SUBSTRUCTURE, DYNAMICS AND EVOLUTION IN CLUSTERS OF GALAXIES

Abstract

ABSTRACT Most models for the formation of large scale structure imply that clusters of galaxies are currently forming. Because the Hubble time is on the order of a few dynamical timescales for clusters, there is some hope that their structure will retain signatures of their formation and initial conditions. In order to objectivly identify the different sorts of substructure which may exist in clusters of galaxies at the present time, I present a variety of statistical diagnostics which measure deviations of cluster galaxy velocities and positions from those expected in a relaxed system. These tests are applied to a database of rich clusters with at least 50 measured redshifts. Depending on the diagnostic considered, 30-70% of all well-studied clusters possess significant substructure. Only one system in the cluster database, A496, does not have any significant deviations from the structure expected for a relaxed, dynamically-evolved cluster. Once the importance of substructure in any particular cluster has been assessed, a maximum likelihood technique is used to assign individual galaxies to their host subclusters. These statistical techniques are applied to the study of central galaxy peculiar velocities. Formation models for central dominant galaxies require them to be preferentially located at the bottom of their cluster potential well. Without correcting for substructure, 50% of the cD-type systems in the cluster database have central galaxies which are displaced from their potential minimum (as defined by the cluster average velocity). These systems are much more likely to exhibit correlations between galaxy velocities and positions than clusters in which the central galaxy is at rest, providing evidence that substructure may be responsible for the high velocity offsets. If dynamical models which include subcluster assignments are used to determine the peculiar velocities, only 20% of the cD-type clusters have significant velocity offsets.