  As part of the HST/ACS Coma Cluster Treasury Survey, we have undertaken a
Keck/LRIS spectroscopic campaign to determine membership for faint dwarf
galaxies. In the process, we discovered a population of Ultra Compact Dwarf
galaxies (UCDs) in the core region of the Coma cluster. At the distance of
Coma, UCDs are expected to have angular sizes 0.01 < R_e < 0.2 arcsec. With ACS
imaging, we can resolve all but the smallest ones with careful fitting.
Candidate UCDs were chosen based on magnitude, color, and degree of resolution.
We spectroscopically confirm 27 objects as bona fide UCD members of the Coma
cluster, a 60% success rate for objects targeted with M_R < -12. We attribute
the high success rate in part to the high resolution of HST data and to an
apparent large population of UCDs in Coma. We find that the UCDs tend to be
strongly clustered around giant galaxies, at least in the core region of the
cluster, and have a distribution and colors that are similar to globular
clusters. These findings suggest that UCDs are not independent galaxies, but
rather have a star cluster origin. This current study provides the dense
environment datapoint necessary for understanding the UCD population.
