Right ascension 23 54 47.7 Apparent magnitude (V) 9.9 ± 0.6 Constellation Andromeda | Declination +42° 28′ 15″ Type dSph | |
Distance 859 ± 51 kiloparsecs (2.80 ± 0.17 Mly) Notable features Satellite galaxy
of Andromeda People also search for Andromeda XI, Andromeda XIX | ||
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.
This large satellite of the Andromeda Galaxy (M31) has a half-light radius of nearly 1 kpc.
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 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.
Although moderately bright (MV=-9.9 ± 0.6), it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.