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Local Group

Group of galaxies that includes the Milky Way From Wikipedia, the free encyclopedia

Local Group
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"Local group" redirects here. For other uses, see Local group (disambiguation).

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 (3×10^6 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.

Quick facts Observation data (Epoch J2000), Parent structure ...
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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:

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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 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.

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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.

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Component galaxies

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Local Group (clickable map)

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Structure

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Overview of the structure and trajectory of the Local Group











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 Stream, a stream formed from a dwarf galaxy.
  • Helmi 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.[41] There is debate over whether ellipticity might be the immediate structure of the combined galaxy right after the collision or whether ellipticity might only emerge after a theoretical intermediate period of retaining a spiralling structure directly following the collision. Some even theorise a permanent superspiral or a transition toward a more Lenticular galaxy, rather than a more elliptical or spiralled distribution, as the future of the Local Group's galactic merger.

A NASA conception of the collision using computer-generated imagery
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Location

Main article: Laniakea Supercluster
This paragraph is an excerpt from Laniakea Supercluster.[edit]
The Laniakea Supercluster (/ˌlɑːni.əˈkeɪ.ə/; Hawaiian for "open skies" or "immense heaven")[42] 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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A diagram of our location in the observable universe. (Alternative image.)
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See also

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