Andromeda XVIII, discovered in 2008, is a

dwarf spheroidal galaxy 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 as systems that are c ...

(has no rings, low

luminosity Luminosity is an absolute measure of radiated electromagnetic radiation, electromagnetic energy per unit time, and is synonymous with the radiant power emitted by a light-emitting object. In astronomy, luminosity is the total amount of electroma ...

, much

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

, little gas or dust), which is a satellite of the

Andromeda Galaxy The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a Galaxy#Isophotal diameter, D25 isop ...

(M31). It is one of the 14 known dwarf galaxies orbiting M31. It is relatively isolated, being about 1.8 million light-years (579 kpc) away. However, for an isolated dwarf galaxy it is also unusually quiescent. This suggests that Andromeda XVIII is a backsplash galaxy, a galaxy that once had a close orbital encounter with a more massive galaxy which stripped it of much of its star-forming matter. However, alternative hypotheses are also possible for Andromeda XVIII. It was announced in 2010 that the orbiting galaxies lie close to a plane running through M31's center.

References

Dwarf spheroidal galaxies Andromeda Subgroup Andromeda (constellation) {{Elliptical-galaxy-stub

See also

References