X-ray morphologies of Abell clusters

Abstract

We have determined the orientations and ellipticities of the X-ray images of a sample of 49 Abell clusters. The techniques used in determining the level of noise in the images (and removing it) are described, and a variety of approaches to the problem of defining reliable cluster morphological parameters are discussed and compared. For each cluster, we (1) determine overall parameters (using the faintest flux levels) and (2) investigate the variation of those parameters with flux level. We find that around one-half of the cluster images show unambiguous elongation, approximately one-quarter exhibit isophotal twisting, and the remaining 25% are either too irregular or too uncertain to allow definitive statements to be made. A short catalog of cluster properties is presented, and some of the clusters with significant substructure are discussed in more detail. The results of this study are applied to the question of large-scale cluster alignments in a separate paper.