List of Andromeda's satellite galaxies

The Andromeda Galaxy (M31) has satellite galaxies just like the Milky Way. Orbiting M31 are at least 13 dwarf galaxies: the brightest and largest is M110, which can be seen with a basic telescope. The second-brightest and closest one to M31 is M32. The other galaxies are fainter, and were mostly discovered only starting from the 1970s.

The Andromeda Galaxy with M110 at upper left and M32 to the right of the core.

On January 11, 2006, it was announced that Andromeda Galaxy's faint companion galaxies lie on or close to a single plane running through the Andromeda Galaxy's center. This unexpected distribution is not obviously understood in the context of current models for galaxy formation. The plane of satellite galaxies points toward a nearby group of galaxies (M81 Group), possibly tracing the large-scale distribution of dark matter.

Table of known satellites

Andromeda Galaxy's satellites are listed here by discovery (orbital distance is not known). Andromeda IV is not included in the list, as it was discovered to be roughly 10 times further than Andromeda from the Milky Way in 2014, and therefore a completely unrelated galaxy.

Andromeda Galaxy's satellites
Name Type Distance from Sun
(million ly)		 Right Ascension** Declination** Absolute Magnitude[1] Magnitude Mass-to-light ratio Year
discovered		 Notes
M32 dE2 2.48  00h 42m 41.877s +40° 51′ 54.71″ +9.2 1749
M110 dE6 2.69  00h 40m 22.054s +41° 41′ 08.04″ +9.4 1773
NGC 185 dE5 2.01  00h 38m 57.523s +48° 20′ 14.86″ +11 1787
NGC 147 dE5 2.2  00h 33m 12.131s +48° 30′ 32.82″ +12 1829
Andromeda I dSph 2.43  00h 45m 39.264s +38° 02′ 35.17″ -11.8 +13.2 31 ± 6[1] 1970
Andromeda II dSph 2.13  01h 16m 28.136s +33° 25′ 50.36″ -12.6 +13 13 ± 3[1] 1970
Andromeda III dSph 2.44  00h 35m 31.777s +36° 30′ 04.19″ -10.2 +10.3 19 ± 12[1] 1970
Andromeda V dSph 2.52  01h 10m 16.952s +47° 37′ 40.12″ -9.6 +15.4 78 ± 50[2] 1998
Pegasus Dwarf Spheroidal
(Andromeda VI)		 dSph 2.55  23h 51m 46.516s +24° 34′ 55.69″ -11.5 +14.5 12 ± 5[2] 1998
Cassiopeia Dwarf
(Andromeda VII)		 dSph 2.49  23h 26m 33.321s +50° 40′ 49.98″ -13.3 +12.9 7.1 ± 2.8[1] 1998
Andromeda VIII dSph 2.7  00h 42m 06s +40° 37′ 00″ +9.1 2003
Andromeda IX dSph 2.5  00h 52m 52.493s +43° 11′ 55.66″ -8.3 +16.2 2004
Andromeda X dSph 2.9  01h 06m 34.740s +44° 48′ 23.31″ -8.1 +16.2 63 ± 40[1] 2005
Andromeda XI[3] dSph  00h 46m 20s +33° 48′ 05″ -7.3 2006
Andromeda XII[3] dSph  00h 47m 27s +33° 22′ 29″ -6.4 2006
Andromeda XIII[3] dSph  00h 51m 49.555s +33° 00′ 31.40″ -6.9 2006
Andromeda XIV[4] dSph  00h 41m 35.219s +29° 41′ 45.87″ -8.3 102 ± 71[1] 2007
Andromeda XV[4] dSph  01h 14m 18.7s +38° 07′ 02.9″ -9.4 2007
Andromeda XVI[4] dSph  00h 59m 29.843s +32° 22′ 27.96″ -9.2 2007
Andromeda XVII[4] dSph  00h 37m 07s +44° 19′ 20″ -8.5 2008
Andromeda XVIII[4] dSph/Sm  00h 02m 15.184s +45° 05′ 19.78″ 2008
Andromeda XIX[4] dSph  00h 19m 32.1s +35° 02′ 37.1″ -9.3 2008
Andromeda XX[4] dSph  00h 07m 30.530s +35° 07′ 45.94″ -6.3 2008
Andromeda XXI[4] dSph  23h 54m 47.7s +42° 28′ 15″ -9.9 2009
Andromeda XXII[4] dSph  00h 27m 40s +28° 05′ 25″ -7.0 2009
Andromeda XXIII[4] dIrr  01h 29m 21.944s +38° 43′ 05.97″ 2011
Andromeda XXIV[4]  01h 18m 30s +46° 21′ 58″ 2011
Andromeda XXV[4]  00h 30m 08.9s +46° 51′ 07″ 2011
Andromeda XXVI[4]  00h 23m 45.6s +47° 54′ 58″ 2011
Andromeda XXVII[4]  00h 37m 27.1s +45° 23′ 13″ 2011
Andromeda XXVIII[5] dSph  22h 32m 41.449s +31° 12′ 59.10″ 2011
Andromeda XXIX[5] dIrr  23h 58m 55.440s +30° 45′ 22.09″ 2011
Tidal Stream Northwest
(Tidal Stream E and F)[6]		  00h 20m 00s +46° 00′ 00″ 2009
Tidal Stream Southwest[6]  00h 30m 00s +37° 30′ 00″ 2009
Triangulum Galaxy*
(M33)		 SA(s)cd 2.59  01h 33m 50.883s +30° 39′ 36.54″ +6.27 1654? Exact distance and relation to Andromeda uncertain

* It is uncertain whether it is a companion galaxy of the Andromeda Galaxy.

** RA/DEC values marked in Italics are rough estimates.

See also

References
  1. The SPLASH Survey: Internal Kinematics, Chemical Abundances, and Masses of the Andromeda I, II, III, VII, X, and XIV Dwarf Spheroidal Galaxies, Kalirai, J. S. et al., 2010
  2. The scatter about the 'Universal' dwarf spheroidal mass profile: a kinematic study of the M31 satellites And V and And VI, Collins, M.L.M. et al., 2011.
  3. Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy, N. F. Martin et.al, 2006.
  4. Discovery of the Local Group Galaxies
  5. Moore, Nicole Casal (7 Nov 2011), "Newly found dwarf galaxies could help reveal the nature of dark matter", News Service, University of Michigan, retrieved 10 Dec 2011 (reprinted in R&D Magazine)
  6. New tidal streams found in Andromeda reveal history of galactic mergers
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