Interactions and Mergers of Cluster Galaxies

The high-density environment of galaxy clusters is ripe for collisional encounters of galaxies. While the large velocity dispersion of clusters was originally thought to preclude slow encounters, the infall of smaller groups into the cluster environment provides a mechanism for promoting slow encounters and even mergers within clusters. The dynamical and star-forming response of galaxies to a close encounter depends on both their internal structure and on the collisional encounter speed -- fast encounters tend to trigger modest, disk-wide responses in luminous spirals, while slow encounters are more able to drive instabilities that result in strong nuclear activity. While the combined effects of the cluster tidal field and ram pressure stripping make it difficult for individual cluster galaxies to participate in many merger-driven evolutionary scenarios, infalling groups represent a natural site for these evolutionary processes and may represent a "preprocessing" stage in the evolution of cluster galaxies. Meanwhile, the efficiency of tidal stripping also drives the formation of the diffuse intracluster light in galaxy clusters; deep imaging of clusters is beginning to reveal evidence for significant substructure in the intracluster light.