Quasars and Superclusters

Abstract

The topic of quasars and superclusters is only a few years old. Although the first pairs of quasars with small angular separations on the sky were found ten years ago (Stockton 1972, Wampler et al. 1973), the pair members had very different redshifts. The surface density of samples with available redshifts at that time was far too low for cases of quasars with both small angular separations and small redshift differences to turn up. Setti and Woltjer (1977) pointed out that if quasars occur in the nuclei of giant elliptical galaxies, then clustering should be apparent at 20th magnitude and fainter. In 1979 Walsh, Carswell and Weymann found a very close pair with identical redshifts; that, of course, was the first discovery of a gravitational lens. Also in 1979 Arp, Sulentic, and di Tullio showed that some of the quasars near NGC 3389 had similar redshifts, although at that time they did not discuss the hypothesis of the quasars being associated with superclusters. Subsequently Burbidge et al. (1980) confirmed that a compact (5 minutes of arc) group of 3 quasars found by Hoag on a 4m grating prism plate of the M82 field had very similar redshifts. Indeed, the group had the dimensions of a galaxy cluster, not a supercluster. Oort, Arp, and de Ruiter (1981) then specifically called attention to the fact that enough pairs of quasars with similar redshifts were known to suggest that quasar associations on the scale of galaxy superclusters do exist.