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; 9×1019kilometres),[1] and a total mass of the order of 2×1012solar 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 5×1010M☉ (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 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.
Second largest galaxy in the group, which may or may not be the most massive galaxy of the group.[13] Diameter (D25 isophote): 87,400 light-years Mass: (1.54±0.1)×1012M☉ Number of stars: (2.5±1.5)×1011.
Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy. Diameter (D25 isophote): 60,000 light-years Mass: 5×1010M☉ Number of stars: 4×1010.
Fourth largest member of the group, satellite of Milky Way and only Magellanic Spiral Galaxy in the local group Mass: 1×1010M☉ Diameter (D25 isophote): 32,200 light-years
Once considered to be associated with M31. Its distance is now known to be 22 to 24 million light years (not close to the Andromeda Galaxy at all).[27]
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
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.[37]
^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 1012M☉,
and Peñarrubia et al. (2014) estimate (2.3±0.7)×1012M☉ for the Local Group,
but Karachentsev and Kashibadze (2006) estimate the somewhat lower value of (1.29±0.14)×1012M☉.
^Kalirai, Jason S.; Beaton, Rachael L.; Geha, Marla C.; Gilbert, Karoline M.; Guhathakurta, Puragra; Kirby, Evan N.; Majewski, Steven R.; Ostheimer, James C.; Patterson, Richard J. (17 February 2010). "The Splash Survey: Internal Kinematics, Chemical Abundances, and Masses of the Andromeda I, Ii, III, Vii, X, and Xiv Dwarf Spheroidal Galaxies". The Astrophysical Journal. 711 (2): 671–692. arXiv:0911.1998. Bibcode:2010ApJ...711..671K. doi:10.1088/0004-637X/711/2/671. ISSN0004-637X. S2CID43188686.
^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. 805 (2): 130. arXiv:1503.02079. Bibcode:2015ApJ...805..130K. doi:10.1088/0004-637X/805/2/130. S2CID118267222.
^"The Local Group". NASA's High Energy Astrophysics Science Archive Research Center (HEASARC). NASA. Retrieved 5 May 2015.
^Zucker, D. B.; Belokurov, V.; Evans, N. W.; Wilkinson, M. I.; Irwin, M. J.; Sivarani, T.; Hodgkin, S.; Bramich, D. M.; Irwin, J. M.; Gilmore, G.; Willman, B.; Vidrih, S.; Fellhauer, M.; Hewett, P. C.; Beers, T. C.; Bell, E. F.; Grebel, E. K.; Schneider, D. P.; Newberg, H. J.; Wyse, R. F. G.; Rockosi, C. M.; Yanny, B.; Lupton, R.; Smith, J. A.; Barentine, J. C.; Brewington, H.; Brinkmann, J.; Harvanek, M.; Kleinman, S. J.; Krzesinski, J. (2006). "A New Milky Way Dwarf Satellite in Canes Venatici". The Astrophysical Journal. 643 (2): L103. arXiv:astro-ph/0604354. Bibcode:2006ApJ...643L.103Z. doi:10.1086/505216. S2CID119421888.
^Martin, N. F.; De Jong, J. T. A.; Rix, H. W. (2008). "A Comprehensive Maximum Likelihood Analysis of the Structural Properties of Faint Milky Way Satellites". The Astrophysical Journal. 684 (2): 1075–1092. arXiv:0805.2945. Bibcode:2008ApJ...684.1075M. doi:10.1086/590336. S2CID17838966.
^ abHomma, Daisuke; Chiba, Masashi; Komiyama, Yutaka; Tanaka, Masayuki; Okamoto, Sakurako; Tanaka, Mikito; Ishigaki, Miho N; Hayashi, Kohei; Arimoto, Nobuo; Lupton, Robert H; Strauss, Michael A; Miyazaki, Satoshi; Wang, Shiang-Yu; Murayama, Hitoshi (8 June 2024). "Final results of the search for new Milky Way satellites in the Hyper Suprime-Cam Subaru Strategic Program survey: Discovery of two more candidates". Publications of the Astronomical Society of Japan. arXiv:2311.05439. doi:10.1093/pasj/psae044. ISSN0004-6264.
^Tolstoy, Eline (1999). "Detailed Star-Formation Histories of Nearby Dwarf Irregular Galaxies using HST". In Patricia Whitelock; Russell Cannon (eds.). The stellar content of Local Group galaxies, Proceedings of the 192nd symposium of the International Astronomical Union. Vol. 192. Astronomical Society of the Pacific. p. 218. Bibcode:1999IAUS..192..218T. ISBN978-1886733824.
^Jensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; et al. (February 2003). "Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations". Astrophysical Journal. 583 (2): 712–726. arXiv:astro-ph/0210129. Bibcode:2003ApJ...583..712J. doi:10.1086/345430. S2CID551714.
^Grocholski, Aaron J.; Aloisi, Alessandra; van der Marel, Roeland P.; Mack, Jennifer; et al. (20 October 2008). "A New Hubble Space Telescope Distance to NGC 1569: Starburst Properties and IC 342 Group Membership". Astrophysical Journal Letters. 686 (2): L79–L82. arXiv:0808.0153. Bibcode:2008ApJ...686L..79G. doi:10.1086/592949. S2CID9877496.
^"Pal3". simbad.u-strasbg.fr. Retrieved 26 August 2017.
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