Cluster Evolution

Abstract

Using a time-resolved approach, we compare the dynamical evolutions of groups and clusters of galaxies, as expected from direct hierarchical collapses and from aggregations and mergings. We find that aggregations dominate in bound environments, and in fact easily lead to a merging runaway that describes growth of cD galaxies in groups and erasure of substructures in forming clusters. In the ‘open’ field, instead, direct collapses set the dominant scales. Fast antievolution for the X-ray luminosities as indicated by the existing data may be understood in terms of the intracluster gas breaking the self-similarity associated with the dynamical evolution.