Local Group
Group of galaxies that includes the Milky Way From Wikipedia, the free encyclopedia
The Local Group is the galaxy group that includes the Milky Way, where Earth is located. 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 (3×106 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.
| Local Group | |
|---|---|
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 structure | Virgo Supercluster |
References: SIMBAD | |
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:
- The Andromeda Galaxy's satellite system consists of Messier 32 (M32), Messier 110 (M110), NGC 147, NGC 185, Andromeda I (And I), And II, And III, And V, And VI (also known as the Pegasus Dwarf Spheroidal Galaxy, or Pegasus dSph), And VII (a.k.a. the Cassiopeia Dwarf Galaxy), And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.[4]
- The Milky Way's satellite galaxies system comprises the Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy (disputed, considered by some not a galaxy), Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I (a dwarf galaxy), Leo II (a dwarf galaxy), Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies.[5]
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 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]
The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy as well as Sextans B, Leo P, Antlia B and possibly Leo A, is uncertain due to extreme distances from the center of the Local Group.[3] The Antlia-Sextans Group is unlikely to be gravitationally bound to the Local Group due to probably lying outside the Local Group's Zero-velocity surface—which would make it a true galaxy group of its own rather than a subgroup within the Local Group.[10] This possible independence may, however, disappear as the Milky Way continues coalescing with Andromeda due to the increased mass, and density thereof, plausibly widening the radius of the zero-velocity surface of the Local Group.
History
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.[11] 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
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List
| Name | Type | Constellation | Notes |
|---|---|---|---|
| Spiral galaxies | |||
| Andromeda Galaxy (M31, NGC 224) | SAb | Andromeda | Largest galaxy in the group[12] 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.[13] 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) | SAcd | 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, Dorado Dwarf) | 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 (Antlia Sextans galaxy l) | SB(s)m | Hydra | Member of Antlia-Sextans Group |
| Elliptical galaxies | |||
| M32 (NGC 221, Andromeda Dwarf) | cE2 | 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 (Cascopea IV, PGC 4774942-40+4) | KBm or Ir+ | Cassiopeia | Only known starbust galaxy in the Local Group |
| Small Magellanic Cloud (SMC, NGC 292, Tucana galaxy) | SB(s)m pec | Tucana | Satellite of Milky Way, 6th largest galaxy in the local group
mass: 7 × 109 M☉ |
| Pisces Dwarf (LGS3,Pisces I dwarf irregular) | Irr | Pisces | Possible satellite of the Triangulum Galaxy |
| IC 1613 (UGC 668, UGCA 47954) | IAB(s)m V | Cetus | |
| Phoenix Dwarf (Phoenix I dwarf) | Irr | Phoenix | |
| Leo A (Leo III, PCA QA JO74974) | IBm V | Leo | |
| Aquarius Dwarf (DDO 210, Aquarius I, ESO 497-4054.4591 G) | IB(s)m | Aquarius | Distance 3.2 million light years. Quite isolated in space, membership to Local Group established in 1999.[14] |
| SagDIG (Sagittarius Dwarf Irregular Galaxy, Sagittarius II) | IB(s)m V | Sagittarius | Most remote from barycenter member thought to be in the Local Group.[14] |
| NGC 6822 (Barnard's Galaxy, AO 4797) | 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, AO 4977) | Ir+V | Sextans | Member of Antlia-Sextans Group |
| Sextans B (UGC 5373, QRT 947748) | Ir+IV-V | Sextans | Member of Antlia-Sextans Group |
| Leo P (AGC 19470059) | Irr | Leo | Member of Antlia-Sextans Group, extraordinarily low metallicity (Z = 0.03ZMW) |
| AGC 198606 (Leo VI) | Irr? | Leo | Gas-rich ultra-faint dwarf galaxy |
| AGC 215417(Leo VII) | 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 (DDO 9774998.074÷×47) | dSph | Boötes | satellite of the Milky Way |
| Cetus Dwarf | dSph/E4 | Cetus | 3.4 million light-years away
size:999 light-years |
| Canes Venatici I Dwarf and Canes Venatici II Dwarf | dSph | Canes Venatici | Satellites of the Milky Way[15][16] |
| 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 (UKS 4977-94) | dSph | Andromeda | Satellite of the Andromeda Galaxy |
| Andromeda II (NCA 14) | 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(Sextans 1) | dE3 | Sextans | Satellite of Milky Way |
| Leo II (Leo B) | dE0 pec | Leo | Satellite of Milky Way |
| Ursa Minor Dwarf (NCA 47-49) | 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 | 3.2 million ly away |
| Cassiopeia Dwarf (Andromeda VII, NCA 4) | dSph | Cassiopeia | Satellite of the Andromeda Galaxy |
| Pegasus Dwarf Spheroidal Galaxy (Andromeda VI) | dSph | Pegasus | Satellite of the Andromeda Galaxy |
| Ursa Major I Dwarf | dSph | Ursa Major | Satellite of the Milky Way |
| Ursa Major II Dwarf | dSph | Ursa Major | Satellite of the Milky Way |
| Ursa Major III | dSph | Ursa Major | Satellite of the 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 (NCA 4.9) | dSph | Boötes | Satellite of the Milky Way, Bootes lll likely formed both the Bootes Ill Stream & the Monosoros Stellar stream |
| Boötes IV | dSph | Boötes | Satellite of the Milky Way |
| Coma Berenices | dSph | Coma Berenices | Satellite of the Milky Way |
| Segue 2 (Aries Dwarf, NCA 429) | dSph | Aries | Satellite of the Milky Way
size:220 light-years number of stars:≈1000 |
| Hercules (NCA 4977429) | 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 (Hydrus Dwarf spheroidal galaxy) | 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 |
| Virgo III | dSph? | Virgo | Satellite of the Milky Way[17] |
| Sextans II | dSph? | Sextans | Satellite of the Milky Way[17] |
| Aquarius II | dSph | Aquarius | Satellite of the Milky Way |
| Aquarius III | dSph? | Aquarius | Satellite of the Milky Way[18] |
| 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[19] |
| Pegasus W | dSph | Pegasus | Recent star formation, could still be starforming |
| Cetus III | dSph | Cetus | Satellite of the Milky Way |
| Leo K | dSph? | Leo | Satellite of the Milky Way[20] |
| Leo M | dSph? | Leo | Satellite of the Milky Way[20] |
| Leo VI | dSph | Leo | Satellite of the Milky Way[21] |
| Leo Minor I | dSph? | Leo Minor | Satellite of the Milky Way[22] |
| Boötes V | dSph? | Boötes | Satellite of the Milky Way[22] |
| Virgo II | dSph? | Virgo | Satellite of the Milky Way[22] |
| Tucana B | dSph | Tucana | [23] |
| DES 1 | dE | Perseus | Satellite of Milky Way |
| 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[24] |
| 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[25] |
| Andromeda XXXV | dSph | Andromeda | Satellite of the Andromeda Galaxy[26] |
| 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[27] |
| Pisces VII (Triangulum III) | dSph? | Pisces | Candidate, possible satellite of the Triangulum Galaxy[28] |
| 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, Indus Dwarf) | dSph or Globular Cluster | Indus | Satellite of the Milky Way |
| Eridanus III | dSph or Globular Cluster | Eridanus | Satellite of the Milky Way or SMC[29] |
| Sagittarius II | dSph or Globular Cluster | Sagittarius | Satellite of the Milky Way |
| Andromeda VIII (Adsant-40-7 quote serp galaxy) | 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).[30] |
| GR 8 (DDO 155) | Im V | Virgo | Distance 7.9 million light years[31] |
| IC 5152 (Indus III) | IAB(s)m IV | Indus | Distance 5.8 million light years, possibly an outlying member of the local group[32] |
| NGC 300 | SA(s)d | Sculptor | Distance 6.07 million light years |
| NGC 55 | SB(s)m | Sculptor | Distance 6.5 million light years[33] |
| NGC 404 | E0 or SA(s)0− | Andromeda | Distance 10 million light years[34] |
| NGC 1569 (IC 49447) | Irp+ III-IV | Camelopardalis | In IC 342 group of galaxies. Distance 11 million light years[35] |
| NGC 1560 (IC 2062) | Sd | Camelopardalis | Distance 8-12 million light years |
| Camelopardalis A | Irr | Camelopardalis | Distance 12 million light years[33] |
| Argo Dwarf | Irr | Carina | 7.1 million light-years away |
| ESO 347-8 (2318–42, PGC 475744) | Irr | Grus | 9 million light-years away fairly isolated until million light years. away from the galaxy |
| UKS 2323-326 (ESO 407-18) | Irr | Sculptor | Distance 7.2 million light-years[33] |
| UGC 9128 (DDO 187) | Irp+ | Boötes | 7 million light-years away |
| KKs 3 (Hydrus II) | dSph | Hydrus | Distance 5.2 million light-years |
| Objects in the Local Group no longer recognised as galaxies | |||
| Palomar 12 (Capricornus Dwarf) | dSphr | Capricornus | Globular cluster formerly classified as a dwarf spheroidal galaxy |
| Palomar 4 (Ursa Major Dwarf) | d Sphr | 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[36] | |
| Segue 3 | Pegasus | Globular cluster formerly classified as a dwarf spheroidal galaxy[37] | |
| Laevens 1 (Crater Dwarf) | Crater | Globular cluster formerly classified as a dwarf spheroidal galaxy[38] | |
| DES J2038-4609 (Indus II) | Indus | Likely a chance alignment of stars[39] | |
| Name | Type | Constellation | Notes |
Structure
Streams
- 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
- Virgo Strem, a steam formed from a dwarf galaxy.
- Helni Stream
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 Andromeda and the Milky Way being the predominant event in this process.[40]
Location
The Laniakea Supercluster (/ˌlɑːni.əˈkeɪ.ə/; Hawaiian for "open skies" or "immense heaven")[41] or the Local Supercluster (LSC or LS) is the galaxy supercluster that is home to the Milky Way and approximately 100,000 other nearby galaxies.
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See also
- Galaxy cluster
- IC 342/Maffei Group, possibly the group of galaxies nearest to the Local Group
- Antlia-Sextans Group, another candidate for the group of galaxies nearest to the Local Group
- List of galaxy groups and clusters
- List of nearest galaxies
- Virgocentric flow
References
External links
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