An Etymological Dictionary of Astronomy and Astrophysics
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An small → irregular galaxy undergoing → violent star formation activity. These objects appear blue by reason of containing clusters of hot, → massive stars which ionize the surrounding interstellar gas. They are chemically unevolved since their → metallicity is only 1/3 to 1/30 of the solar value. Same as → H II galaxy.

→ blue; → compact; → dwarf; → galaxy.

A small, low luminosity galaxy that is associated with a larger spiral galaxy and may make up part of a galactic halo. There are many of them in the Local Group, and often orbit around larger galaxies such as the Milky Way and the Andromeda Galaxy. There are three main types of them: → dwarf elliptical galaxy (dE), → dwarf irregular galaxy (dI), and → dwarf spiral galaxy (dSA).

→ dwarf; → galaxy.

A self-gravitating entity which has been formed from tidal material expelled during interactions between larger galaxies. TDGs are typically found at the tip of tidal tails at distances between 20 and 100 kpc from the merging galaxies, of which at least one should be a gas-rich galaxy. They are gas-rich objects that can be as massive as the Magellanic Clouds, form stars at a rate which might be as high as in blue compact dwarf galaxies and seem dynamically independent from their parent galaxies.

→ tidal; → dwarf; → galaxy.

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.