Collisional Evolution of Galaxy Clusters and the Growth of Common Halos

Abstract

We investigated the dynamical evolution of clusters of galaxies in virial equilibrium using Fokker-Planck models and self-consistent N-body models. In particular, we focused on the growth of a common halo, which is a cluster-wide halo formed by matter stripped from galaxies, and the development of a central density cusp. The Fokker-Planck models include the effects of two-body gravitational encounters both between galaxies and between galaxies and common halo particles. The effects of tidal mass stripping from the galaxies due to close galaxy-galaxy encounters and accompanying dissipation of the orbital kinetic energies of the galaxies were also taken into account in the Fokker-Planck models. We find that the results of the Fokker-Planck models are in excellent agreement with those of the N-body models regarding the growth of the common halo mass and the evolution of the cluster density profiles. In the central region of the cluster, a shallow density cusp, approximated by $\rho (r)\propto r^{-\alpha }$ ($\alpha \sim$ 1), develops. This shallow cusp results from the combined effects of two-body relaxation and tidal stripping. The cusp steepness, $\alpha$, weakly depends on the relative importance of the tidal stripping. When the effect of stripping is important, the central velocity dispersion decreases as the central density increases and, consequently, a shallow ($\alpha <2$) cusp is formed. In the limit of no stripping, usual gravothermal core collapse occurs, i.e. the central velocity dispersion increases as the central density increases with a steep ($\alpha >2$) cusp left. We conclude from our consideration of the origin of the cusp demonstrated here that shallow cusps should develop in real galaxy clusters.

revised, 21 pages, 16 figures, to appear in PASJ, 54, No.1 (2002)

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Cited by in corpus (2)

• The ESO Nearby Abell Cluster Survey. XII. The mass and mass-to-light-ratio profiles of rich clusters
• The radial profiles of the different mass components in galaxy clusters