Environmental Effects on Evolution of Cluster Galaxies in a Λ‐dominated Cold Dark Matter Universe

Abstract

We investigate environmental effects on evolution of bright clustergalaxies (L>L*) in a Λ-dominated cold dark matteruniverse using a combination of dissipationless N-body simulations and asemianalytic galaxy formation model. The N-body simulations enable us tocalculate orbits of galaxies in simulated clusters. Therefore, we canincorporate stripping of cold gas from galactic disks by ram pressure(RP) from the intracluster medium into our model. In this paper we studyhow ram pressure stripping (RPS) and small starburst induced by a minormerger affect colors, star formation rates (SFRs), and morphologies ofcluster galaxies. These processes are new ingredients in our model andhave not been studied sufficiently. We find that the RPS is notimportant for colors and SFRs of galaxies in the cluster core if thestar formation timescale is properly chosen, because the star formationis sufficiently suppressed by consumption of the cold gas in the disks.Then observed color and SFR gradients can be reproduced without the RPS.The small starburst triggered by a minor merger hardly affects the SFRsand colors of the galaxies as well. We also examine whether these twoprocesses can resolve the known problem that the hierarchical clusteringmodels based on the major merger-driven bulge formation scenario predicttoo few galaxies of intermediate bulge-to-total luminosity ratio (B/T)in clusters. When the minor burst is taken into account, theintermediate B/T population is increased, and the observed morphologygradients in clusters are successfully reproduced. Without the minorburst, the RPS cannot increase the intermediate B/T population. On theother hand, when the minor burst is considered, the RPS also plays animportant role in formation of the intermediate B/T galaxies. We presentredshift evolution of morphological fractions predicted by our models.The predicted number ratios of the intermediate B/T galaxies to thebulge-dominated galaxies show nearly flat or slightly increasing trendswith increasing redshift. We conclude that these trends are inevitablewhen bulges are formed through mergers. We discuss whether our resultsconflict with observationally suggested NS0/NEevolution in clusters, which is a decreasing function of redshift.