blue compact dwarf galaxy
kahkešân-e kutule-ye âbi-ye hampak
Fr.: galaxie naine bleue compacte
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.
Fr.: naine T
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.