Faint stellar systems in the Virgo Cluster: The connection between dwarf galaxies and globular clusters

We present observations of faint stellar systems in the Virgo Cluster that have properties similar to both globular clusters and dwarf elliptical galaxies. These systems are not only very faint but are under-represented in the local Universe, factors that make their observation and analysis not only compelling but also challenging. However, using state-of-the-art observing facilities, we can undertake the first ever study of the connection between these two types of stellar systems.; We use optical and near-infrared Hubble Space Telescope images from the Advanced Camera for Survey Virgo Cluster Survey program, and spectroscopy obtained with the Echelle Spectrograph and Imager on the Keck 10-m telescope.; We perform careful dynamical and kinematical analyses, in which the properties of these "Dwarf Globular Transition Objects" (DGTOs) are compared to those of globular clusters and dwarf elliptical galaxies in the Local Supercluster. A few of our DGTOS are just "normal", old, metal-rich globular clusters, although much brighter than the previously known equivalent systems in our Galaxy.; The majority of the DGTOs have properties unlike those of normal globular clusters or dwarf galaxies. We consider them to belong to a recently identified class of faint stellar system, called ultracompact dwarf galaxies (UCDs). On the whole, the UCDs resemble the nuclei of nucleated dwarf elliptical galaxies in the Virgo Cluster. Our analysis generally support two possible scenarios for UCD formation: tidal stripping of the stellar envelopes of nucleated dwarf galaxies, and the amalgamation of multiple young massive clusters.; We find that there is a transition between globular clusters and ultracompact dwarf galaxies in terms of total system mass, at M ∼ (2-3) x 106 M⊙ . This transition is reflected in the onset of a size-luminosity relation for the UCDs, while there is none for the globular clusters. We suggest a quantitative classification scheme for DGTOs based on half-mass relaxation time, where UCDs have relaxation times that exceed a Hubble Time.; The strong concentration of UCDs to the center of the Virgo Cluster (and M87 in particular) suggests that proximity to the cluster center may be of key importance in the formation of UCDs.