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Local Group

Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way

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.

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:

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.

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

Clickable map

lyMilky WayMilky WayNGC 6822NGC 6822Sextans BSextans BSextans ASextans ANGC 3109NGC 3109Antlia DwarfAntlia DwarfLeo ALeo ALeo I (dwarf galaxy)Leo I (dwarf galaxy)Leo II (dwarf galaxy)Canes DwarfCanes DwarfPhoenix DwarfPhoenix DwarfTucana DwarfTucana DwarfWolf-Lundmark-MelotteWolf-Lundmark-MelotteCetus DwarfCetus DwarfIC 1613IC 1613SagDIGSagDIGAquarius DwarfAquarius DwarfLGS 3LGS 3Pegasus DwarfPegasus DwarfAndromeda GalaxyAndromeda GalaxyTriangulum GalaxyTriangulum GalaxyNGC 185NGC 185NGC 147NGC 147M110M110IC 10IC 10M32Andromeda IIAndromeda IIAndromeda IIIAndromeda IIIAndromeda IAndromeda I
Local Group (clickable map)

List

Structure

Overview of the structure and trajectory of the Local Group

Streams

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 Messier 31 and the Milky Way being the predominant event in this process.[34]

A NASA conception of the collision using computer-generated imagery

Location

The Laniakea Supercluster (/ˌlɑːni.əˈkeɪ.ə/; Hawaiian for "open skies" or "immense heaven")[35] 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.
A diagram of our location in the observable universe. (Alternative image.)

See also

References

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  2. ^ The mass of the Local Group is essentially accounted for by the mass of the Milky Way and the Andromeda Galaxy. Estimates for the mass of each galaxy are compatible with 1012 M☉, and Peñarrubia et al. (2014) estimate (2.3±0.7)×1012 M☉ for the Local Group, but Karachentsev and Kashibadze (2006) estimate the somewhat lower value of (1.29±0.14)×1012 M☉.
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