The Milky Way's biggest star cluster may be the heart of a swallowed galaxy
Omega Centauri looks like the Milky Way's largest globular cluster. Astronomers now suspect it is something stranger: the leftover core of a dwarf galaxy, complete with a central black hole, that our galaxy absorbed long ago. More than 20 other dwarf galaxies still circle the Milky Way.
Dwarf galaxies hold anywhere from about a thousand stars to several billion, compared with the Milky Way's 100 to 400 billion. The boundary is fuzzy: the Large Magellanic Cloud, with over 30 billion stars, is called a dwarf by some astronomers and a full galaxy by others. The faintest of all, ultra-faint dwarfs, contain from a few hundred to a hundred thousand stars; they resemble globular clusters but are more spread out and packed with dark matter, and were first found in 2005 through digital sky surveys, especially the Sloan Digital Sky Survey.
Larger neighbours dominate their lives. Big spirals tug dwarfs apart into stellar streams and eventually merge with them. A 2007 paper suggested many dwarfs were born from tidal debris during the early growth of the Milky Way and Andromeda, since colliding galaxies can fling out streams of material that condense into tidal dwarfs. A 2018 study argued that some local dwarfs formed remarkably early, within the first billion years after the Big Bang, while blue, metal-poor, star-forming dwarfs nearby seem to have made most of their stars in the last billion years.
Their origins are debated. One view holds that galaxies grow with dark matter or from gas enriched with heavy elements, yet NASA's Galaxy Evolution Explorer spotted dwarfs forming from metal-poor gas in the Leo Ring, a hydrogen and helium cloud around two big galaxies. The Subaru telescope's Hyper Suprime-Cam survey found that 30 to 50 percent of dwarfs in sparse surroundings have largely stopped forming stars, pointing to internal effects from stars or active nuclei as well as outside influences.
Types include dwarf ellipticals, dwarf spheroidals, now counted as their own class, and dwarf irregulars. Blue compact dwarfs such as NGC 1705 are lumpy clusters of hot, young, massive stars that burn through gas furiously. Tracking individual stars in nearby systems like the Sculptor dwarf lets researchers test the standard cosmological model and study processes such as ram pressure stripping.
Source: Dwarf galaxy
