Andromeda XXI

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.

Andromeda XXI
Observation data
Constellation	Andromeda
Right ascension	23^h 54^m 47.7^s[1]
Declination	+42° 28′ 15″[1]
Distance	859 ± 51 kiloparsecs (2.80 ± 0.17 Mly)[2]
Apparent magnitude (V)	9.9 ± 0.6
Characteristics
Type	dSph
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 nearly 1 kpc.[2]

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 (M_V=-9.9 ± 0.6), it has low surface brightness. This indicates that numerous relatively luminous M31 satellites remain undiscovered.

See also

• Low surface brightness galaxy (LSB galaxy)
• Dwarf spheroidal galaxy (dSph)
• Satellite galaxy
• Andromeda Galaxy
• Andromeda constellation

References

1. "NAME Andromeda XXI". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2 April 2011.
2. 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, Volume 705, Issue 1, pp. 758-765 (2009); doi:10.1088/0004-637X/705/1/758; Bibcode: 2009ApJ...705..758M; arXiv:0909.0399;