1.5 Two-dimensional and three-dimensional cluster surveys

An early projection of the distribution of clusters of anagalactic nebulae is given by Lundmark (1927). In 1958 Abell published The Distribution of Rich Clusters of Galaxies, the first catalogue devoted entirely to the listing of positions and characteristics of prominent clusters of galaxies. For this, Abell developed and applied a classification scheme of richness classes and distance classes. Abell's presentation of the cluster distribution is shown in Fig. 11. Three years later the first volume of the Catalogue of Galaxies and Clusters of Galaxies (Zwicky et al. 1961-1968) was published. Recently, a southern extension of the Abell Catalogue became available (Olowin 1987). For more detail on the work of Zwicky and Abell, see Sect. 2.1.

Figure 11. Distribution of clusters of galaxies (Abell 1958). ``The distribution in galactic co-ordinates of the catalogued clusters in richness groups 1-5 and distance groups 1-6, inclusive. The plot is on an Aitoff equal-area projection.''

Three-dimensional cluster surveys are largely based on the Abell catalogue and its southern extension. For very distant clusters information is also drawn from radio data, since strong radio galaxies are frequently found at the center of these clusters. Redshifts have now reached z = 1.5 for radio galaxies and z = 0.9 for radio-quiet galaxies (Kron 1988).