'Galaxy associations' as possible common features of galaxy clusters

The results are here presented of an analysis of the subclustering in a sample of galaxy clusters from the European Southern Observatory (ESO) Nearby Abell Cluster Survey (ENACS), along with complementary data from other studies. The analysis is performed by using the S-tree method, enabling us to study the hierarchical properties of clusters by the detection of the main physical cluster and of its subgroups. The results indicate (a) systematically lower genuine cluster velocity dispersions than were known from previous studies, and (b) the existence of 2–3 subgroups in each cluster. As a result of certain properties of a new class of dynamical entities, we denote these subgroups as galaxy associations ; these may become an essential challenge for the formation mechanisms of galaxy clusters.