Studies of Substructure in Galaxy Clusters: A Two‐dimensional Analysis

I explore a procedure for the detection and quantification of substructure in the projected positions of galaxies in clusters. The method is first tested by application to the 56 well-studied galaxy clusters that make up the morphological sample of Dressler (1980). This method is then applied to a much larger sample of 119 Abell clusters originally identified of Hoessel, Gunn, & Thuan (1980). This sample includes all Abell clusters with distance class ≤4 and richness class greater than 0 with . Galaxy positions are obtained from digitized copies FbF 1 30 of the POSS I plates carried out by the Automated Plate Scanner (APS) project at the University of Minnesota. Adaptive-kernel contour maps (Silvermann 1986) are constructed for each sample cluster and possible secondary peaks identified within 1 Abell radius (1.5 h Mpc with H 5 100 0 h km s Mpc) of the cluster center. Two tests for substructure, one parametric and one nonparametric, are then applied to the galaxy positions, and the results are compared. The KMM algorithm (McLachlan & Basford 1988) partitions the data into Gaussian subpopulations and estimates their statistical significance via a hypothesis test. The DEDICA algorithm (Pisani 1996) is a nonparametric technique that identifies peaks in the projected galaxy density and determines their significance with respect to the background. To avoid background contamination, a K-S test is applied to the magnitude distribution of each subcluster identified as significant at the 99% level. It is found that 20% of the significant subclusters can be rejected as likely background or foreground groups. After removal of these background/foreground groups, of the large cluster 64% 5 15% sample is found to contain significant substructure. Radial number-density profiles are calculated for each of the sample clusters using a maximum penalized likelihood estimator (Merritt & Tremblay 1994). An average core radius of h kpc is obtained. This is however, likely to be an 112 5 75 upper limit due to misspecification of the cluster centers, chosen to be the position of the maximum projected density. Inside of 1 Mpc, the space density is found to vary as after 21.950. 3 r ∝ r a correction is made for background contamination. The large fraction of clusters with presently detectable substructure, as well as the shallow space-density profiles, are used to argue that rich clusters of galaxies are still in the process of formation during the present epoch through accretion of smaller groups of galaxies and are not in general well described by equilibrium models. With the assumption that substructure in the projected galaxy distribution is erased on a timescale of four cluster crossing times, . On the other hand, if Q * 0.4 , as currently predicted, then substructure in the proQ ≈ 0.2 0 jected galaxy positions is erased on average in six to seven cluster crossing times. Future work will include expanding the sample size to include ACO clusters in the southern sky using galaxy positions from COSMOS. These results will then be compared to other, more objectively determined cluster catalogs currently being constructed using POSS II as well as clusters selected based on their diffuse X-ray emissions.