An Etymological Dictionary of Astronomy and Astrophysics
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A type of very bright compact → stellar system (-14 ≤ M_V≥ -12) that is intermediate between → globular clusters (GCs) and → compact elliptical galaxies (cEs). With masses of M > 2 × 10⁶ Msun 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 > 10⁷ Msun. The most massive UCD discovered to date, M59-UCD3 (M_* ~ 2 × 10⁸ Msun, radius ~ 25 pc), hosts a SMBH (Ahn et al., 2018, arxiv/1804.02399, and references therein).

→ ultracompact; → dwarf; → galaxy.