The time evolution of bias

Abstract

ABSTRACT We investigate the time evolution of bias of cosmic density fields. We perform numerical simulations of the evolution of the cosmic web for the conventional Λ cold dark matter (ΛCDM) model. The simulations cover a wide range of box sizes $L=256\rm{-}1024\,h^{-1}\, {\rm Mpc}$, and epochs from very early moments z = 30 to the present moment z = 0. We calculate spatial correlation functions of galaxies, ξ(r), using dark matter particles of the biased Lambda cold dark mattter (ΛCDM) simulation. We analyse how these functions describe biasing properties of the evolving cosmic web. We find that for all cosmic epochs the bias parameter, defined through the ratio of correlation functions of selected samples and matter, depends on two factors: the fraction of matter in voids and in the clustered population, and the luminosity (mass) of galaxy samples. Gravity cannot evacuate voids completely, thus there is always some unclustered matter in voids, and the bias parameter of galaxies is always greater than unity, over the whole range of evolution epochs. We find that for all cosmic epochs bias parameter values form regular sequences, depending on galaxy luminosity (particle density limit), and decreasing with time.