A radio perspective on galaxy evolution in the cluster environment

Using the recently-completed NRAO VLA Sky Survey (Condon et al. 1998) and new radio continuum observations, a comprehensive catalog of active galaxies in twenty nearby Abell clusters has been created. Basing the catalog on radio emission insures that both active galactic nuclei (AGN) as well as galaxies forming stars at rates comparable to the Milky Way are identified. The catalog also canvasses large physical areas within each cluster, from their cores out to radii where the local galaxy density is consistent with the field. Velocities determined from a rigorous program of follow-up optical spectroscopy and from public sources were used to finalize lists of cluster and non-cluster members. In total, 401 cluster members were confirmed, 159 radio galaxies were identified as foreground/background objects, and only 23 radio galaxies lack velocities. Consequently, a statistical database of active galaxies in a broad range of cluster environment has been established. This database was used to explore galaxy evolution in clusters, including studies within the identified sample and comparisons with clusters of galaxies at higher redshift. In summary, a picture develops wherein large-scale influences govern the evolution of galaxies in clusters. Outlying groups are accreted by clusters, and over relatively short timescales star formation in the group members has been quenched. In dramatic cases, entire clusters may merge and drive numerous outlying groups and field galaxies into the cluster environment during the process. Such structure formation processes were more common in the past, explaining the changes seen in clusters from high redshift up until the present. (Abstract shortened by UMI.)