SUBCLUSTERING AND LUMINOUS-DARK-MATTER SEGREGATION IN GALAXY CLUSTERS

We have performed a series of high-resolution N-body experiments on a Connection Machine CM-5 in order to simulate the formation of galaxy clusters gravitationally dominated by a massive dark background. In accordance with previous authors we find an extremely inhomogeneous evolution where subcondensations are continually formed and merged. Final distributions do not present subclustering and galaxies are more centrally condensed than dark matter particles. We have found that such a luminous-dark matter segregation persists even when inelastic encounters of dark halos around galaxies are taken into account. We show rigorously that such a segregation is due to dynamical friction within the subclusters, while an interpretation of this effect as a biasing process in initial conditions, is ruled out. We are then interested in the origin of subclustering. We find that galaxy-dark matter segregation is mainly due to the substructures formed from local gravitational perturbations produced by the presence itself of massive bodies (galaxies), while the subclusters formed from initial Poisson density fluctuations. Consequently, the final amount of segregation depends only very slightly on the initial fluctuation spectrum. Physical parameters characterizing the model of protocluster have, however, an important influence on the final result. This dependence has been qualitatively explained by means of a simple dynamical model and has been numerically analyzed. We find that final distributions without segregation are only possible for extreme values of these physical parameters. We then conclude that such a segregation effect cannot be in general avoided in any hierarchical clustering scenario. Consequently, all observational analyses assuming that galaxies in clusters trace mass are misleading and their conclusions have to be reconsidered.