Spatial and Velocity Biases

We give a summary of our recent studies of spatial and velocity biases of galaxy-size halos in cosmological models. Recent progress in numerical techniques made it possible to simulate halos in large volumes with a such accuracy that halos survive in dense environments of groups and clusters of galaxies. While the very central parts of clusters still may have been affected by numerical effects, halos in simulations look like real galaxies, and, thus, can be used to study the biases – differences between galaxies and the dark matter. For the standard LCDMmodel we find that the correlation function and the power spectrum of galaxy-size halos at z = 0 are antibiased on scales r < 5h−1Mpc and k ≈ (0.15− 30)hMpc−1. The biases depend on scale, redshift, and circular velocities of selected halos. Two processes seem to define the evolution of the spatial bias: (1) statistical bias (or peak bias) and merger bias (merging of galaxies, which happens preferentially in groups, reduces the number of galaxies, but does not affect the clustering of the dark matter). There are two kinds of velocity bias. The pair-wise velocity bias is b12 = 0.6−0.8 at r < 5h −1Mpc, z = 0. This bias mostly reflects the spatial bias and provides almost no information on the relative velocities of the galaxies and the dark matter. One-point velocity bias is a better measure of the velocities. Inside clusters the galaxies should move slightly faster (bv = 1.1− 1.3) than the dark matter. Qualitatively this result can be understood using the Jeans equations of the stellar dynamics.