Local Group
From Wikipedia, the free encyclopedia
The Local Group is the group of galaxies that includes the Milky Way, among others. It comprises more than 54 galaxies, including numerous dwarf galaxies. Its gravitational center is located somewhere between the Milky Way and the Andromeda galaxies. The Local Group covers a diameter of 10 megalight-years (3.1 megaparsecs) (see 1 E+23 m for distance comparisons) and has a binary (dumbbell)[1] distribution. The group is estimated to have a total mass of 1.29±0.14 ×1012 M☉[1] and has a velocity dispersion of 61±8 km/s.[2] The group itself is part of the Virgo Supercluster (i.e. the Local Supercluster).[3]
The two most massive members of the group are the Milky Way and Andromeda galaxies. These two spiral galaxies each have a system of satellite galaxies.
- 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 I Dwarf and Ursa Major II Dwarf.
- Andromeda's satellite system consists of M32, M110, NGC 147, NGC 185, 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 XII, And XIII, And XIV, And XV, And XVI, And XVII, And XVIII, And XIX, And XX, And XXI, And XXII, And XXIII, And XXIV, And XXV, And XXVI, And XXVII, And XXVIII, and And XXIX.
- 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.
History
The term, "The Local Group" was introduced by Edwin Hubble in Chapter VI of his book The Realm of the Nebulae (Hubble 1936, pp. 124–151). 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 twelve to thirty-six by way of the discovery of almost two dozen low-luminosity galaxies.[5]
Component galaxies
Map
Local Group (clickable map)
Galactic bodies
| Spiral galaxies | |||
|---|---|---|---|
| name | type | constellation | notes |
| Milky Way | SBbc | Sagittarius (centre) | Largest, a recent study done by NASA and the BBC shows that the Milky Way is indeed more massive than the Andromeda Galaxy, even though Andromeda has roughly twice the number of stars.[6] |
| Andromeda Galaxy (M31, NGC 224) | SA(s)b | Andromeda | Second largest member of the group, recently (2006) discovered to also be a barred spiral. Proved to be less massive than the Milky Way in 2012.[7] |
| 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 | |
| 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 (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 |
| 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 | |
| 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 8000 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
.jpg)
