Non-linear Stochastic Galaxy Biasing in Cosmological Simulations

We study the biasing relation between dark-matter halos or galaxies and the underlying mass distribution, using cosmological $N$-body simulations in which galaxies are modelled via semi-analytic recipes. The nonlinear, stochastic biasing is quantified in terms of the mean biasing function and the scatter about it as a function of time, scale and object properties. The biasing of galaxies and halos shows a general similarity and a characteristic shape, with no galaxies in deep voids and a steep slope in moderately underdense regions. At \sim 8\hmpc, the nonlinearity is typically \lsim 10 percent and the stochasticity is a few tens of percent, corresponding to $\sim 30$ percent variations in the cosmological parameter $\beta=\Omega^{0.6}/b$. Biasing depends weakly on halo mass, galaxy luminosity, and scale. The time evolution is rapid, with the mean biasing larger by a factor of a few at $z\sim 3$ compared to $z=0$, and with a minimum for the nonlinearity and stochasticity at an intermediate redshift. Biasing today is a weak function of the cosmological model, reflecting the weak dependence on the power-spectrum shape, but the time evolution is more cosmology-dependent, relecting the effect of the growth rate. We provide predictions for the relative biasing of galaxies of different type and color, to be compared with upcoming large redshift surveys. Analytic models in which the number of objects is conserved underestimate the evolution of biasing, while models that explicitly account for merging provide a good description of the biasing of halos and its evolution, suggesting that merging is a crucial element in the evolution of biasing.Comment: 27 pages, 21 figures, submitted to MNRA

Evidence for a Positive Cosmological Constant from Flows of Galaxies and Distant Supernovae

Recent observations of high-redshift supernovae seem to suggest that the global geometry of the Universe may be affected by a `cosmological constant', which acts to accelerate the expansion rate with time. But these data by themselves still permit an open universe of low mass density and no cosmological constant. Here we derive an independent constraint on the lower bound to the mass density, based on deviations of galaxy velocities from a smooth universal expansion. This constraint rules out a low-density open universe with a vanishing cosmological constant, and together the two favour a nearly flat universe in which the contributions from mass density and the cosmological constant are comparable. This type of universe, however, seems to require a degree of fine tuning of the initial conditions that is in apparent conflict with `common wisdom'.Comment: 8 pages, 1 figure. Slightly revised version. Letter to Natur

The velocity function in the local environment from LCDM and LWDM constrained simulations

Using constrained simulations of the local Universe for generic cold dark matter and for 1keV warm dark matter, we investigate the difference in the abundance of dark matter halos in the local environment. We find that the mass function within 20 Mpc/h of the Local Group is ~2 times larger than the universal mass function in the 10^9-10^13 M_odot/h mass range. Imposing the field of view of the on-going HI blind survey ALFALFA in our simulations, we predict that the velocity function in the Virgo-direction region exceeds the universal velocity function by a factor of 3. Furthermore, employing a scheme to translate the halo velocity function into a galaxy velocity function, we compare the simulation results with a sample of galaxies from the early catalog release of ALFALFA. We find that our simulations are able to reproduce the velocity function in the 80-300 km/s velocity range, having a value ~10 times larger than the universal velocity function in the Virgo-direction region. In the low velocity regime, 35-80 km/s, the warm dark matter simulation reproduces the observed flattening of the velocity function. On the contrary, the simulation with cold dark matter predicts a steep rise in the velocity function towards lower velocities; for V_max=35 km/s, it forecasts ~10 times more sources than the ones observed. If confirmed by the complete ALFALFA survey, our results indicate a potential problem for the cold dark matter paradigm or for the conventional assumptions about energetic feedback in dwarf galaxies.Comment: 24 pages, 14 figures, 1 table, accepted for publication in Ap

The VIMOS VLT Deep Survey: Evolution of the non-linear galaxy bias up to z=1.5

We present the first measurements of the Probability Distribution Function (PDF) of galaxy fluctuations in the VIMOS-VLT Deep Survey (VVDS) cone, covering 0.4x0.4 deg between 0.4<z<1.5. The second moment of the PDF, i.e. the rms fluctuations of the galaxy density field, is with good approximation constant over the full redshift baseline investigated: we find that, in redshift space, sigma_8 for galaxies brighter than M=-20+5log h has a mean value of 0.94\pm0.07 in the redshift interval 0.7<z<1.5. The third moment, i.e. the skewness, increases with cosmic time: we find that the probability of having underdense regions is greater at z~0.7 than it was at z~1.5. By comparing the PDF of galaxy density contrasts with the theoretically predicted PDF of mass fluctuations we infer the redshift-, density-, and scale-dependence of the biasing function b(z, \delta, R) between galaxy and matter overdensities up to redshift z=1.5. Our results can be summarized as follows: i) the galaxy bias is an increasing function of redshift: evolution is marginal up to z~0.8 and more pronounced for z>0.8; ii) the formation of bright galaxies is inhibited below a characteristic mass-overdensity threshold whose amplitude increases with redshift and luminosity; iii) the biasing function is non linear in all the redshift bins investigated with non-linear effects of the order of a few to 10% on scales >5Mpc.Comment: 30 pages, 17 figs, Accepted by A&

The density field of the 10k zCOSMOS galaxies

We use the current sample of ~10,000 zCOSMOS spectra of sources selected with I(AB) < 22.5 to define the density field out to z~1, with much greater resolution in the radial dimension than has been possible with either photometric redshifts or weak lensing. We apply new algorithms that we have developed (ZADE) to incorporate objects not yet observed spectroscopically by modifying their photometric redshift probability distributions using the spectroscopic redshifts of nearby galaxies. This strategy allows us to probe a broader range of galaxy environments and reduce the Poisson noise in the density field. The reconstructed overdensity field of the 10k zCOSMOS galaxies consists of cluster-like patterns surrounded by void-like regions, extending up to z~1. Some of these structures are very large, spanning the ~50 Mpc/h transverse direction of the COSMOS field and extending up to Delta z~0.05 in redshift. We present the three dimensional overdensity maps and compare the reconstructed overdensity field to the independently identified virialised groups of galaxies and clusters detected in the visible and in X-rays. The distribution of the overdense structures is in general well traced by these virialised structures. A comparison of the large scale structures in the zCOSMOS data and in the mock catalogues reveals an excellent agreement between the fractions of the volume enclosed in structures of all sizes above a given overdensity between the data and the mocks in 0.2<z<1.Comment: 35 pages, 20 figures; submitted to ApJ; the high resolution pdf available at http://www.exp-astro.phys.ethz.ch/kovac/public/zCOSMO