Studies of Clusters of Galaxies Using Measurements by Cosmos--I PROPERTIES OF SIX RICH SOUTHERN CLUSTERS

Abstract

Measurements have been made with the COSMOS automatic plate-measuring machine at the Royal Observatory Edinburgh on six rich southern clusters of galaxies. Separation of the images into galaxies and stars has been carried out by computer and the distributions of the resulting non-stellar components are analysed with a view to examining the cluster properties. The results indicate that all six clusters are unambiguously elongated. The presence of large scale luminosity segregation (i.e. > so per cent difference in core radii between the distribution of bright and faint members) has been detected in one cluster, the brighter galaxies also being in a more disklike configuration. There is some indication that the degree of segregation may increase with increasing elongation, indicating that more flattened systems are generally more relaxed. There is also the suggestion in the results of an increase in segregation with concentration, indicating that clusters with high concentrations are more relaxed, their extensive halos being lost in the general background of galaxies. Comparison of the results for one cluster on a red and on a blue plate reveals that the galaxies are more centrally concentrated and in a more flattened configuration on the blue plate than on the red. The observations are consistent with a picture for the structure of the cluster in which the bluer galaxies form a flattened core while the redder galaxies form a more spherical halo.