Nonlinear Stochastic Biasing of Galaxies and Dark Halos in Cosmological Hydrodynamic Simulations

We perform an extensive analysis of nonlinear and stochastic biasing ofgalaxies and dark halos in spatially flat low-density CDM universe usingcosmological hydrodynamic simulations. We compare their biasing properties withthe predictions of an analytic halo biasing model. Dark halos in oursimulations exhibit reasonable agreement with the predictions only on scaleslarger than 10h^{-1}Mpc, and on smaller scales the volume exclusion effect ofhalos due to their finite size becomes substantial. Interestingly the biasingproperties of galaxies are better described by extrapolating the halo biasingmodel predictions. We also find the clear dependence of galaxy biasing on their formation epoch;the distribution of old populations of galaxies tightly correlates with theunderlying mass density field, while that of young populations is slightly morestochastic and anti-biased relative to dark matter. The amplitude of two-pointcorrelation function of old populations is about 3 times larger than that ofthe young populations. Furthermore, the old population of galaxies residewithin massive dark halos while the young galaxies are preferentially formed insmaller dark halos. Assuming that the observed early and late-type galaxiescorrespond to the simulated old and young populations of galaxies,respectively, all of these segregations of galaxies are consistent withobservational ones for the early and late-type of galaxies such as themorphology--density relation of galaxies.Comment: 28 pages, 14 figures, accepted for publication in ApJ, Abstract abridged. For preprint with higher-resolution PS files, see ftp://www.kusastro.kyoto-u.ac.jp/pub/kohji/ytjs2001