Environmental Effects on Evolution of Cluster Galaxies in a Λcdm Universe

We investigate environmental effects on evolution of cluster galaxies under hierarchical structure formation scenario using combination of a dissipationless N-body simulation and a semi-analytic model of galaxy formation. The N-body simulation enables us to calculate orbits of galaxies in simulated clusters. Therefore we can include stripping of cold gas from the galactic disks by ram pressure from an ICM in our model. In this paper we consider galaxy mergers, stripping of diffuse hot gas from galactic halos as they infall into larger virialized objects, and ram-pressure stripping of cold gas from galactic disks. We find that observed color and star formation gradients in galaxy clusters are explained by the hot gas stripping as previous work has shown. Since the hot gas stripping sufficiently suppress star formation in clusters, the ram-pressure stripping of cold gas hardly affects the global properties of the cluster galaxies. We also examine whether the ram-pressure stripping can resolve the problem that the simulated clusters contain too few galaxies with intermediate bulge-to-disk ratio under the hierarchical galaxy formation. We, however, find that the ram-pressure stripping gives little change in morphologies of the cluster galaxies. It suggests that the additional processes other than major mergers and ram-pressure stripping, which affect the morphological evolution of disk-dominated galaxies, should be considered to reproduce the observed intermediate population in clusters. Subject headings: galaxies: formation — galaxies: evolution — galaxies: clusters: general — galaxies: halos — galaxies: interactions