SUBSTRUCTURE IN ABELL CLUSTERS

We present the results of a study of substructure in a sample of 107 Abell clusters selected according to the following criteria: Abell richness class R greater-than-or-equal-to 1, redshift, z less-than-or-equal-to 0.1, 10h less-than-or-equal-to alpha less-than-or-equal-to 18h, delta > -25-degrees, b greater-than-or-equal-to 30-degrees. Using the Leiden plate scanning machine we have scanned Palomar Sky Survey Plates to produce lists of galaxies in these clusters down to approximately 19th magnitude. In this paper we present the results of six tests to search for different kinds of clumpiness or substructure in two dimensional galaxy distributions. Two of the tests are based on the percolation or cluster analysis method, one test uses the minimal spanning tree algorithm, one test uses the distribution of angular spacings, one test is sensitive to deviations from symmetry about the cluster center and finally the Lee statistic searches for clusters whose members are grouped in two clumps. No individual test detects substructure in more than 10% of clusters at the 99% confidence level. In total we find that 28 clusters show significant substructure, 4 clusters show significant substructure in more than one test. This is less than in previous studies but note that; (1) we are looking at the central 4 MPc (H0 = 50 km s-1 Mpc-1) of the clusters, (2) we use 100 galaxies per cluster, (3) comprehensive studies of substructure require radial velocities as well as positions on the sky.