Connecting galaxy formation and galaxy clustering

Abstract

We study the environmental dependence of the formation history of dark matter haloes in a large dark matter simulation, the Millennium Run. Adopting a sensitive test of this dependence —— the marked correlation function —— reveals highly significant evidence that haloes of a given mass form earlier in denser regions. We explore the effect further using a new variant of this statistic, and confirm our results using some simpler tests made possible by the size and resolution of the simulation. We go on to study the effect of this environmental dependence on the galaxy population generated by a recent semi-analytic model run in the Millennium Run. We show that environmentally dependent halo formation imparts a small but cleanly detected change to the correlation function and void probability function of galaxies. We can model this change by applying a modulation based on local density to the halo occupation distribution of galaxies. We also note that having the correct placement scheme for galaxies within haloes is at least as important as correctly accounting for environmental effects. Two more dark matter simulations are run, and their outputs are appropriately relabelled and rescaled to represent different cosmologies. We generate consistent semi- analytic galaxy populations in these simulations, using two versions of each of three variants of our semi-analytic model. We compare the predictions for the galaxy clustering from these models to the projected two-point correlation function of the SDSS, obtaining a constraint on the amplitude of the fluctuations in the mass, σ(_8) = 0.96 士 0.05. We find that environmental effects do not significantly affect this estimate, but discuss other possible effects which might. We remark on how this result compares to other recent determinations of σ(_8).