Substructure in the Coma Cluster: Giants versus Dwarfs

The processes that form and shape galaxy clusters, such as infall, mergersand dynamical relaxation, tend to generate distinguishable differences betweenthe distributions of a cluster's giant and dwarf galaxies. Thus the dynamics ofdwarf galaxies in a cluster can provide valuable insights into its dynamicalhistory. With this in mind, we look for differences between the spatial andvelocity distributions of giant (b<18) and dwarf (b>18) galaxies in the Comacluster. Our redshift sample contains new measurements from the 2dF and WYFFOSspectrographs, making it more complete at faint magnitudes than any previouslystudied sample of Coma galaxies. It includes 745 cluster members - 452 giantsand 293 dwarfs. We find that the line-of-sight velocity distribution of thegiants is significantly non-Gaussian, but not that for the dwarfs. A battery ofstatistical tests of both the spatial and localised velocity distributions ofthe galaxies in our sample finds no strong evidence for differences between thegiant and dwarf populations. These results rule out the cluster as a wholehaving moved significantly towards equipartition, and they are consistent withthe cluster having formed via mergers between dynamically-relaxed subclusters.Comment: 23 pages, 6 figures, to appear in Ap