Evolution of Clusters of Galaxies

Abstract We have investigated the evolution of clusters of galaxies using direct N-body simulations in which each galaxy is modeled by many particles. We found that the positive correlation between the masses and the distances of galaxies from the center of a cluster develops as the result of perturbations due to close encounters with other galaxies and the tidal field of the potential of the parent cluster. In the inner region of the cluster, the number density of galaxies is higher than that in the outer region, and the tidal field of the cluster is stronger compared to the outer region. Therefore, the masses of the galaxies in the inner region decrease more rapidly than do those in the outer region. The effect of mass segregation is not sufficiently strong to cancel out this tendency. We also found that the galaxies evolve so as to satisfy the Faber-Jackson relation. This implies that the Faber-Jackson relation is a result of the evolution of galaxies driven by interactions with other galaxies and the tidal field of the parent cluster. The coefficient of the L-σ relation changes as the cluster evolves. Therefore, the Faber-Jackson relation might not be a good standard candle to determine the distance to galaxies. Particles which escape from galaxies form a background halo, which is somewhat less centrally condensed than the distribution of the remaining galaxies. Both of heavier and lighter galaxies sink towards the center of the cluster, since they lose kinetic energy through inelastic collisions and the dynamical friction from halo particles. The density profile of a cluster including both galaxies and halo particles does not significantly change for more than 8 two-body relaxation times in the one-mass component model. This is caused by decreases in the masses of the galaxies, which slow down the speed of the two-body relaxation.