DARK-MATTER IN HIERARCHICAL CLUSTER EVOLUTION

We have found that the distribution of X-ray core radii for clusters i n bimodal systems, subclusters nearly equal in luminosity which are se parated by approximately 1 Mpc in projection, have a larger mean value (0.53 Mpc) than various comparison samples consisting of clusters wit h 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 paramet er. If the bimodal system subclusters, which are observed before merge r, represent the final stage prior to the hierarchical formation of th e clusters that make up the four comparison samples, then the X-ray co re radius of subclusters decreases during the merger process. For an i sothermal 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 distribut ion. Since the cluster mass distribution is dominated by dark matter, the decrease in core radius during cluster merger may result from a ch ange in the dark matter distribution during the merger, implying that the mass becomes more concentrated to the center of the cluster. Compa rison of the results to various n-body simulations and to the mean opt ical core radius implies that rich clusters may be characterized by a constant mass-to-light ratio before merger, and by a mass-to-light rat io that is higher in the central region after merging.