Physical Bias of Galaxies From Large-Scale Hydrodynamic Simulations

We analyze a new large-scale ($100h^{-1}$Mpc) numerical simulation of the popular $\Lambda$CDM cosmological model including in our treatment dark matter, gas and galaxies formed on the basis of standard physical processes. The method, applied with a numerical resolution of $<200h^{-1}$kpc (which is small compared to nearest neighbor separation of galaxies except in clusters), attempts to determine where and when galaxies form. We then compare the galaxy distribution with the mass distribution to determine the "bias" defined as $b\equiv (\delta M/M)_{gal}/(\delta M/M)_{total}$. We find that (holding all variables constant except the quoted one) bias increases with decreasing galactic separation, with increasing galactic age or metallicity and with increasing redshift of observations. At $8h^{-1}$Mpc comoving bias (for bright galaxies) is 1.35 at $z=0$ reaching to 3.6 at $z=3$, both numbers being consistent with extant observations. We also find that $(10-20)h^{-1}$Mpc voids in the bright galaxy distributions are as observed (i.e., observed voids are not an argument against CDM-like models) and finally that the late type galaxies should show a colder Hubble flow than do the early type galaxies (a testable proposition). Surprisingly, little evolution is found in the amplitude of the galaxy-galaxy correlation function (as a function of {\it comoving} separation). Testing this prediction vs observations will allow a comparison between this work and that of Kauffmann et al which is based on a different physical modeling method.