Loading AI tools
Dwarf spheroidal galaxy in constellation Andromeda From Wikipedia, the free encyclopedia
Andromeda XXI (And 21, And XXI) is a moderately bright dwarf spheroidal galaxy about 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly) away from the Sun in the constellation Andromeda. It is the fourth largest Local Group dwarf spheroidal galaxy. The discovery arose from the first year data of a photometric survey of the M31/M33 subgroupings of the Local Group by the Pan-Andromeda Archaeological Survey (PAndAS). This survey was conducted with the Megaprime/MegaCam wide-field camera mounted on the Canada-France-Hawaii Telescope.
Andromeda XXI | |
---|---|
Observation data (J2000 epoch) | |
Constellation | Andromeda |
Right ascension | 23h 54m 47.7s[1] |
Declination | +42° 28′ 15″[1] |
Distance | 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly)[2] |
Apparent magnitude (V) | 15.5 ± 0.3[3] |
Absolute magnitude (V) | −9.1 ± 0.3[3] |
Characteristics | |
Type | dSph |
Half-light radius (physical) | 1005±175 pc[3] |
Half-light radius (apparent) | 4.1+0.8 −0.4′[3] |
Notable features | Satellite galaxy of Andromeda |
Other designations | |
Andromeda XXI, And XXI, And 21 |
This large satellite of the Andromeda Galaxy (M31) has a half-light radius of roughly 1 kpc.[2]
Andromeda XXI appears as a spatial overdensity of stars. It has red giant branches at the distance of M31/M33, and follows metal-poor, [Fe/H]=-1.8 when plotted in a color-magnitude diagram.
Like other dwarf spheroidal galaxies, Andromeda XXI shows no sign of current star formation and appears to have had 90% of its stars formed 5.8 billion years ago.[3] Its central dark matter density is lower than expected from the ΛCDM model, but this could be explained if it lost most of its mass in a previous tidal stripping event or had experience tidal shocks from an eccentric orbit.[3]
Although moderately bright (MV = −9.1 ± 0.3),[3] it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.