A REANALYSIS OF SUBSTRUCTURE IN CLUSTERS OF GALAXIES AND THEIR SURROUNDINGS

Subclustering in rich clusters of galaxies is studied using a variety of statistical tests which analyze information available from both the projected distribution of galaxies and their radial velocities. New techniques are presented to evaluate more uniformly the frequency with which substructure occurs. Galaxy positional data are used to construct smoothed images of clusters from which structural parameters can be extracted via the methods of surface photometry. These parameters can then be used to test for the presence of subclustering. Galaxy radial velocity data are used to develop a statistic which is sensitive to correlations between local kinematics and galaxy position that would signal the presence of substructure. Application of these new techniques to ∼70 clusters with good positional and/or radial velocity data shows that, while little significant substructure is found in the inner regions of most clusters, many clusters do exhibit more than one dynamical component, although it is usually in the regions surrounding clusters that the greatest degree of subclustering can be found. This tendency for subclustering to occur at relatively large distances from the cluster center (≥ 1h-1 Mpc) creates an unfortunate level of ambiguity regarding the present dynamical state of nearby clusters. This ambiguity is also reflected by the observed distribution of line-of-sight velocities of galaxies in clusters, which we argue provides little constraining power.