Andromeda XXI (And 21, And XXI) is a moderately bright

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 ...

about away from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Andromeda. It is the fourth largest

Local Group The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a " dumbbell" shape; the Milky Way ...

dwarf spheroidal galaxy. The discovery arose from the first year data of a

photometric survey Photometry can refer to: * Photometry (optics), the science of measurement of visible light in terms of its perceived brightness to human vision * Photometry (astronomy), the measurement of the flux or intensity of an astronomical object's electro ...

of the M31/ M33 subgroupings of the Local Group by the

Pan-Andromeda Archaeological Survey Pan-Andromeda Archaeological Survey (PAndAS) is a large-scale astronomical survey using the Canada-France-Hawaii Telescope. The survey is exploring the structure and content of the Andromeda Galaxy (M31) and its neighbour, the Triangulum Galaxy ( ...

(PAndAS). This survey was conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope. This large

satellite A satellite or an artificial satellite is an object, typically a spacecraft, placed into orbit around a celestial body. They have a variety of uses, including communication relay, weather forecasting, navigation ( GPS), broadcasting, scient ...

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) has a half-light radius of roughly 1 kpc.Nicolas F. Martin, Alan W. McConnachie, Mike Irwin, Lawrence M. Widrow, Annette M. N. Ferguson, Rodrigo A. Ibata, John Dubinski, Arif Babul, Scott Chapman, Mark Fardal, Geraint F. Lewis, Julio Navarro, R. Michael Rich; "PAndAS' cubs: discovery of two new dwarf galaxies in the surroundings of the Andromeda and Triangulum galaxies";

The Astrophysical Journal ''The Astrophysical Journal'' (''ApJ'') is a peer-reviewed scientific journal of astrophysics and astronomy, established in 1895 by American astronomers George Ellery Hale and James Edward Keeler. The journal discontinued its print edition and ...

, Volume 705, Issue 1, pp. 758-765 (2009); ; ; ; Andromeda XXI appears as a spatial overdensity of stars. It has red giant branches at the distance of M31/M33, and follows metal-poor, e/H-1.8 when plotted in a color-magnitude diagram. Like other dwarf spheroidal galaxies, Andromeda XXI shows no sign of current

star formation Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...

and appears to have had 90% of its stars formed 5.8 billion years ago. Its central

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 ...

density is lower than expected from the

ΛCDM model The Lambda-CDM, Lambda cold dark matter, or ΛCDM model is a mathematical model of the Big Bang theory with three major components: # a cosmological constant, denoted by lambda (Λ), associated with dark energy; # the postulated cold dark mat ...

, but this could be explained if it lost most of its mass in a previous

tidal stripping Tidal stripping occurs when a larger galaxy pulls stars and other stellar material from a smaller galaxy because of strong tidal forces. An example of this scenario is the interacting pair of galaxies NGC 2207 and IC 2163, which are currently in t ...

event or had experience tidal shocks from an eccentric orbit. Although moderately bright (M_V = −9.1 ± 0.3), it has low

surface brightness In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on ...

. This indicates that numerous relatively luminous M31 satellites remain undiscovered.

References

{{DEFAULTSORT:Andromeda XXI Dwarf spheroidal galaxies Andromeda Subgroup Andromeda (constellation)

See also

References