Local Group
The Local Group is the group of galaxies that includes our galaxy, the Milky Way. The group comprises over 35 galaxies, with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy. The galaxies of the Local Group cover a 10 million light-year diameter (see 1 E+22 m for distance comparisons) and have a binary (dumbbell)[1] shape. The group is estimated to have a total mass of (1.29 ± 0.14)×1012M☉.[1] The group itself is one of many within the Virgo Supercluster (i.e. the Local Supercluster).[2]
The two most massive members of the group are the Milky Way and the Andromeda Galaxy. These two barred spirals each have a system of satellite galaxies.
- The Milky Way's satellite system consists of 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 Dwarf.
- Andromeda's satellite system comprises M32, M110, NGC 147, NGC 185, And I, And II, And III, And IV, And V, Pegasus dSph, Cassiopeia Dwarf, And VIII, And IX, and And X.
- The Triangulum Galaxy, the third largest and only ordinary spiral galaxy in the Local Group, may or may not be a companion to the Andromeda galaxy but probably has Pisces Dwarf as a satellite.
The other members of the group are gravitationally secluded from these large subgroups: IC1613, Phoenix Dwarf, Leo A, Tucana Dwarf, Cetus Dwarf, Pegasus Dwarf Irregular, Wolf-Lundmark-Melotte, Aquarius Dwarf, and Sagittarius Dwarf Irregular.
History
Edwin Hubble was the first to identify the Local Group in Chapter VI of his book The Realm of Nebulae (Hubble 1936, pp. 124-151). There he describes it as "a typical small group of nebulae which is isolated in the general field." In that book, he delineated by decreasing luminosity its members to be M31, the Milky Way, M33, the Large Magellanic Cloud, the 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. In the ~70 years since his work, the number of known Local Group members has increased from his initial 12 to 36 in 2003. This was by way of almost two dozen low-luminosity galaxies.[3]
Component galaxies
Map
Galactic bodies
| Spiral galaxies | |||
|---|---|---|---|
| name | type | constellation | notes |
| Milky Way | SBbc | n/a | Second largest, though possibly most massive galaxy in the group. |
| Andromeda Galaxy (M31, NGC 224) | SA(s)b | Andromeda | Largest member of the group, recently (2006) discovered to also be a barred spiral. May be less massive than the Milky Way. |
| Triangulum Galaxy (M33, NGC 598) | SAc | Triangulum | only ordinary spiral galaxy and possible satellite of the Andromeda Galaxy? |
| Elliptical galaxies | |||
| name | type | constellation | notes |
| M110 (NGC 205) | E6p | Andromeda | satellite of the Andromeda Galaxy |
| 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 | 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 |
| 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.[4] |
| 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 Dwarf | dSph | Boötes | |
| Cetus Dwarf | dSph/E4 | Cetus | |
| Canes Venatici Dwarf | dSph | Canes Venatici | |
| 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 | 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 (And VI) | dSph | Pegasus | satellite of the Andromeda Galaxy |
| Ursa Major Dwarf | dSph | Ursa Major | satellite of 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 |
| UGC-A 86 (0355+66) | Irr, dE or S0 | Camelopardalis | |
| UGC-A 92 (EGB0427+63) | Irr or S0 | Camelopardalis | |
| 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 | |
| 2318-42 | Irr | Grus | |
| UKS 2323-326 | Irr | Sculptor | |
| UGC 9128 (DDO 187) | Irp+ | Boötes | |
| 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 | |
| Sextans C | |||
Other notable objects
- Smith's Cloud - High velocity cloud, between 32,000 and 49,000 light years from Earth[5], and 8000 light years the disk of the Milky Way Galaxy[6].
- HVC 127-41-330 - High velocity cloud, 2.3 million light-years from Earth
See also
References
- ^ a b Karachentsev, I. D.; Kashibadze, O. G. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics. 49 (1): 3–18. doi:10.1007/s10511-006-0002-6.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ R. B. Tully (1982). "The Local Supercluster". Astrophysical Journal. 257: 389–422. doi:10.1086/159999.
- ^ 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
{{citation}}: CS1 maint: date and year (link) - ^ 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
{{citation}}: CS1 maint: date and year (link) - ^ Wakker, B. P. (2008). "Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1". The Astrophysical Journal. 672: 298. 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. doi:10.1086/316548.
| Location | Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow (→) may be read as "within" or "part of". |  | |||||
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| Structure |
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| Satellite galaxies | |||||||
| Related | |||||||
| Included | Earth → Solar System → Local Interstellar Cloud → Local Bubble → Gould Belt → Orion Arm → Milky Way → Milky Way subgroup → Local Group → Local Sheet → Virgo Supercluster → Laniakea Supercluster → Local Hole → Observable universe → Universe Each arrow (→) may be read as "within" or "part of". |
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| Related | |