The Elongation of Abell Clusters. III. Rich Clusters and Initial Conditions

We present the results of a study of the elongation and orientation of a sample of 107 rich clusters of galaxies. The data consist of galaxy lists derived from digitised images of the Palomar Sky Survey red plate copies. We work with three measures of the cluster orientation (1) the cluster position angle derived from the Fourier transform of the azimuthal galaxy distribution, (2) the first-ranked galaxy position angle derived from the moments of the digitized galaxy image, and (3) the cluster position angle derived from the x-ray surface brightness distribution [Einstein data]. We have examined the orientation of clusters with respect to their nearest neighbors as a function of distance to the nearest neighbor. The orientation of clusters with respect to members of the same supercluster has also been examined. We find that alignment effects are present for clusters residing in superclusters. We also find that cD galaxies are significantly aligned with the direction to the nearest neighbor cluster. These results are discussed in the context of current ideas on galaxy formation.