2.11. Extensions of clustering
Rich clusters of galaxies represent only a portion of a spectrum of clustering (Peebles, 1974), which ranges from individual galaxies and binary galaxies to enormous regions of enhanced density ('superclusters') or reduced density ('voids').
2.11.1. Poor clusters
Lists of poor clusters and groups have been given by Sandage and Tammann (l975), de Vaucouleurs (1975), Turner and Gott (1976b), Hickson (1982), Beers et al. (1982), and Huchra and Geller (1982). Of particular interest are the poor clusters containing possible cD galaxies which have been catalogued by Morgan, Kayser, and White (l975, MKW) and Albert, White, and Morgan (l977, AWM). In Section 2.10.1 observations of the cDs in these poor clusters were used to constrain models for the formation of cD galaxies. Bahcall (1980) has studied the optical properties (richness, galaxy distribution, and galactic content) and finds that they represent a smooth continuation to lower richness of the properties of the Abell clusters. Recent optical studies of these clusters include Beers et al. (1984) and Malumuth and Kriss (1986).
2.11.2. Superclusters and voids
Clustering does not stop at the well-defined rich clusters, but extends to a much larger scale (Peebles, 1974). These superclusters appear in the distribution of galaxies , or in the distribution of clusters of galaxies. Recent reviews of superclustering include those of Rood (1981) and Oort (1983). Our own galaxy appears to lie within the Local Supercluster, a flattened system dominated by the Virgo cluster (de Vaucouleurs, 1953).
The recognition of distinct superclusters and of voids (nearly empty regions between superclusters) has largely become possible as larger samples of redshifts have become available for galaxies and clusters. With redshifts, one can study the three-dimensional distribution of galaxies, rather than just their angular distribution on the sky, and the confusing effects of projection can be reduced. Such studies have shown that Coma (A1656) and A1367 form part of a large ( 30 Mpc) Coma supercluster (Rood et al., 1972; Chincarini and Rood, 1976; Tifft and Gregory, 1976; Gregory and Thompson, 1978), and that the Perseus cluster is embedded in the Perseus supercluster (Gregory et al., 1981). Other possible superclusters identified as groupings of clusters have been cataloged by Abell (1961), Rood (1976), Murray et al. (1978), and Thuan (1980).
Similar large scale clustering is observed directly in the distribution of galaxies in redshift surveys in small regions of the sky by Kirshner, Oemler, and Schechter (1978) and by the more extensive survey of Davis et al. (1982). Many of the superclusters appear to be highly elongated.
Large voids have also been found to lie between the superclusters. Such voids appear in front of the Coma, Perseus, and Hercules superclusters (Rood, 1981). An extremely large void (100-200 Mpc in size) in the galaxy distribution in the direction of Bootes has been discovered by Kirshner et al. (1981). A similarly large void in the distribution of Abell clusters was discovered recently by Bahcall and Soneira (1982).