Merging History as a Function of Halo Environment

According to the hierarchical scenario, galaxies form via merging and accretion of small objects. Using N -body simulations, we study the frequency of merging events in the history of the halos. We find that at z ∼< 2 the merging rate of the overall halo population can be described by a simple power law (1+z). The main emphasis of the paper is on the effects of environment of halos at the present epoch (z = 0). We find that the halos located inside clusters have formed earlier (∆z ≈ 1) than isolated halos of the same mass. At low redshifts (z < 1), the merger rate of cluster halos is 3 times lower than that of isolated halos and 2 times lower than merger rate of halos that end up in groups by z = 0. At higher redshifts (z ∼ 1 − 4), progenitors of cluster and group halos have 3–5 times higher merger rates than isolated halos. We briefly discuss implications of our results for galaxy evolution in different environments.