Empirical indicators of the evolution of clusters of galaxies

Current hypotheses on the dynamic processes in clusters of galaxies (CG) are discussed. The morphological classification schemes currently in use are reviewed, and the three-phase, three-region model of CG evolution developed by von Hoerner (1976), based on a description of a CG as a self-gravitating system of N bodies of equal mass, is presented. A comparison of the weighted density distributions of six CGs with model predictions (Schmidt and Baier, 1979) reveals good agreement, except that the dissipation expected in the third evolutionary phase has not yet been observed. X-ray-luminosity and X-ray-temperature measurements of CG components obtained with HEAO-B, when plotted against the evolution modulus of von Hoerner (Schmidt, 1980), verify the compatibility of the evolutionary hypothesis with the model of CG-gas temperature and density distribution proposed by Perrenod (1978).