Gerhard Börner

The dependence of clustering on galaxy properties

We use a sample of similar to 200 000 galaxies drawn from the Sloan Digital Sky Survey (SDSS) with 0.01 5 h(-1) Mpc). This large-scale clustering dependence is not seen for the parameters C or mu*. On small scales ( 1.5. In contrast, the dependence of the amplitude of wp(r(p)) on concentration on scales less than 1 h(-1) Mpc is strongest for disc-dominated galaxies with C < 2.6. This demonstrates that different processes are required to explain environmental trends in the structure and in the star formation history of galaxies.

Luminosity dependence of the spatial and velocity distributions of galaxies: semi-analytic models versus the sloan digital sky survey

By comparing semi-analytic galaxy catalogues with data from the Sloan Digital Sky Survey (SDSS), we show that current galaxy formation models reproduce qualitatively the dependence of galaxy clustering and pairwise peculiar velocities on luminosity, but some subtle discrepancies with the data still remain. The comparisons are carried out by constructing a large set of mock galaxy redshift surveys that have the same selection function as the SDSS Data Release Four (DR4). The mock surveys are based on two sets of semi-analytic catalogues presented by Croton et al. and Kang et al. From the mock catalogues, we measure the redshift-space projected two-point correlation function w(p)(r(p)), the power spectrum P(k) and the pairwise velocity dispersion (PVD) in Fourier space sigma(12)(k) and in configuration space sigma(12)(r(p)), for galaxies in different luminosity intervals. We then compare these theoretical predictions with the measurements derived from the SDSS DR4. On large scales and for galaxies brighter than L*, both sets of mock catalogues agree well with the data. For fainter galaxies, however, both models predict stronger clustering and higher pairwise velocities than observed. We demonstrate that this problem can be resolved if the fraction of faint satellite galaxies in massive haloes is reduced by similar to 30 per cent compared to the model predictions. A direct look into the model galaxy catalogues reveals that a significant fraction (15 per cent) of faint galaxies (-18 10(13) M-., and this population is predominantly red in colour. These faint red galaxies are responsible for the high PVD values of low-luminosity galaxies on small scales.

Spatial correlation functions and the pairwise peculiar velocity dispersion of galaxies in the Point Source Catalog Redshift Survey: Implications for the galaxy biasing in cold dark matter models

We report on the measurement of the two-point correlation function and the pairwise peculiar velocity of galaxies in the IRAS Point Source Catalog Redshift (PSCz) Survey. We compute these statistics first in redshift space and then obtain the projected functions that have simple relations to the real-space correlation functions on the basis of the method developed earlier in analyzing the Las Campanas Redshift Survey (LCRS) by Jing, Mo, & Borner. We find that the real space two-point correlation function can be fitted to a power law xi(r) = (r(o)/r)(gamma) with gamma = 1.69 and r(o) = 3.70 h(-1) Mpc. The pairwise peculiar velocity dispersion (PVD) sigma(12)(r(p)) is close to 400 km s(-1) at r(p) = 3 h(-1) Mpc and decreases to about 150 km s(-1) at r(p) approximate to 0.2 h(-1) Mpc. These values are significantly lower than those obtained from the LCRS. In order to understand the implications of those measurements on the galaxy biasing, we construct mock samples for a low-density spatially flat cold dark matter model (Omega(o) = 0.3, lambda(o) = 0.7, Gamma = 0.2, sigma(8) = 1), using a set of high-resolution N-body simulations in a box size of 100, 300, and 800 h(-1) Mpc. Applying a stronger cluster underweight biasing (proportional toM(-0.25)) than for the LCRS (proportional toM(-0.08)), we are able to reproduce these observational data, except for the strong decrease of the pairwise peculiar velocity at small separations. This is qualitatively ascribed to the different morphological mixture of galaxies in the two catalogs. Disk-dominated galaxy samples drawn from the theoretically constructed GIF catalog yield results rather similar to our mock samples with the simple cluster underweight biasing. We further apply the phenomenological biasing model in our N-body mock samples, which takes account of the density-morphology relation of galaxies in clusters. The model does not reduce the velocity dispersions of galaxies to the level measured in the PSCz data either. Thus, we conclude that the PVDs of the PSCz galaxies require a biasing model that substantially reduces the PVD on small scales relative to their spatial clustering.

An analytical model for the non-linear redshift-space power spectrum

We use N-body simulations to test the predictions of redshift distortion in the power spectrum given by the halo model in which the clustering of dark matter particles is considered as a result both of the clustering of dark haloes in space and of the distribution of dark matter particles in each individual dark halo. The predicted redshift distortion depends sensitively on several model parameters in a different way from the real-space power spectrum. An accurate model of the redshift distortion can be constructed if the following properties of the halo population are modelled accurately: the mass function of dark haloes, the velocity dispersion among dark haloes, and the non-linear nature of halo bias on small scales. The model can be readily applied to interpreting the clustering properties and velocity dispersion of different populations of galaxies once a cluster-weighted bias (or equivalently an halo occupation number model) is specified for the galaxies. Some non-trivial bias features observed from redshift surveys of optical galaxies and of IRAS galaxies relative to the standard low-density cold dark matter model can be easily explained in the cluster weighted bias model. The halo model further indicates that a linear bias can be a good approximation only for k less than or equal to 0.1 h Mpc(-1).

The dependence of the pairwise velocity dispersion on galaxy properties

We present measurements of the pairwise velocity dispersion (PVD) for different classes of galaxies in the Sloan Digital Sky Survey (SDSS). For a sample of about 200 000 galaxies with redshifts in the interval 0.01 < z < 0.3, and r-band magnitudes M-0.1r between - 16 and - 23, we study the dependence of the PVD on galaxy properties such as luminosity, stellar mass (M-*), colour (g - r), 4000-angstrom break strength (D-4000), concentration index (C), and stellar surface mass density (mu(*)). The luminosity dependence of the PVD is in good agreement with the results of Jing & Borner for the two-degree Field Galaxy Redshift Survey catalogue, once the photometric bandpass difference between the two surveys is taken into account. The value of sigma(12) measured at k = 1 h Mpc(-1) decreases as a function of increasing galaxy luminosity for galaxies fainter than L*, before increasing again for the most luminous galaxies in our sample. Each of the galaxy subsamples selected according to luminosity or stellar mass is divided into two further subsamples according to colour, D4000, C and mu(*). We find that galaxies with redder colours and higher D4000, C, and mu(*) values have larger PVDs on all scales and at all luminosities/stellar masses. The dependence of the PVD on the parameters related to recent star formation ( colour, D4000) is stronger than on the parameters related to galaxy structure (C, mu(*)), especially on small scales and for faint galaxies. The reddest galaxies and galaxies with high surface mass densities and intermediate concentrations have the highest pairwise peculiar velocities, that is, these move in the strongest gravitational fields. We conclude that the faint red population located in rich clusters is responsible for the high PVD values that are measured for low-luminosity galaxies on small scales.

Spatial and Dynamical Biases in Velocity Statistics of Galaxies

We present velocity statistics of galaxies and their biases inferred from the statistics of the underlying dark matter using a cosmological hydrodynamic simulation of galaxy formation in low-density and spatially flat (Ohm(0) = 0.3 and lambda(0) = 0.7) cold dark matter cosmogony. Our simulation is based on a particle-particle-particle-mesh ((PM)-M-3) N-body Poisson solver and smoothed particle hydrodynamics. Galaxies in our simulation are identified as clumps of cold and dense gas particles and classified as young and old galaxies according to their formation epochs. We find that the pairwise velocity dispersion (PVD) of all galaxies is significantly lower than that of the dark matter particles and that the PVD of the young galaxies is lower than that of the old types, and even of all galaxies together, especially at small separations. These results are in reasonable agreement with the recent measurements of PVDs in the Las Campanas redshift survey, the IRAS Point Source Catalogue Redshift Survey, and the Sloan Digital Sky Survey data. We also find that the low PVD of young galaxies is due to the effects of dynamical friction as well as the different spatial distribution, while the difference in the PVD between all galaxies and dark matter can be mostly ascribed to their different spatial distributions. We also consider the mean infall velocity and the POTENT density reconstruction that are often used to measure the cosmological parameters, and investigate the effects of spatial bias and dynamical friction. In our simulation, the mean infall velocity of young galaxies is significantly lower than that of all the galaxies or of the old galaxies, and the dynamical bias becomes important on scales less than 3 h(-1) Mpc. The mass density field reconstructed from the velocity field of young galaxies using the POTENT-style method suffers in accuracy from both the spatial bias and the dynamical friction on the smoothing scale of R-s = 8 h(-1) Mpc. On the other hand, in the case of R-s = 12 h(-1) Mpc, which is typically adopted in the actual POTENT analysis, the density reconstruction based on various tracers of galaxies is reasonably accurate. We also analyze the motions of central galaxies and the velocity dispersion of galaxies within halos and discuss their implications for the motion of cD galaxies and the determination of the mass of galaxy groups.

Rotating neutron star models and pulsars

Recently developed equations of state are used to construct fully relativistic models of rotating neutron stars. All of these stable neutron star models are bound. Consideration of binding energies suggests that very-low-mass neutron stars will not be formed in supernova explosions. Some criteria for equations of state are derived from a comparison with observations. Limits for the physical parameters of the Crab pulsar are obtained. In addition, we find that cosmic-ray protons cannot be produced by pulsars.

Stellar mass and color dependence of the three-point correlation function of galaxies in the local universe

The three-point correlation function (3PCF) for galaxies provides an opportunity to measure the non-Gaussianity generated from nonlinear structure formation and also probes information about galaxy formation and evolution that is generally not available from the two-point correlation function (2PCF). We measure the 3PCF of the Sloan Digital Sky Survey DR7 main sample galaxies in both redshift and projected spaces on scales up to 40 h –1 Mpc. We explore the dependence of the 3PCF on galaxy stellar mass and color in order to constrain the formation and evolution for galaxies of different properties. The study of the dependence on these properties also helps better constrain the relation between galaxy stellar mass and color and the properties of their hosting dark-matter halos. We focus on the study of the reduced 3PCF, Q, defined as the ratio between the 3PCF and the sum of the products of the 2PCFs. We find a very weak stellar-mass dependence of Q in both redshift and projected spaces. On small scales, more massive galaxies tend to have slightly higher amplitudes of Q. The shape dependence of Q is also weak on these small scales, regardless of stellar mass and color. The reduced 3PCF has a strong color dependence for the low-mass galaxies, while no significant dependence on color is found for the high-mass galaxies. Low-mass red galaxies have higher amplitudes and stronger shape dependence of the reduced 3PCF than the blue galaxies, implying that these low-mass red galaxies tend to populate filamentary structures. The linear galaxy bias model fails to interpret the color dependence of Q, emphasizing the importance of a nonvanishing quadratic bias parameter in the correct modeling of the galaxy color distribution.

The importance of gravitational self-field effects in binary systems with compact objects. II. The “static two-body problem” and the attraction law in a post-post-Newtonian approximation of general relativity

As a first step toward a proper treatment of compact objects in binary systems the attraction force of two massive bodies connected by a rod is calculated in a post-Newtonian expansion. Contrary to a calculation by Weyl und Bach we start without specializing the internal structure of the bodies. We consider general anisotropic pressures and do not require axial symmetry for the bodies. We calculate the attraction force first in a post-Newtonian approximation and then (in paper II) we shall be concerned with the post-post-Newtonian approximation. In both approximations we obtain Newtons attraction forceMS1MS2/R2 plus terms of order 1/R3 and higher, whereMS1,MS2 are the Schwarzschild masses of the bodies.

X-rays from neutron stars

Abstract The basic theoretical ideas in the models of regularly pulsating X-ray sources are discussed, and put in relation to the observations. The topics covered include physics of the magnetosphere of an accreting neutron star, hydrodynamics of the accretion column, physical processes close to the surface of the neutron star such as proton-electron collisions, photon-electron interactions.