Quantitative analysis of galaxy-galaxy lensing

Gravitational light deflection due to mass along the line-of-sight will distort the images of background sources. This effect has been used successfully to investigate the mass distribution of galaxy clusters. Although an individual galaxy is not massive enough to cause a detectable lensing distortion in the background population, this effect can be measured statistically for a population of galaxies, and a first detection was reported recently by Brainerd, Blandford and Smail (BBS). In this paper we explore a quantitative and efficient method to constrain the halo properties of distant galaxy populations through galaxy-galaxy lensing and show that the mean masses and sizes of halos can be estimated accurately, without excessive data requirements. Specifically, we propose a maximum-likelihood analysis which takes full account of the actual image ellipticities, positions and apparent magnitudes. We apply it to simulated observations, using the same model for the lensing galaxy population as in BBS, where the galaxy halos are described by isothermal spheres with velocity dispersion #sigma#, truncated at a radius s. Both parameters are assumed to scale with the luminosity of the galaxy. The best fitting values, #sigma#* and s*, corresponding to an L*-galaxy, are then determined with the maximum-likelihood analysis28 refs.Available from TIB Hannover: RR 4697(928) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Probing the thermal history of the intergalactic medium with Ly#alpha# absorption lines

The Doppler parameter distribution of Ly#alpha# absorption is calculated for a set of different reionization histories. The differences in temperature between different reionization histories are as large as a factor three to four depending on the spectrum of the ionizing sources and the redshift of helium reionization. These temperature differences result in observable differences in the Doppler parameter distribution. Best agreement with the observed Doppler parameter distribution between redshift two and four is found if hydrogen and helium are reionized simultaneously at or before redshift five with a quasar-like spectrum. (orig.)Available from TIB Hannover: RR 4697(1033) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The structure of cold dark matter halos

We use N-body simulations to investigate the structure of dark halos in the standard Cold Dark Matter cosmogony. Halos are excised from simulations of cosmologically representative regions and are resimulated individually at high resolution. We study objects with masses ranging from those of dwarf galaxy halos to those of rich galaxy clusters. The spherically averaged density profiles of all our halos can be fit over two decades in radius by scaling a simple 'universal' profile. The characteristic overdensity of a halo, or equivalently its concentration, correlates strongly with halo mass in a way which reflects the mass dependence of the epoch of halo formation. Halo profiles are approximately isothermal over a large range in radii, but are significantly shallower than r"-"2 near the center and steeper than r"-"2 near the virial radius. Matching the observed rotation curves of disk galaxies requires disk mass-to-light ratios to increase systematically with luminosity. Further, it suggests that the halos of bright galaxies depend only weakly on galaxy luminosity and have circular velocities significantly lower than the disk rotation speed. This may explain why luminosity and dynamics are uncorrelated in observed samples of binary galaxies and of satellite/spiral systems. For galaxy clusters, our halo models are consistent both with the presence of giant arcs and with the observed structure of the intracluster medium, and they suggest a simple explanation for the disparate estimates of cluster core radii found by previous authors. Our results also highlight two shortcomings of the CDM model. CDM halos are too concentrated to be consistent with the halo parameters inferred for dwarf irregulars, and the predicted abundance of galaxy halos is larger than the observed abundance of galaxies. The first problem may imply that the core structure of dwarf galaxies was altered by the galaxy formation process, the second that galaxies failed to form (or remain undetected) in many dark halos. (orig.)SIGLEAvailable from TIB Hannover: RR 4697(884) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Damped Ly#alpha# absorbers at high redshift - large disks or galactic building blocks?

We investigate the nature of the physical structures giving rise to damped Ly#alpha# absorption systems (DLAS) at high redshift. In particular, we examine the suggestion that rapidly rotating large disks are the only viable explanation for the characteristic observed asymmetric profiles of low ionization absorption lines. Using hydrodynamic simulations of galaxy formation in a cosmological context we demonstrate that irregular protogalactic clumps can reproduce the observed velocity width distribution and asymmetries of the absorption profiles equally well. The velocity broadening in the simulated clumps is due to a mixture of rotation, random motions, infall and merging. The observed velocity width correlates with the virial velocity of the dark matter halo of the forming protogalactic clump (#DELTA#v #approx# 0.6 v_v_i_r for the median values, with a large scatter of order a factor two between different lines-of-sight). The typical virial velocity of the halos required to give rise to the DLAS population is about 100 kms"-"1 and most standard hierarchical structure formation scenarios can easily account even for the largest observed velocity widths. We conclude that the evidence that DLAS at high redshift are related to large rapidly rotating disks with v_c_i_r_c >or#approx# 200 kms"-"1 is not compelling. (orig.)Available from TIB Hannover: RR 4697(1029) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

The effects of a photoionizing UV background on the formation of disk galaxies

We use high resolution N-body/gasdynamical simulations to investigate the effects of a photoionizing UV background on the assembly of disk galaxies in hierarchically clustering universes. We focus on the mass and rotational properties of gas that can cool to form centrifugally supported disks in dark matter halos of different mass. Photoheating can significantly reduce the amount of gas that can cool in galactic halos. Depending on the strength of the UV background field, the amount of cooled gas can be reduced by up to 50% in systems with circular speeds in the range 80-200 km s"-"1. The magnitude of the effect, however, is not enough to solve the ''overcooling'' problem that plagues hierarchical models of galaxy formation if the UV background is chosen to be consistent with estimates based on recent observations of QSO absorption systems. Photoionization has little effect on the collapse of gas at high redshift and affects preferentially gas that is accreted at late times. Since disks form inside-out, accreting higher angular momentum gas at later times, disks formed in the presence of a UV background have spins that are even smaller than those formed in simulations that do not include the effects of photoionization. This exacerbates the angular momentum problem that afflicts hierarchical models of disk formation. We conclude that photoionization cannot provide the heating mechanism required to reconcile hierarchically clustering models with observations. Energy feeback and enrichment processes from the formation and evolution of stars must therefore be indispensable ingredients for any successful model of the formation of disk galaxies. (orig.)Available from TIB Hannover: RR 4697(953) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

A universal density profile from hierarchical clustering

We use high-resolution N-body simulations to study the equilibrium density profiles of dark matter halos in hierarchically clustering universes. We find that all such profiles have the same shape, independent of halo mass, of initial density fluctuation spectrum, and of the values of the cosmological parameters. Spherically averaged equilibrium profiles are well fit over two decades in radius by a simple formula originally proposed to describe the structure of galaxy clusters in a cold dark matter universe. In any particular cosmology the two scale parameters of the fit, the halo mass and its characteristic density, are strongly correlated. Low-mass halos are significantly denser than more massive systems, a correlation which reflects the higher collapse redshift of small halos. The characteristic density of an equilibrium halo is proportional to the density of the universe at the time it was assembled. A suitable definition of this assembly time allows the same proportionality constant to be used for all the cosmologies that we have tested. We compare our results to previous work on halo density profiles and show that there is good agreement. We also provide a step-by-step analytic procedure, based on the Press-Schechter formalism, which allows accurate equilibrium profiles to be calculated as a function of mass in any hierarchical model. (orig.)25 refs.Available from TIB Hannover: RR 4697(997) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Non-LTE line formation for neutral oxygen Model atom and first results on A-type stars

Available from: http://www.mpa-garching.mpg.de / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman