Local Group
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Distribution of the iron content (in logarithmic scale) in four dwarf neighbouring galaxies of the Milky Way
The Local Group is the galaxy group that includes the Milky Way. The Local Group comprises more than 54 galaxies, most of them dwarf galaxies. Its gravitational center is located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group covers a diameter of 10 Mly (3.1 Mpc) (about 1023 meters) and has a binary (dumbbell)[1] distribution. The group itself is a part of the larger Virgo Supercluster, which in turn may be a part of the Laniakea Supercluster.
The two most massive members of the group are the Milky Way and Andromeda galaxies.[2] These two spiral galaxies have each a system of satellite galaxies.
- Milky Way's satellite galaxies system comprises Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf, Ursa Minor Dwarf, Draco Dwarf, Carina Dwarf, Sextans Dwarf, Sculptor Dwarf, Fornax Dwarf, Leo I, Leo II, and Ursa Major I Dwarf and Ursa Major II Dwarf, plus several additional ultra-faint dwarf spheroidal galaxies.[3]
- 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 IV, And V, Pegasus dSph (aka And VI), Cassiopeia Dwarf (aka And VII), And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.
- The Triangulum Galaxy, the third largest and only unbarred spiral galaxy in the Local Group, may or may not be a companion to the Andromeda Galaxy. Pisces Dwarf is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.[4]
- The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf, is uncertain due to extreme distances from the center of the Local Group.
- The other members of the group are gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf–Lundmark–Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.
History
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.[5] There, he described it as "a typical small group of nebulae which is isolated in the general field". He 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 Local Group member. By 2003, the number of known Local Group members has increased from his initial 12 to 36 by way of the discovery of almost two dozen low-luminosity galaxies.[6]
Component galaxies
Map
Local Group (clickable map)
List of galactic bodies
| Spiral galaxies | |||
|---|---|---|---|
| name | type | constellation | notes |
| Milky Way | SBbc | Sagittarius (centre) | Where we reside; the second largest galaxy in the group.[7] |
| Andromeda Galaxy (M31, NGC 224) | SA(s)b | Andromeda | ~220 kly in diameter, it is the largest and most massive galaxy in the group. |
| Triangulum Galaxy (M33, NGC 598) | SA(s)cd | Triangulum | Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy. |
| Elliptical galaxies | |||
| name | type | constellation | notes |
| M32 (NGC 221) | E2 | Andromeda | satellite of the Andromeda Galaxy |
| Irregular galaxies | |||
| name | type | constellation | notes |
| Wolf–Lundmark–Melotte (WLM, DDO 221) | Ir+ | Cetus | |
| IC 10 | KBm or Ir+ | Cassiopeia | |
| Small Magellanic Cloud (SMC, NGC 292) | SB(s)m pec | Tucana | satellite of Milky Way |
| Canis Major Dwarf | Irr | Canis Major | satellite of Milky Way |
| Pisces Dwarf (LGS3) | Irr | Pisces | satellite of the Triangulum Galaxy? |
| IC 1613 (UGC 668) | IAB(s)m V | Cetus | |
| Phoenix Dwarf | Irr | Phoenix | |
| Large Magellanic Cloud (LMC) | Irr/SB(s)m | Dorado | Fourth largest member of the group, satellite of Milky Way |
| Leo A (Leo III) | IBm V | Leo | |
| Sextans B (UGC 5373) | Ir+IV-V | Sextans | |
| NGC 3109 | Ir+IV-V | Hydra | |
| Sextans A (UGCA 205) | Ir+V | Sextans | |
| Dwarf elliptical galaxies | |||
| name | type | constellation | notes |
| M110 (NGC 205) | dE6p | Andromeda | satellite of the Andromeda Galaxy |
| NGC 147 (DDO 3) | dE5 pec | Cassiopeia | satellite of the Andromeda Galaxy |
| SagDIG (Sagittarius Dwarf Irregular Galaxy) | IB(s)m V | Sagittarius | Most remote from barycenter member thought to be in the Local Group.[8] |
| NGC 6822 (Barnard's Galaxy) | IB(s)m IV-V | Sagittarius | |
| Pegasus Dwarf (Pegasus Dwarf Irregular, DDO 216) | Irr | Pegasus | |
| Dwarf spheroidal galaxies | |||
| name | type | constellation | notes |
| Boötes I | dSph | Boötes | |
| Cetus Dwarf | dSph/E4 | Cetus | |
| Canes Venatici I Dwarf and Canes Venatici II Dwarf | dSph | Canes Venatici | |
| KKs 3 | dSph | ||
| 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 |
| Antlia Dwarf | dE3/dSph/Irr? | Antlia | |
| 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 |
| SagDEG (Sagittarius Dwarf Elliptical 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 |
| 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 |
| Eridanus II | dSph | Eridanus | satellite of the Milky Way |
| Grus | dSph | Grus | satellite of the Milky Way |
| Tucana II | dSph | Tucana | satellite of the Milky Way |
| Identification Unclear | |||
| name | type | constellation | notes |
| Virgo Stellar Stream | dSph (remnant)? | Virgo | In the process of merging with the Milky Way |
| Willman 1 | dwarf Spherical galaxy or Globular cluster? |
Ursa Major | 147,000 light-years away |
| Andromeda IV | Irr? | Andromeda | probably not a galaxy |
| UGCA 86 (0355+66) | Irr, dE or S0 | Camelopardalis | |
| UGCA 92 (EGB0427+63) | Irr or S0 | Camelopardalis | |
| Horologium | 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 | dSph or Globular Cluster | Indus | satellite of the Milky Way |
| Eridanus III | dSph or Globular Cluster | Eridanus | satellite of the Milky Way |
| Probable non-members | |||
| name | type | constellation | notes |
| GR 8 (DDO 155) | Im V | Virgo | |
| IC 5152 | IAB(s)m IV | Indus | |
| NGC 55 | SB(s)m | Sculptor | |
| Aquarius Dwarf (DDO 210) | Im V | Aquarius | |
| NGC 404 | E0 or SA(s)0− | Andromeda | |
| NGC 1569 | Irp+ III-IV | Camelopardalis | |
| NGC 1560 (IC 2062) | Sd | Camelopardalis | |
| Camelopardalis A | Irr | Camelopardalis | |
| Argo Dwarf | Irr | Carina | |
| ESO 347-8 (2318–42) | Irr | Grus | |
| UKS 2323-326 | Irr | Sculptor | |
| UGC 9128 (DDO 187) | Irp+ | Boötes | |
| Sextans C | |||
| objects in local group no longer recognised as galaxies | |||
| name | type | constellation | notes |
| Palomar 12 (Capricornus Dwarf) | Capricornus | a globular cluster formerly classified as a dwarf spheroidal galaxy | |
| Palomar 4 (originally designated Ursa Major Dwarf) | Ursa Major | a globular cluster formerly classified as a dwarf spheroidal galaxy | |
Other notable objects
- Smith's Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth,[9] and 8,000 light years from the disk of the Milky Way Galaxy[10]
- HVC 127-41-330, a high-velocity cloud, 2.3 million light-years from Earth
- Monoceros Ring, a ring of stars around the Milky Way which is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy
Diagram
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A diagram of our location in the observable universe. (Click here for smaller image.)
See also
- Galaxy cluster
- List of nearest galaxies
- List of galaxy clusters
- IC 342/Maffei Group, the group of galaxies nearest to the Local Group.
- Local Supercluster
- List of Andromeda's satellite galaxies
- List of Milky Way's satellite galaxies
References
- ↑ Karachentsev, I. D.; Kashibadze, O. G.0[ (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics 49 (1): 3–18. Bibcode:2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6.
- ↑ "The Local Group". NASA's High Energy Astrophysics Science Archive Research Center (HEASARC). Retrieved 2015-05-05.
- ↑ Sergey E. Koposov, Vasily Belokurov, Gabriel Torrealba, N. Wyn Evans (10 March 2015). "Beasts of the Southern Wild. Discovery of a large number of Ultra Faint satellites in the vicinity of the Magellanic Clouds". The Astrophysical Journal. arXiv:1503.02079. Bibcode:2015arXiv150302079K.
- ↑ Miller, Bryan W.; et al. (December 2001), "The Star Formation History of LGS 3", The Astrophysical Journal 562 (2): 713–726, arXiv:astro-ph/0108408, Bibcode:2001ApJ...562..713M, doi:10.1086/323853
- ↑ Hubble, E.P. (1936). The realm of the nebulae (PDF). Mrs. Hepsa Ely Silliman memorial lectures, 25. New Haven: Yale University Press. ISBN 9780300025002. OCLC 611263346. Archived from the original on 2012-09-29.(pp. 124–151)
- ↑ van den Bergh, Sidney (May 2003). "History of the Local Group". To be published in: "The Local Group as an Astrophysical Laboratory" (Cambridge University Press): 5042. arXiv:astro-ph/0305042. Bibcode:2003astro.ph..5042V.
- ↑ http://www.atlasoftheuniverse.com/galaxies.html
- ↑ van den Bergh, Sidney (April 2000). "Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548.
- ↑ Wakker, B. P.; York, D. G.; Wilhelm, R.; Barentine, J. C.; Richter, P.; Beers, T. C.; Ivezić, Ž.; Howk, J. C. (2008). "Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1". The Astrophysical Journal 672 (1): 298–319. arXiv:0709.1926. Bibcode:2008ApJ...672..298W. doi:10.1086/523845.
- ↑ "Massive Gas Cloud Speeding Toward Collision With Milky Way". Retrieved 2008-06-06.
External links
 |
Wikimedia Commons has media related to Local Group. |
- The Local Group of Galaxies, SEDS Messier pages
- A Survey of the Resolved Stellar Content of Nearby Galaxies Currently Forming Stars, Lowell Observatory
- van den Bergh, Sidney (2000). "Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific 112 (770): 529–536. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548.
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