2008MNRAS.386L..57S

In this Letter, we investigate the very large-scale environments of cluster-scale triplets of type 1 Seyfert galaxies, lower luminosity counterparts of quasars, detected at z < 0.2 in the fourth Data Release of the Sloan Digital Sky Survey. The search for cluster scale triplets (object-object separations <2.0h^–1Mpc and <2000km/s) in a combined sample of Seyferts and quasars resulted in only 7 triplets, 3 of which are at z < 0.2. We investigated their environments and found a strong association of z < 0.2 triplets with the richest central parts of superclusters. In two out of three cases, the members of the triplets have been found on the periphery of an extremelly rich galaxy cluster (Abell class 2 and 3). The third triplet also appears associated with a very rich cluster of galaxies but on a somewhat larger scale. Nevertheless, on a small scale, it is still associated with a cluster but of lesser richness. These results suggest that triplets of type 1 Seyfert galaxies at low redshifts are excellent tracers of very rich clusters. Following these results we conclude that triplets of quasars, bright counterparts of type 1 Seyfert galaxies, may be the most successful means of finding distant, very rich, galaxy clusters.