Evidence for substructure in rich clusters of galaxies from radial-velocity measurements

Abstract

New radial-velocity measurements in 15 clusters of galaxies are used to look for substructure. With a new statistical test that looks for deviations of the local velocity mean and dispersion from the global values, it is found that most of the clusters in the sample show significant subclustering. In 30-40 percent of the cases, the subclusters contain a large fraction of the galaxies found in the main body of the cluster. This result is in good agreement with earlier work by Geller and Beers (1982) based on number counts. The large fraction of clusters with significant substructure, along with evidence based on radial-velocity dispersion profiles, suggest that many, perhaps most, rich clusters did not collapse in single, coherent events.