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 type of → brown dwarf with an → effective temperature between about 1200 K and 500 K, i.e. colder than the preceding type → L dwarf. The spectra of T dwarfs are characterized by the presence of → methane (CH₄) bands in the → near infrared. The presence of these bands, broad H₂O features, and H₂ collision-induced absorption radically alter the spectral energy distributions of T dwarfs compared to a black body at the same temperature. Hence near-infrared colors become increasingly blue (J - K ~ 0) as compared to L dwarfs. The first T dwarf, called → Gl 229B, was discovered by Nakajima et al. (1995, Nature 378, 463). The spectral classification scheme (subtypes T0 to T9) currently used was defined by Burgasser et al. (2002, ApJ 564, 421).

For the reasoning behind the choice of the letter T, see Kirkpatrick et al. 1993, ApJ 406, 701; → dwarf.

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.