Ultra-Compact Dwarfs in the Core of the Coma Cluster
  We have discovered both a red and a blue subpopulation of Ultra-Compact Dwarf
(UCD) galaxy candidates in the Coma galaxy cluster. We analyzed deep F475W
(Sloan g) and F814W (I) Hubble Space Telescope images obtained with the
Advanced Camera for Surveys Wide Field Channel as part of the Coma Cluster
Treasury Survey and have fitted the light profiles of ~5000 point-like sources
in the vicinity of NGC 4874, one of the two central dominant galaxies of the
Coma cluster. Although almost all of these sources are globular clusters that
remain unresolved, we found that 52 objects have effective radii between ~10
and 66 pc, in the range spanned by Dwarf Globular Transition Objects (DGTO) and
UCDs. Of these 52 compact objects, 25 are brighter than M_V ~-11 mag, a
magnitude conventionally thought to separate UCDs and globular clusters. The
UCD/DGTO candidates have the same color and luminosity distribution as the most
luminous globular clusters within the red and blue subpopulations of the
immensely rich NGC 4874 globular cluster system. Unlike standard globular
clusters, blue and red UCD/DGTO subpopulations have the same median effective
radius. The spatial distribution of UCD/DGTO candidates reveal that they
congregate towards NGC 4874, and are not uniformly distributed. We find a
relative deficit of UCD/DGTOs compared with globular clusters in the inner 15
kpc around NGC 4874, however at larger radii UCD/DGTO and globular clusters
follow the same spatial distribution.
