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

We have used new radial-velocity measurements in 15 clusters of galaxies to look for substructure. With a new statistical test that looks for deviations of the local velocity mean and dispersion from the global values, we find that most of the clusters in our sample show significant subclustering. In 30%-40% 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 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.