The HST/ACS Coma Cluster Survey: V - Compact Stellar Systems in the Coma
  Cluster
  The HST ACS Coma Cluster Treasury Survey is a deep two passband imaging
survey of the nearest very rich cluster of galaxies, covering a range of galaxy
density environments. The imaging is complemented by a recent wide field
redshift survey of the cluster conducted with Hectospec on the 6.5m MMT. Among
the many scientific applications for this data are the search for compact
galaxies. In this paper, we present the discovery of seven compact (but quite
luminous) stellar systems, ranging from M32-like galaxies down to ultra-compact
dwarfs (UCDs)/dwarf to globular transition objects (DGTOs). We find that all
seven compact galaxies require a two-component fit to their light profile and
have measured velocity dispersions that exceed those expected for typical
early-type galaxies at their luminosity. From our structural parameter analysis
we conclude that three of the sample should be classified as compact
ellipticals or M32-like galaxies, the remaining four being less extreme
systems. The three compact ellipticals are all found to have old luminosity
weighted ages (> 12 Gyr), intermediate metallicities (-0.6 < [Fe/H] < -0.1) and
high [Mg/Fe] (> 0.25). Our findings support a tidal stripping scenario as the
formation mode of compact galaxies covering the luminosity range studied here.
We speculate that at least two early-type morphologies may serve as the
progenitor of compact galaxies in clusters.
