Ultra-compact Dwarfs: A missing link between galaxies and globular clusters?

In the course of an "all-object" spectroscopic survey of an area of the Fornax Cluster, we have detected, in addition to 'conventional' fairly low surface brightness dwarf galaxies, a new population of physically very small (and therefore high surface brightness) galaxies. We have named these "ultra-compact dwarfs" or UCDs. In this paper we present HST imaging of these UCDs, demonstrating that they have half-light radii of typically 20pc. This places them in a formerly unoccupied region of parameter space between dwarf galaxies and globular clusters.