Non-linear stochastic galaxy biasing in cosmological simulations

We study the biasing relation between dark matter haloes or galaxies and th e underlying mass distribution, using cosmological N-body simulations in wh ich galaxies are modelled via semi-analytic recipes. The non-linear, stocha stic biasing is quantified in terms of the mean biasing function and the sc atter about it as a function of time, scale and object properties. The bias ing of galaxies and haloes shows a general similarity and a characteristic shape, with no galaxies in deep voids and a steep slope in moderately under dense regions. At a comoving scale of similar to8 h(-1) Mpc, the non-linear ity in the biasing relation is typically less than or similar to 10 per cen t and the stochasticity is a few tens of per cent, corresponding to similar to 30 per cent variations in the cosmological parameter beta=Omega (0.6)/b . Biasing depends weakly on halo mass, galaxy luminosity, and scale. The ob served trend with luminosity is reproduced when dust extinction is included . The time evolution is rapid, with the mean biasing larger by a factor of a few at z similar to3 compared with z=0, and with a minimum for the non-li nearity and stochasticity at an intermediate redshift. Biasing today is a w eak function of the cosmological model, reflecting the weak dependence on t he power-spectrum shape, but the time evolution is more cosmology-dependent , reflecting the effect of the growth rate. We provide predictions for the relative biasing of galaxies of different type and colour, to be compared w ith upcoming large redshift surveys. Analytic models in which the number of objects is conserved underestimate the evolution of biasing, while models that explicitly account for merging provide a good description of the biasi ng of haloes and its evolution, suggesting that merging is a crucial elemen t in the evolution of biasing.