Correlation between supercluster membership and richness for Abell clusters

Abstract

THE Abell catalogue¹ contains 2,712 clusters, of which 1,682 form a homogeneous statistical sample, with clusters of richness R = 1 to 5. Murray et al.² have analysed the entire sample of 2,712 Abell clusters with a set of uniform criteria which identify 21 superclusters. Their criteria are: (1) that clusters which belong to the supercluster are approximately equidistant from us; M₁₀, the magnitude of the tenth brightest cluster galaxy, is allowed to range over ≤0.3 mag for the member clusters; (2) six or more such clusters are contained in a circle of radius θ centred on one of the clusters; and (3) θ ∼ 1.5° for a distance class-6 cluster and scales inversely with distance according to M₁₀. These criteria identify high-multiplicity (n ≥ 6) superclusters, rather than groups which are predominantly binary and triplet clusters. While ∼40% of nearby Abell clusters are in low-multiplicity (n = 2, 3) groups^3,4, only ∼6% of all Abell clusters are contained in the superclusters of high multiplicity. This letter shows (1) that clusters of galaxies which are located in superclusters (second-order clusters of ∼30 Mpc radius) are systematically richer than those in the ‘field’, and (2) that the expected higher X-ray luminosities of these richer clusters may, therefore, account for much, if not all, of the possibly large X-ray luminosities from superclusters.