Position Angles and Alignments of Galaxy Clusters

Abstract

The position angles of a large number of Abell and Shectman clusters (in total 637 clusters), identified in the Lick map as surface density enhancements, are estimated. Using published redshifts for 277 of these I have detected strong nearest neighbour alignments (2.5 - 3 sigma level), up to about 15 Mpc (Ho = 100). Weaker alignments but still significant are found up to larger separations. An even more significant alignment (4 sigma) is found between all neighbours residing in superclusters and having separations <= 10 Mpc. Again weaker but significant alignments are found when larger separations are considered. I have performed various tests to check whether systematic effects could induce an artificial alignment signal but I find no evidence for the presence of such effects. Since my cluster sample is neither volume limited nor redshift complete, a fact that would tend to wash-out any real alignment signal, the alignments detected should reflect a real and possibly a stronger underline effect.