TRACING THE ASSEMBLY HISTORIES OF GALAXY CLUSTERS IN THE NEARBY UNIVERSE

We have compiled a sample of 67 nearby (z < 0.15) clusters of galaxies, for which on average more than 150 spectroscopic members are available and, by applying different methods to detect substructures in their galaxy distribution, we have studied their assembly history. Our analysis confirms that substructures are present in 70% of our sample, having a significant dynamical impact in 57% of them. A classification of the assembly state of the clusters based on the dynamical significance of their substructures is proposed. In 19% of our clusters, the originally identified brightest cluster galaxy is not the central gravitationally dominant galaxy (CDG), but turns out to be either the second-rank, or the dominant galaxy of a substructure (a SDG, in our classification), or even a possible “fossil” galaxy in the periphery of the cluster. Moreover, no correlation was found in general between the projected offset of the CDG from the X-ray peak and its peculiar velocity.