The ACS Virgo Cluster Survey. VII. Resolving the Connection between Globular Clusters and Ultracompact Dwarf Galaxies

We investigate the connection between globular clusters and ultra-compactdwarf galaxies (UCDs) by examining the properties of several compact objectsassociated with M87, all of which were previously classified as globularclusters. Combining imaging from the Hubble Space Telescope with ground-basedKeck spectroscopy, we find two objects to have half-light radii, velocitydispersions and mass-to-light ratios that are consistent with the predictionsof population synthesis models for old, metal-rich, luminous globular clusters.Three other objects are much larger, with half-light radii of approximately20pc, and have V-band mass-to-light ratios in the range 6-9. These objects,which we consider to be UCDs, resemble the nuclei of nucleated dwarf ellipticalgalaxies in Virgo, having similar mass-to-light ratios, luminosities andcolors. These UCDs are found to obey the extrapolated scaling relations ofgalaxies more closely than those of Galactic globular clusters. There appearsto be a transition between the two types of stellar systems at a mass of abouttwo million solar masses. If the UCDs are gravitationally bound, then wesuggest that the presence of dark matter is the fundamental propertydistinguishing globular clusters from UCDs. More than half of the UCDcandidates uncovered in the ACS Virgo Cluster Survey are associated with asingle galaxy -- M87 -- which suggests that proximity to the Virgo center maybe of critical importance for the formation of these objects. These resultsshow that distinguishing bonafide UCDs from bright globular clusters requires acareful analysis of their detailed structural and dynamical properties,particularly their mass-to-light ratios. (ABRIDGED)