extragalactic H II region
extragalactic H II region(eks-tră-gă-lak -tik) A cloud of hot ionized gas that is isolated in extragalactic space and is believed to consist of intergalactic gas that has begun to condense into stars and will probably become a dwarf irregular galaxy. Extragalactic H II regions were identified in 1970. They have no old stars, and a low metallicity indicates that some (like the compact galaxy IZw18) are less than 108 years old. These are the only ‘galaxies' significantly different in age from our Galaxy.
Collins Dictionary of Astronomy © Market House Books Ltd, 2006