An evolutionary continuum from nucleated dwarf galaxies to star clusters.

Systematic studies have revealed hundreds of ultra-compact dwarf galaxies (UCDs) in the nearby Universe. With half-light radii r of approximately 10-100 parsecs and stellar masses M≈ 10-10solar masses, UCDs are among the densest known stellar systems. Although similar in appearance to massive globular clusters, the detection of extended stellar envelopes, complex star formation histories, elevated mass-to-light ratio and supermassive black holes suggest that some UCDs are remnant nuclear star clusters of tidally stripped dwarf galaxies, or even ancient compact galaxies.