Galaxy Dynamics in Clusters

Abstract

We use high-resolution simulations to study the formation anddistribution of galaxies within a cluster that forms hierarchically. Wefollow both dark matter and a gas component that is subject to thermalpressure, shocks, and radiative cooling. Galaxy formation is identifiedwith the dissipative collapse of the gas into cold, compact knots. Weexplore two extreme numerical representations of these galaxies duringsubsequent cluster evolution, one purely gaseous and the other purelystellar, and we find that the results are quite sensitive to thischoice. Simulations in which galaxies remain gaseous appear to sufferfrom an "overmerging" problem, but this problem is much less severe ifthe gas is allowed to turn into stars. We compare the kinematics of thegalaxy population in these two representations to the kinematics of darkhalos and of the underlying dark matter distribution. Galaxies in thestellar representation are positively biased (i.e., overrepresented inthe cluster) both by number and by mass fraction. Both representationspredict the galaxies to be more centrally concentrated than the darkmatter, whereas the dark halo population is more extended. A modestvelocity bias also exists in both representations, with the largesteffect, σ_gal_/σ_DM_ ~ 0.7, found for the more massive stargalaxies. Phase diagrams show that the galaxy population in the stellarcase is roughly in hydrostatic equilibrium. Virial mass estimators canunderestimate the true cluster mass by up to a factor of 5 because ofthese various bias effects. The discrepancy is largest if only the mostmassive galaxies are used, reflecting significant mass segregation. Abinding energy analysis suggests that this segregation is primarily aresult of dynamical friction. We discuss briefly the relevance of theseresults both to real clusters and to the general problem of simulatingthe formation and clustering of galaxies. The incorporation of arealistic star formation algorithm within future simulations is the keyto further progress.