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Concept
Dwarf spheroidal galaxy
Summary
A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are companions to the Andromeda Galaxy (M31). While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape and generally have lower luminosity.
Despite the radii of dSphs being much larger than those of globular clusters, they are much more difficult to find due to their low luminosities and surface brightnesses. Dwarf spheroidal galaxies have a large range of luminosities, and known dwarf spheroidal galaxies span several orders of magnitude of luminosity. Their luminosities are so low that Ursa Minor, Carina, and Draco, the known dwarf spheroidal galaxies with the lowest luminosities, have mass-to-light ratios (M/L) greater than that of the Milky Way. Dwarf spheroidals also have little to no gas with no obvious signs of recent star formation. Within the Local Group, dSphs are primarily found near the Milky Way and M31.
The first dwarf spheroidal galaxies discovered were Sculptor and Fornax in 1938. The Sloan Digital Sky Survey has resulted in the discovery of 11 more dSph galaxies as of 2007 By 2015, many more ultra-faint dSphs were discovered, all satellites of the Milky Way. Nine potentially new dSphs were discovered in the Dark Energy Survey in 2015. Each dSph is named after constellations they are discovered in, such as the Sagittarius dwarf spheroidal galaxy, all of which consist of stars generally much older than 1–2 Gyr that formed over the span of many gigayears.
For example, 98% of the stars in the Carina dwarf spheroidal galaxy are older than 2 Gyr, formed over the course of three bursts around 3, 7 and 13 Gyr ago. The stars in Carina have also been found to be metal-poor.
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