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

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Abstract
We perform an extensive analysis of nonlinear and stochastic biasing of galaxies and dark halos in a spatially flat, low-density cold dark matter universe (Ω0 = 0.3, λ0 = 0.7, h = 0.7, and σ8 = 1) using cosmological hydrodynamic simulations. We identify galaxies by linking cold and dense gas particles that satisfy the Jeans criterion. We compare their biasing properties with the predictions of an analytic halo biasing model. Dark halos in our simulations exhibit reasonable agreement with the predictions only on scales larger than ~10 h-1 Mpc; on smaller scales, the volume exclusion effect of halos due to their finite size becomes substantial. Interestingly, the biasing properties of galaxies are better described by extrapolating the halo biasing model predictions. The clustering amplitudes of galaxies are almost independent of the redshift between z = 0 and 3, as reported in previous simulations. This in turn leads to a rapidly evolving biasing factor; we find that bcov 1 at redshift z 0 and bcov 3-4 at z = 3, where bcov is a biasing parameter defined from the linear regression of galaxy and dark matter density fields. Those values are consistent with the observed clustering of Lyman break galaxies. We also find the clear dependence of galaxy biasing on formation epoch; the distribution of old populations of galaxies tightly correlates with the underlying mass density field while that of young populations is slightly more stochastic and antibiased relative to dark matter. The amplitude of the two-point correlation function of old populations is about 3 times larger than that of young populations. Furthermore, the old population of galaxies resides within massive dark halos while the young galaxies are preferentially formed in smaller dark halos. Assuming that the observed early- and late-type galaxies correspond to the simulated old and young populations of galaxies, respectively, all of these segregations of galaxies are consistent with observational ones for early- and late-type galaxies such as, e.g., the morphology-density relation of galaxies.
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