Andromeda XXI

"Andromeda 21" redirects here. For the star, see 21 Andromedae.

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 (M_V = −9.1 ± 0.3),^[3] it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.

See also

• Low surface brightness galaxy (LSB galaxy)