DARK-MATTER IN HIERARCHICAL CLUSTER EVOLUTION

We have found that the distribution of X-ray core radii for clusters in bimodal systems, subclusters nearly equal in luminosity which are separated by approximately 1 Mpc in projection, have a larger mean value (0.53 Mpc) than various comparison samples consisting of clusters with and without optical or X-ray substructure (mean core radius of 0.26 Mpc). There is 0.01%-0.3% probability that the bimodal and any of the comparison samples are related using the core radius as a test parameter. If the bimodal system subclusters, which are observed before merger, represent the final stage prior to the hierarchical formation of the clusters that make up the four comparison samples, then the X-ray core radius of subclusters decreases during the merger process. For an isothermal gas or a nonisothermal gas in which the temperature profile has a similar core radius to the density profile, a smaller X-ray core radius indicates a more centrally concentrated cluster mass distribution. Since the cluster mass distribution is dominated by dark matter, the decrease in core radius during cluster merger may result from a change in the dark matter distribution during the merger, implying that the mass becomes more concentrated to the center of the cluster. Comparison of the results to various n-body simulations and to the mean optical core radius implies that rich clusters may be characterized by a constant mass-to-light ratio before merger, and by a mass-to-light ratio that is higher in the central region after merging.