Local Group

The Local Group is the galaxy group that includes the Milky Way. It has a total diameter of roughly 3 megaparsecs (9.8 Mly), and a total mass of the order of 2×1012 solar masses (4.0×1042 kg).[1] It consists of two clusters 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 clusters are separated by about 0.8 Mpc (2.5×1022 m) and are moving toward one another with a velocity of 123 km/s.[2] The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster. The total number of galaxies in the Local Group is unknown as some are occluded by the Milky Way; however, at least 80 such objects are known (most of which are dwarf galaxies).

Local Group of galaxies, including the massive members Messier 31 (Andromeda Galaxy) and Milky Way, as well as other nearby galaxies.
Distribution of the iron content (in logarithmic scale) in four neighbouring dwarf galaxies of the Milky Way

The two largest members, the Andromeda Galaxy and the Milky Way, are both spiral galaxies with masses of about 1012 solar masses each, and each have their own system of satellite galaxies:

The Triangulum Galaxy is the third-largest member of the Local Group, with a mass of approximately 5×1010 M☉, and is the third spiral galaxy.[5] It is unclear whether the Triangulum Galaxy is a companion of the Andromeda Galaxy, although the two galaxies 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.[6]

The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy, is uncertain due to extreme distances from the center of the Local Group.[2] 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.[7]

History

The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae.[8] 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

Map
Local Group (clickable map)

List of galactic bodies
Spiral galaxies
name type constellation notes
Andromeda Galaxy (M31, NGC 224) SA(s)b Andromeda Largest galaxy in the group[9]

Diameter: 220 kly

mass: (1.5 ± 0.5) × 10¹²M☀️

Number of stars:

(1.5±0.5)×1012 M☉ Number of stars: ca. 1012.

Milky Way Galaxy SBbc Sagittarius (centre)

Second Largest Galaxy in the group, May or may be the most massive galaxy of the group.[10]

Diameter: 175±25 kly

mass: (1.54±0.1)×1012 M☉

number of stars: (2.5±1.5)×1011.

Triangulum Galaxy (M33, NGC 598) SA(s)cd Triangulum Third largest, only unbarred spiral galaxy and possible satellite of the Andromeda Galaxy.

Diameter: 60 kly mass: 5×1010 M☉ number of stars: 4×1010.

Magellanic Spiral Galaxies
Large Magellanic Cloud (LMC) Irr/SB(s)m Dorado Fourth largest member of the group, satellite of Milky Way and only Magellanic Spiral Galaxy in the local group

mass: 1 × 10¹⁰M☀️ diameter: 14 kly

Elliptical galaxies
name type constellation notes
M32 (NGC 221) E2 Andromeda Satellite of the Andromeda Galaxy, Shows signs of a Supermassive Black Hole
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, 6th largest galaxy in the local group

mass: 7 × 10⁹

M☉

Canis Major Dwarf Irr Canis Major Satellite of Milky Way
Pisces Dwarf (LGS3) Irr Pisces Possible satellite of the Triangulum Galaxy
IC 1613 (UGC 668) IAB(s)m V Cetus
Phoenix Dwarf Irr Phoenix
Leo A (Leo III) IBm V Leo
Aquarius Dwarf (DDO 210) IB(s)m Aquarius Distance 3.2 million light years. Quite isolated in space, membership to Local Group established in 1999.[11]
Dwarf elliptical galaxies
name type constellation notes
M110 (NGC 205) dE6p Andromeda Satellite of the Andromeda Galaxy and 5th largest galaxy with the mass of 9.3 billion solar masses.
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.[11]
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
KKs 3 dSph Hydrus
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
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
SagDSG (Sagittarius Dwarf Spheroidal 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
Leo IV dSph Leo Satellite of the Milky Way
Leo V dSph Leo Satellite of the Milky Way
Leo T dSph/Irr Leo Satellite of the Milky Way
Boötes II dSph Boötes Satellite of the Milky Way
Boötes III dSph Boötes Satellite of the Milky Way
Coma Berenices dSph Coma Berenices Satellite of the Milky Way
Segue 2 dSph Aries Satellite of the Milky Way
Hercules dSph Hercules Satellite of the Milky Way
Pisces II dSph Pisces Satellite of the Milky Way
Reticulum II dSph Reticulum Satellite of the Milky Way
Eridanus II dSph Eridanus Probable satellite of the Milky Way
Grus dSph Grus Satellite of the Milky Way
Tucana II dSph Tucana Satellite of the Milky Way
Perseus I Dwarf Galaxy (Andromeda XXXIII) dE Perseus Satellite of Andromeda
Identification unclear
name type constellation notes
Virgo Stellar Stream dSph (remnant)? Virgo In the process of merging with the Milky Way
Willman 1 dSph or Globular Cluster Ursa Major 147,000 light-years away
UGCA 86 (0355+66) Irr, dE or S0 Camelopardalis
UGCA 92 (EGB0427+63) Irr or S0 Camelopardalis
Horologium dSph or Globular Cluster Horologium Satellite of the Milky Way. Not to be confused with the Horologium Supercluster.
Pictoris dSph or Globular Cluster Pictor Satellite of the Milky Way
Phoenix II dSph or Globular Cluster Phoenix Satellite of the Milky Way
Indus dSph or Globular Cluster Indus Satellite of the Milky Way
Eridanus III dSph or Globular Cluster Eridanus Satellite of the Milky Way
Probable non-members
name type constellation notes
Sextans B (UGC 5373) Ir+IV-V Sextans Now considered as member of Antlia-Sextans Group
NGC 3109 Ir+IV-V Hydra Now considered as member of Antlia-Sextans Group
Sextans A (UGCA 205) Ir+V Sextans Now considered as member of Antlia-Sextans Group
Antlia Dwarf dE3/dSph/Irr? Antlia Now considered as member of Antlia-Sextans Group
GR 8 (DDO 155) Im V Virgo Distance 7.9 million light years[12]
IC 5152 IAB(s)m IV Indus Distance 5.8 million light years, possibly an outlying member of the local group[13]
NGC 300 SA(s)d Sculptor Distance 6.07 million light years
NGC 55 SB(s)m Sculptor Distance 6.5 million light years
NGC 404 E0 or SA(s)0− Andromeda Distance 10 million light years[14]
Andromeda IV Irr Andromeda 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).[15]
NGC 1569 Irp+ III-IV Camelopardalis In IC 342 group of galaxies. Distance 11 million light years[16]
NGC 1560 (IC 2062) Sd Camelopardalis Distance 8-12 million light years
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
Objects in the Local Group no longer recognised as galaxies
name type constellation notes
Palomar 12 (Capricornus Dwarf) Capricornus Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 4 (Ursa Major Dwarf) Ursa Major Globular cluster formerly classified as a dwarf spheroidal galaxy
Palomar 3 (Sextans C) Sculptor Globular cluster formerly classified as a dwarf spheroidal galaxy[17]

Other objects
  • Smith's Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth[18] and 8,000 light years from the disk of the Milky Way galaxy[19]
  • 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 that is proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy
A diagram of our location in the observable universe. (Alternative image.)

See also

References
  1. 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 1023 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☉.
  2. 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. Bibcode:2006Ap.....49....3K. doi:10.1007/s10511-006-0002-6. S2CID 120973010.
  3. 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. ISSN 0004-637X. S2CID 43188686.
  4. 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. S2CID 118267222.
  5. "The Local Group". NASA's High Energy Astrophysics Science Archive Research Center (HEASARC). NASA. Retrieved 5 May 2015.
  6. Miller, Bryan W.; et al. (December 2001). "The Star Formation History of LGS 3". The Astrophysical Journal. 562 (2): 713–726. arXiv:astro-ph/0108408. Bibcode:2001ApJ...562..713M. doi:10.1086/323853. ISSN 0004-637X. S2CID 119089499.
  7. 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.; Wolf, Joe (2012). "The observed properties of dwarf galaxies in and around the Local Group". The Astronomical Journal. 144 (1): 4. arXiv:1204.1562. Bibcode:2012AJ....144....4M. doi:10.1088/0004-6256/144/1/4. S2CID 118515618.
  8. Hubble, E.P. (1936). The realm of the nebulae. Mrs. Hepsa Ely Silliman memorial lectures, 25. New Haven: Yale University Press. ISBN 9780300025002. OCLC 611263346. Alt URL(pp. 124–151)
  9. 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.; Wolf, Joe (2018). "Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions". The Astrophysical Journal. 873 (2): 118. arXiv:1804.11348. Bibcode:2019ApJ...873..118W. doi:10.3847/1538-4357/ab089f. S2CID 85463973.
  10. Watkins, Laura L.; van der Marel, Roeland P.; Sohn, Sangmo Tony; Evans, N. Wyn (8 February 2019). "Evidence for an Intermediate-Mass Milky Way from Gaia DR2 Halo Globular Cluster Motions". arXiv:1804.11348 [astro-ph]. arXiv:1804.11348. doi:10.3847/1538-4357/ab089f.
  11. 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. arXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. doi:10.1086/316548. S2CID 1805423.
  12. 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. The Stellar Content of Local Group Galaxies. 192. Astronomical Society of the Pacific. p. 218. Bibcode:1999IAUS..192..218T. ISBN 978-1886733824.
  13. Ziljstra, A. A.; Minniti, Dante (April 1999). "A Dwarf Irregular Galaxy at the Edge of the Local Group: Stellar Populations and Distance of IC 5152". Astronomical Journal. 117 (4): 1743–1757. arXiv:astro-ph/9812330. Bibcode:1999AJ....117.1743Z. doi:10.1086/300802. S2CID 14737502.
  14. 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. S2CID 551714.
  15. Nowakowski, Tomasz (22 December 2015). "Andromeda IV turns out to be a solitary gas-rich dwarf galaxy". physorg. Retrieved 25 December 2015.
  16. 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. S2CID 9877496.
  17. "Pal3". simbad.u-strasbg.fr. Retrieved 26 August 2017.
  18. Wakker, B. P.; York, D. G.; Wilhelm, R.; Barentine, J. C.; Richter, P.; Beers, T. C.; Ivezić, Ž.; Howk, J. C. (2008). "Distances to Galactic High‐Velocity Clouds. I. Cohen Stream, Complex GCP, Cloud g1". The Astrophysical Journal. 672 (1): 298–319. arXiv:0709.1926. Bibcode:2008ApJ...672..298W. doi:10.1086/523845. S2CID 12764246.
  19. "Massive Gas Cloud Speeding Toward Collision With Milky Way". Retrieved 6 June 2008.
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