Local Group

The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly 3 megaparsecs (10 million light-years; 9×1019 kilometres),[1] and a total mass of the order of 2×1012 solar masses (4×1042 kg).[2] It consists of two collections of galaxies in a "dumbbell" shape; the Milky Way and its satellites form one lobe, and the Andromeda Galaxy and its satellites constitute the other. The two collections are separated by about 800 kiloparsecs (3×10^6 ly; 2×1019 km) and are moving toward one another with a velocity of 123 km/s.[3] The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster. The exact number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 members are known, most of which are dwarf galaxies.

For other uses, see Local group (disambiguation).
Not to be confused with Observable universe.
Local Group
Local Group of galaxies, including the massive members Messier 31 (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
Local Group of galaxies, including the massive members Messier 31 (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
Observation data (Epoch J2000)
Parent structureVirgo Supercluster
Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way

The two largest members, the Andromeda and the Milky Way galaxies, are both spiral galaxies with masses of about 1012 solar masses each. Each has its own system of satellite galaxies:

Visual size comparison of the six largest Local Group galaxies, with details

The Triangulum Galaxy (M33) is the third-largest member of the Local Group, with a mass of approximately 5×1010 M☉ (1×1041 kg), and is the third spiral galaxy.[6] It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy; the two galaxies are 750,000 light years apart,[7] and experienced a close passage 2–4 billion years ago which triggered star formation across Andromeda's disk. The Pisces Dwarf Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.[8]

The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy, is uncertain due to extreme distances from the center of the Local Group.[3] The other members of the group are likely gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf Galaxy, Leo A, Tucana Dwarf Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte, Aquarius Dwarf Galaxy, and Sagittarius Dwarf Irregular Galaxy.[9]

History

The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.[10] There, he described it as "a typical small group of nebulae which is isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147. He also identified IC 10 as a possible part of the Local Group.

Component galaxies

Map
Local Group (clickable map)
Map of the Milky Way with directions of nearby galaxies marked (not to scale)

List
Properties of galactic bodies in and around the Local Group[9]
Name Type Constellation Notes
Spiral galaxies
Andromeda Galaxy (M31, NGC 224) SA(s)b Andromeda Largest galaxy in the group[11]
Diameter (D25 isophote): 152,000 light-years
Mass: (1.5±0.5)×1012 M☉
Number of stars: ca. 1012.
Milky Way Galaxy SBbc Sagittarius (centre) Second largest galaxy in the group, which may or may not be the most massive galaxy of the group.[12]
Diameter (D25 isophote): 87,400 light-years
Mass: (1.54±0.1)×1012 M☉
Number of stars: (2.5±1.5)×1011.
Triangulum Galaxy (M33, NGC 598) SA(s)cd Triangulum Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy.
Diameter (D25 isophote): 60,000 light-years
Mass: 5×1010 M☉
Number of stars: 4×1010.
Magellanic spiral galaxies
Large Magellanic Cloud (LMC) Irr/SB(s)m Dorado Fourth largest member of the group, satellite of Milky Way and only Magellanic Spiral Galaxy in the local group
Mass: 1×1010 M☉
Diameter (D25 isophote): 32,200 light-years
NGC 3109 SB(s)m Hydra Member of Antlia-Sextans Group
Elliptical galaxies
M32 (NGC 221) E2 Andromeda Satellite of the Andromeda Galaxy, shows signs of a supermassive black hole
Irregular galaxies
Wolf–Lundmark–Melotte (WLM, DDO 221) Ir+ Cetus Possible size between Small Magellanic Cloud and Large Magellanic Cloud
IC 10 KBm or Ir+ Cassiopeia
Small Magellanic Cloud (SMC, NGC 292) SB(s)m pec Tucana Satellite of Milky Way, 6th largest galaxy in the local group

mass: 7 × 109 M☉

Pisces Dwarf (LGS3) Irr Pisces Possible satellite of the Triangulum Galaxy
IC 1613 (UGC 668) IAB(s)m V Cetus
Phoenix Dwarf Irr Phoenix
Leo A (Leo III) IBm V Leo
Aquarius Dwarf (DDO 210) IB(s)m Aquarius Distance 3.2 million light years. Quite isolated in space, membership to Local Group established in 1999.[13]
SagDIG (Sagittarius Dwarf Irregular Galaxy) IB(s)m V Sagittarius Most remote from barycenter member thought to be in the Local Group.[13]
NGC 6822 (Barnard's Galaxy) IB(s)m IV-V Sagittarius
Pegasus Dwarf (Pegasus Dwarf Irregular, DDO 216) Irr Pegasus
UGC 4879 (VV124) IAm Ursa Major One of the most isolated galaxies in Local Group. Situated at the edge of the Local Group.
Sextans A (UGCA 205) Ir+V Sextans Member of Antlia-Sextans Group
Sextans B (UGC 5373) Ir+IV-V Sextans Member of Antlia-Sextans Group
Leo P Irr Leo Member of Antlia-Sextans Group, extraordinarily low metallicity (Z = 0.03ZMW)
AGC 198606 Irr? Leo Gas-rich ultra-faint dwarf galaxy
AGC 215417 Irr? Leo Gas-rich ultra-faint dwarf galaxy
AGC 219656 Irr? Leo Gas-rich ultra-faint dwarf galaxy
AGC 249525 Irr? Boötes Gas-rich ultra-faint dwarf galaxy, Situated at the edge of the Local Group
AGC 268069 Irr? Serpens Gas-rich ultra-faint dwarf galaxy
Dwarf elliptical galaxies
M110 (NGC 205) dE6p Andromeda Satellite of the Andromeda Galaxy and 5th largest galaxy with the mass of 9.3 billion solar masses.
NGC 147 (DDO 3) dE5 pec Cassiopeia Satellite of the Andromeda Galaxy
Dwarf spheroidal galaxies
Boötes I dSph Boötes
Cetus Dwarf dSph/E4 Cetus
Canes Venatici I Dwarf and Canes Venatici II Dwarf dSph Canes Venatici Satellites of the Milky Way[14][15]
Andromeda III dE2 Andromeda Satellite of the Andromeda Galaxy
NGC 185 dE3 pec Cassiopeia Satellite of the Andromeda Galaxy
Andromeda I dE3 pec Andromeda Satellite of the Andromeda Galaxy
Sculptor Dwarf (E351-G30) dE3 Sculptor Satellite of Milky Way
Andromeda V dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda II dE0 Andromeda Satellite of the Andromeda Galaxy
Fornax Dwarf (E356-G04) dSph/E2 Fornax Satellite of Milky Way
Carina Dwarf (E206-G220) dE3 Carina Satellite of Milky Way
Leo I (DDO 74) dE3 Leo Satellite of Milky Way
Sextans Dwarf dE3 Sextans Satellite of Milky Way
Leo II (Leo B) dE0 pec Leo Satellite of Milky Way
Ursa Minor Dwarf dE4 Ursa Minor Satellite of Milky Way
Draco Dwarf (DDO 208) dE0 pec Draco Satellite of Milky Way
SagDSG (Sagittarius Dwarf Spheroidal Galaxy) dSph/E7 Sagittarius Satellite of Milky Way
Tucana Dwarf dE5 Tucana
Cassiopeia Dwarf (Andromeda VII) dSph Cassiopeia Satellite of the Andromeda Galaxy
Pegasus Dwarf Spheroidal Galaxy (Andromeda VI) dSph Pegasus Satellite of the Andromeda Galaxy
Ursa Major I Dwarf and Ursa Major II Dwarf dSph Ursa Major Satellite of Milky Way
Leo IV dSph Leo Satellite of the Milky Way
Leo V dSph Leo Satellite of the Milky Way
Leo T dSph/Irr Leo Satellite of the Milky Way
Boötes II dSph Boötes Satellite of the Milky Way
Boötes III dSph Boötes Satellite of the Milky Way
Boötes IV dSph Boötes Satellite of the Milky Way
Coma Berenices dSph Coma Berenices Satellite of the Milky Way
Segue 2 dSph Aries Satellite of the Milky Way
Hercules dSph Hercules Satellite of the Milky Way
Pisces II dSph Pisces Satellite of the Milky Way
Reticulum II dSph Reticulum Satellite of the Milky Way
Reticulum III dSph Reticulum Satellite of the Milky Way
Eridanus II dSph Eridanus Probable satellite of the Milky Way
Grus I dSph Grus Satellite of the Milky Way
Grus II dSph Grus Satellite of the Milky Way
Tucana II dSph Tucana Satellite of the Milky Way
Hydrus I dSph Hydrus Satellite of the Milky Way
Draco II dSph Draco Satellite of the Milky Way
Carina III dSph Carina Satellite of the Milky Way
Triangulum II (Laevens 2) dSph Triangulum Satellite of the Milky Way
Carina II dSph Carina Satellite of the Milky Way
Pictor II dSph Pictor Satellite of the Milky Way
Horologium II dSph Horologium Satellite of the Milky Way
Virgo I dSph Virgo Satellite of the Milky Way
Aquarius II dSph Aquarius Satellite of the Milky Way
Crater II dSph Crater Satellite of the Milky Way
Hydra II dSph Hydra Satellite of the Milky Way
Antlia II dSph Antlia Satellite of the Milky Way
Pegasus III dSph Pegasus Satellite of the Milky Way
Pegasus IV dSph Pegasus Satellite of the Milky Way[16]
Pegasus W dSph Pegasus Recent star formation, could still be starforming
Cetus III dSph Cetus Satellite of the Milky Way
Tucana B dSph Tucana [17]
Perseus I Dwarf Galaxy (Andromeda XXXIII) dE Perseus Satellite of Andromeda
Antlia Dwarf dE3/dSph/Irr? Antlia Member of Antlia-Sextans Group
Andromeda IX dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda X dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XI dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XII dSph Andromeda Possible satellite of the Andromeda Galaxy
Andromeda XIII (Pisces III) dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XIV (Pisces IV) dSph Pisces Possible satellite of the Andromeda Galaxy
Andromeda XV dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XVII dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XIX dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XX dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XXI dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XXII dSph Pisces Possible satellite of the Triangulum Galaxy
Andromeda XXIII dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XXIV dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XXV dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XXVI dSph Andromeda Satellite of the Andromeda Galaxy
Andromeda XXVII dSph Andromeda Satellite of the Andromeda Galaxy, tidally disrupted[18]
Andromeda XXIX dSph Pegasus Satellite of the Andromeda Galaxy
Andromeda XXX (Cassiopeia II) dSph? Cassiopeia Satellite of the Andromeda Galaxy
Andromeda XXXI (Lacerta I) dSph? Lacerta Satellite of the Andromeda Galaxy
Andromeda XXXII (Cassiopeia III) dSph? Cassiopeia Satellite of the Andromeda Galaxy
Andromeda XXXIV (Pegasus V) dSph Pegasus Satellite of the Andromeda Galaxy[19]
Andromeda XVI (Pisces V) dSph Pisces Possible satellite of the Andromeda Galaxy
Andromeda XXVIII dSph? Pegasus Possible satellite of the Andromeda Galaxy
Andromeda XXXIII (Perseus I) dSph? Perseus Possible satellite of the Andromeda Galaxy
Andromeda XVIII dSph Andromeda
Centaurus I dSph Centaurus Satellite of the Milky Way[20]
Pisces VII (Triangulum III) dSph? Pisces Candidate, possible satellite of the Triangulum Galaxy[21]
Identification unclear
Virgo Stellar Stream dSph (remnant)? Virgo In the process of merging with the Milky Way
Canis Major Dwarf Irr? Canis Major Possibly a dwarf galaxy in the process of merging with the Milky Way
Hydra 1 Hydra Possibly a dwarf galaxy in the process of merging with the Milky Way
Tucana III dSph or cluster? Tucana Satellite of the Milky Way, tidally disrupting
Tucana IV dSph or cluster? Tucana Satellite of the Milky Way
Tucana V dSph or cluster? Tucana Possibly non-existent
Columba I dSph or cluster? Columba Satellite of the Milky Way
Segue 1 dSph or Globular Cluster Leo Satellite of the Milky Way
Cetus II Cetus Likely part of Sagittarius tidal stream
Willman 1 dSph or Globular Cluster Ursa Major 147,000 light-years away
Horologium I dSph or Globular Cluster Horologium Satellite of the Milky Way. Not to be confused with the Horologium Supercluster.
Pictoris dSph or Globular Cluster Pictor Satellite of the Milky Way
Phoenix II dSph or Globular Cluster Phoenix Satellite of the Milky Way
Indus I (Kim 2) dSph or Globular Cluster Indus Satellite of the Milky Way
Eridanus III dSph or Globular Cluster Eridanus Satellite of the Milky Way or SMC[22]
Sagittarius II dSph or Globular Cluster Sagittarius Satellite of the Milky Way
Andromeda VIII dSph? Andromeda Satellite of the Andromeda Galaxy, tidally disrupting
Antlia B Antlia Member of Antlia-Sextans Group
Probable non-members
Andromeda IV Irr Andromeda Once considered to be associated with M31. Its distance is now known to be 22 to 24 million light years (not close to the Andromeda Galaxy at all).[23]
GR 8 (DDO 155) Im V Virgo Distance 7.9 million light years[24]
IC 5152 IAB(s)m IV Indus Distance 5.8 million light years, possibly an outlying member of the local group[25]
NGC 300 SA(s)d Sculptor Distance 6.07 million light years
NGC 55 SB(s)m Sculptor Distance 6.5 million light years[26]
NGC 404 E0 or SA(s)0− Andromeda Distance 10 million light years[27]
NGC 1569 Irp+ III-IV Camelopardalis In IC 342 group of galaxies. Distance 11 million light years[28]
NGC 1560 (IC 2062) Sd Camelopardalis Distance 8-12 million light years
Camelopardalis A Irr Camelopardalis Distance 12 million light years[26]
Argo Dwarf Irr Carina
ESO 347-8 (2318–42) Irr Grus
UKS 2323-326 (ESO 407-18) Irr Sculptor Distance 7.2 million light-years[26]
UGC 9128 (DDO 187) Irp+ Boötes
KKs 3 dSph Hydrus
Objects in the Local Group no longer recognised as galaxies
Palomar 12 (Capricornus Dwarf) Capricornus Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 4 (Ursa Major Dwarf) Ursa Major Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 5 (Serpens Dwarf) Serpens Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 3 (Sextans C) Sculptor Globular cluster formerly classified as a dwarf spheroidal galaxy[29]
Segue 3 Pegasus Globular cluster formerly classified as a dwarf spheroidal galaxy[30]
Laevens 1 (Crater Dwarf) Crater Globular cluster formerly classified as a dwarf spheroidal galaxy[31]
DES J2038-4609 (Indus II) Indus Likely a chance alignment of stars[32]

Future

The galaxies of the Local Group are likely to merge together under their own mutual gravitational attractions over a timescale of tens of billions of years into a single elliptical galaxy, with the coalescence of Messier 31 and the Galaxy being the predominant event in this process.[33]

Other objects
  • Magellanic Stream, a stream of gas being stripped off the Magellanic Clouds due to their interaction with the Milky Way
  • Monoceros Ring, a ring of stars around the Milky Way that is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy
A diagram of our location in the observable universe. (Alternative image.)

See also

References
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  2. The mass of the Local Group is essentially accounted for by the mass of the Milky Way and the Andromeda Galaxy. Estimates for the mass of each galaxy are compatible with 1012 M☉, and Peñarrubia et al. (2014) estimate (2.3±0.7)×1012 M☉ for the Local Group, but Karachentsev and Kashibadze (2006) estimate the somewhat lower value of (1.29±0.14)×1012 M☉.
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