Einstein Cluster Alignments Revisited

We examine whether the major axes of rich galaxy clusters tend to point toward their nearest neighboring cluster. We have used the data of Ulmer, McMillan, and Kowalski, who used position angles based on X-ray morphology. We also study a subset of this sample with updated positions and distances from the MX Northern Abell Cluster Survey [for rich clusters (R>=1) with well-known redshifts]. A Kolmogorov-Smirnov (K-S) test shows no significant signal for nonrandom angles on any scale <=100 h^-1 Mpc. However, refining the null hypothesis with the Wilcoxon rank-sum test, we found a high confidence signal for alignment. Confidence levels increase to a high of 99.997%, since only near neighbors that are very close are considered. We conclude that there is a strong alignment signal in the data, consistent with gravitational instability acting on Gaussian perturbations.