The Systematic Properties of Clusters of Galaxies

Citation

Oemler, Augustus, Jr. (1974) The Systematic Properties of Clusters of Galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/BT4A-JT75. https://resolver.caltech.edu/CaltechETD:etd-05272008-091956

Abstract

Fifteen rich clusters of galaxies have been studied on photoelectrically calibrated photographic plates. These plates were measured using new automatic photometry techniques. Data in the literature have been supplemented by new observations to obtain velocity dispersions in nine of these clusters. Clusters can be divided by their galaxy content into three types: spiral rich, spiral poor, and those dominated by a cD galaxy. The luminosity functions of the latter two types are very similar, and differ significantly from that of galaxies in spiral rich clusters. The surface brightness profiles of the spiral poor and cD clusters agree in severalrespects with numerical models of collapsed clusters by Aarseth, but the spiral poor clusters are too irregular and have too little central concentration to be collapsed. All of the clusters have a local minimum in their profiles which cannot be explained. Spiral rich clusters tend to be less dense than average and cD clusters more dense, but, in the mean, all clusters follow a constant density law independent of mass. The cD clusters and others with a high ratio of elliptical to spiral galaxies have mass to light ratios of about 225, while the mass to light ratio of spiral rich clusters is less than half of this value. The collapse times calculated using these mass-luminosity ratios confirm that spiral rich clusters are still collapsing while the others are close to equilibrium. The degree of mass segregation in the collapsed clusters is close to that predicted by the Aarseth models and requires that most of the cluster mass be in the galaxies themselves rather than in a cluster-wide medium. The decrease of the number of spirals towards the center of the collapsed clusters supports the idea that SO galaxies are created by stripping the gas from spirals, and the spiral rich clusters can be expected to evolve into spiral poor clusters after collapse by this mechanism. The high proportion of elliptical galaxies in the cD clusters probably means that they are a distinct type of object, and not just a later evolutionary state of the spiral poor clusters.

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