SUBSTRUCTURE IN GALAXY CLUSTERS - A 2-DIMENSIONAL APPROACH

We discuss a procedure for detecting and quantifying the presence of s ubstructure in the projected positions of galaxies in clusters. New co ntour maps of the surface density of galaxies for the 56 clusters in D ressler's morphological sample are constructed by application of an ad aptive-kernel technique. We employ the KMM algorithm to objectively pa rtition the galaxies in each cluster into sub-populations, identify th e best-fitting alternative to the null hypothesis of a single-populati on model, and estimate the statistical significance of the identified structures. The power of the KMM algorithm for this application is eva luated based on a study of simulated clusters. We estimate that 57% of the Dressier morphological-sample clusters have statistically-signifi cant substructure. In all likelihood, this proportion represents a low er limit to the fraction of clusters with real substructure. We calcul ate the projected median distance of the D or cD galaxy (in the cluste rs which possess them) to the centroid of the nearest subcluster, and demonstrate that the dominant galaxies are located close to the peaks of the galaxy surface-density. (C) 1997 American Astronomical Society.