted for publication in Physical Review D

Some tests of gravity theories - periastron shift, geodetic precession, change in mean motion and gravitational redshift - are applied in solar and stellar systems to constrain the cosmological constant. We thus consider a length scale range from 10^8 to 10^{15} km. Best bounds from the solar system come from perihelion advance and change in mean motion of Earth and Mars, Lambda < 10^{-36} km^{-2}. Such a limit falls very short to estimates from observational cosmology analyses but a future experiment performing radio ranging observations of outer planets could improve it by four orders of magnitude. Beyond the solar system, together with future measurements of periastron advance in wide binary pulsars, gravitational redshift of white dwarfs can provide bounds competitive with Mars data.

 

astro-ph/0602439 [abs, ps, pdf, other] :

Title: Time-Dependent Synchrotron and Compton Spectra from Jets of Microquasars
Authors: Swati Gupta, Markus Boettcher (Ohio Univ.), Charles D. Dermer (NRL)
Comments: Accepted for publication in ApJ; 37 manuscript pages, including 10 eps figures; uses AASTeX macros

Jet models for the high-energy emission of Galactic X-ray binary sources have regained significant interest with detailed spectral and timing studies of the X-ray emission from microquasars, the recent detection by the HESS collaboration of very-high-energy gamma-rays from the microquasar LS~5039, and the earlier suggestion of jet models for ultraluminous X-ray sources observed in many nearby galaxies. Here we study the synchrotron and Compton signatures of time-dependent electron injection and acceleration, adiabatic and radiative cooling, and different jet geometries in the jets of Galactic microquasars. Synchrotron, synchrotron-self-Compton, and external-Compton radiation processes with soft photons provided by the companion star and the accretion disk are treated. An analytical solution is presented to the electron kinetic equation for general power-law geometries of the jets for Compton scattering in the Thomson regime. We pay particular attention to predictions concerning the rapid flux and spectral variability signatures expected in a variety of scenarios, making specific predictions concerning possible spectral hysteresis, similar to what has been observed in several TeV blazars. Such predictions should be testable with dedicated monitoring observations of Galactic microquasars and ultraluminous X-ray sources using Chandra and/or XMM-Newton.

 

astro-ph/0602440 [abs, ps, pdf, other] :

Title: Annihilating dark matter and the galactic positron excess
Authors: Irit Maor
Comments: To appear in the proceedings of the INFN Eloisatron Project 44th Workshop On QCD At Cosmic Energies: The Highest Energy Cosmic Rays And QCD, 29 Aug - 5 Sep 2004, Erice, Italy

The possibility that the Galactic dark matter is composed of neutralinos that are just above half the $Z^o$ mass is examined, in the context of the Galactic positron excess. In particular, we check if the anomalous bump in the cosmic ray positron to electron ratio at $10~GeV$ can be explained with the ``decay'' of virtual $Z^o$ bosons produced when the neutralinos annihilate. We find that the low energy behaviour of our prediction fits well the existing data. Assuming the neutralinos annihilate primarily in the distant density concentration in the Galaxy and allowing combination of older, diffused positrons with young free-streaming ones, produces a fit which is not satisfactory on its own but is significantly better than the one obtained with homogeneous injection.

 

astro-ph/0602441 [abs, ps, pdf, other] :

Title: Spherical collapse with dark energy
Authors: Irit Maor
Comments: To appear in the proceedings of the ``Peyresq Physics 10" Workshop, 19 - 24 June 2005, Peyresq, France

I discuss the work of Maor and Lahav [1], in which the inclusion of dark energy into the spherical collapse formalism is reviewed. Adopting a phenomenological approach, I consider the consequences of - a) allowing the dark energy to cluster, and, b) including the dark energy in the virialization process. Both of these issues affect the final state of the system in a fundamental way. The results suggest a potentially differentiating signature between a true cosmological constant and a dynamic form of dark energy. This signature is unique in the sense that it does not depend on a measurement of the value of the equation of state of dark energy.