Ultracompact dwarf galaxies

A type of very bright compact stellar system (-14 ≤ MV ≥ -12) that is intermediate between globular clusters (GCs) and compact elliptical galaxies (cEs). With masses of M > 2 x 10^6 M_sun and radii > 10 parsecs (pc), UCDs are among the densest stellar systems in the Universe. Nevertheless, the nature and origin of these objects is still widely debated. Early interpretations suggested that UCDs could be the most massive GCs or possibly the tidally stripped remnants of dwarf galaxies. However, there is evidence that both formation mechanisms could contribute to the UCD population. Supermassive black holes (SMBHs) have been confirmed in most UCDs with masses M > 10^7 M_sun. The most massive UCD discovered to date, M59-UCD3 (M_* ~2 x 10^8 M_sun, radius ~ 25 pc), hosts a SMBH.