The Systematic Properties of Clusters of Galaxies. Photometry of 15 Clusters

Fifteen rich clusters of galaxies have been studied using a new automatic method for finding and photometering galaxies on photographic plates. It was found that their properties are such that they may be divided into three groups. Spiral-rich clusters have compositions similar to that of the field, irregular mass distributions of low density and no central concentration, and show no signs of segregation of members according to mass or morphological type. Clusters with cD galaxies are rich in ellipticals, have smooth, spherical mass distributions of high density and central concentration, have a strong deficiency of spirals in the core, and show a considerable tendency toward energy equipartition. Spiral-poor clusters, which are dominated by 50 galaxies, also show segregation by mass and morphological type, but are not as smooth, dense, or centrally condensed as the cD clusters. The outer envelopes of all clusters have a projected density gradient of slope near 3.0. All clusters show a local minimum in the radial density distribution, the effect being strongest in the cD clusters. The mean density of clusters of all types is independent of mass. The luminosity functions of all clusters are similar, with a small dispersion in absolute magnitude. Arguments are presented that spiral-poor clusters represent a later evolutionary state of spiral-rich clusters but that cD clusters, while the most evolved, are intrinsically different. Subject heading: galaxies, clusters of