Galaxy group

A galaxy group[2] or group of galaxies[3] (GrG[4]) is an aggregation of galaxies comprising about 50 or fewer gravitationally bound members, each at least as luminous as the Milky Way (about 1010 times the luminosity of the Sun); collections of galaxies larger than groups that are first-order clustering are called galaxy clusters.[5] The groups and clusters of galaxies can themselves be clustered, into superclusters of galaxies.

The Milky Way galaxy is part of a group of galaxies called the Local Group.[6]

Hubble views bizarre cosmic quartet HCG 16
Four of the seven members of galaxy group HCG 16.[1]


Groups of galaxies are the smallest aggregates of galaxies. They typically contain no more than 50 galaxies in a diameter of 1 to 2 megaparsecs (Mpc).[NB 1] Their mass is approximately 1013 solar masses. The spread of velocities for the individual galaxies is about 150 km/s. However, this definition should be used as a guide only, as larger and more massive galaxy systems are sometimes classified as galaxy groups.[7]

Groups are the most common structures of galaxies in the universe, comprising at least 50% of the galaxies in the local universe. Groups have a mass range between those of the very large elliptical galaxies and clusters of galaxies.[8] In the local universe, about half of the groups exhibit diffuse X-ray emissions from their intracluster media. Those that emit X-rays appear to have early-type galaxies as members. The diffuse X-ray emissions come from zones within the inner 10-50% of the groups' virial radius, generally 50-500 kpc.[9]


There are several subtypes of groups.

Compact Groups

A compact group consists of a small number of galaxies, typically around five, in close proximity and relatively isolated from other galaxies and formations.[10] The first compact group to be discovered was Stephan's Quintet, found in 1877.[11] Stephan's Quintet is named for a compact group of four galaxies plus an unassociated foreground galaxy.[10] Astronomer Paul Hickson created a catalogue of such groups in 1982, the Hickson Compact Groups.[12]

Compact groups of galaxies readily show the effect of dark matter, as the visible mass is greatly less than that needed to gravitationally hold the galaxies together in a bound group. Compact galaxy groups are also not dynamically stable over Hubble time, thus showing that galaxies evolve by merger, over the timescale of the age of the universe.[10]

Fossil Groups

Fossil galaxy groups, fossil groups, or fossil clusters are believed to be the end-result of galaxy merging within a normal galaxy group, leaving behind the X-ray halo of the progenitor group. Galaxies within a group interact and merge. The physical process behind this galaxy-galaxy merger is dynamical friction. The time-scales for dynamical friction on luminous (or L*) galaxies suggest that fossil groups are old, undisturbed systems that have seen little infall of L* galaxies since their initial collapse. Fossil groups are thus an important laboratory for studying the formation and evolution of galaxies and the intragroup medium in an isolated system. Fossil groups may still contain unmerged dwarf galaxies, but the more massive members of the group have condensed into the central galaxy.[9][10]

The closest fossil group to the Milky Way is NGC 6482, an elliptical galaxy at a distance of approximately 180 million light-years located in the constellation of Hercules.[13]


Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters.[14] These contain galaxies and protogalaxies embedded in dark matter haloes that are in the process of fusing into group-formations of singular dark matter halos.[15]


Notable groups
Group Notes
Local Group The group where the Milky Way, including the Earth, is located
Stephan's Quintet One of the most photogenic groups
Bullet Group The merging group exhibits separation of dark matter from normal matter
This lists some of the most notable groups; for more groups, see the list article.

See also


  1. ^ see 1022 m for distance comparisons


  1. ^ "Hubble views a bizarre cosmic quartet". Retrieved 19 June 2015.
  2. ^ Bärbel Koribalski (2004). "The NGC 6221/15 Galaxy Group".
  3. ^ Hartmut Frommert & Christine Kronberg. "Groups and Clusters of Galaxies with Messier objects". SEDS.
  4. ^ "Object classification in SIMBAD". SIMBAD. November 2013.
  5. ^ L.S. Sparke & J.S. Gallagher (2007). Galaxies in the Universe: an Introduction (2nd ed.). Cambridge University Press. p. 278. ISBN 9780521671866.
  6. ^ Mike Irwin. "The Local Group". Retrieved 2009-11-07.
  7. ^ UTK Physics Dept. "Groups of Galaxies". University of Tennessee, Knoville. Retrieved September 27, 2012.
  8. ^ Muñoz, R. P.; Motta, V.; Verdugo, T.; Garrido, F.; et al. (11 December 2012). "Dynamical analysis of strong-lensing galaxy groups at intermediate redshift". Astronomy & Astrophysics (published April 2013). 552: 18. arXiv:1212.2624. Bibcode:2013A&A...552A..80M. doi:10.1051/0004-6361/201118513. A80.
  9. ^ a b Mulchaey, John S. (22 September 2000). "X-ray Properties of Groups of Galaxies". Annual Review of Astronomy and Astrophysics (published 2000). 38: 289–335. arXiv:astro-ph/0009379. Bibcode:2000ARA&A..38..289M. doi:10.1146/annurev.astro.38.1.289.
  10. ^ a b c d Paul Hickson (1997). "Compact Groups of Galaxies". Annual Review of Astronomy and Astrophysics. 35: 357–388. arXiv:astro-ph/9710289. Bibcode:1997ARA&A..35..357H. doi:10.1146/annurev.astro.35.1.357.
  11. ^ M. Stephan (April 1877). "Nebulæ (new) discovered and observed at the observatory of Marseilles, 1876 and 1877, M. Stephan". Monthly Notices of the Royal Astronomical Society. 37 (6): 334. Bibcode:1877MNRAS..37..334S. doi:10.1093/mnras/37.6.334.
  12. ^ Hickson, Paul (April 1982). "Systematic properties of compact groups of galaxies". Astrophysical Journal, Part 1. 255: 382–391. Bibcode:1982ApJ...255..382H. doi:10.1086/159838.
  13. ^ An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482
  14. ^ Yujin Yang (2008). Testing Both Modes of Galaxy Formation: A Closer Look at Galaxy Mergers and Gas Accretion. University of Arizona. ProQuest. p. 205. ISBN 9780549692300.
  15. ^ C. Diener; S. J. Lilly; C. Knobel; G. Zamorani; et al. (9 October 2012). "Proto-groups at 1.8<z<3 in the zCOSMOS-deep sample". The Astrophysical Journal (published March 2013). 765 (2): 11. arXiv:1210.2723. Bibcode:2013ApJ...765..109D. doi:10.1088/0004-637X/765/2/109. 109.CS1 maint: Multiple names: authors list (link)
HCG 87

HCG 87 is a compact group of galaxies listed in the Hickson Compact Group Catalogue. This group is about 400 million light-years away in the constellation Capricornus.

The group distinguishes itself as one of the most compact groups of galaxies, hosting two active galactic nuclei and a starburst among its three members, all of which show signs of interaction. This interaction, which astronomers have called visually, and scientifically, intriguing is being examined to understand the influence of active nuclei on star formation histories.

JKCS 041

JKCS 041 is a group of galaxies with the distinction of being the farthest away galaxy group from Earth ever observed, as of 2009. It is estimated to be 9.9 billion light years away, seen at redshift 1.9. The cluster is located in the constellation Cetus at a photometrically determined redshift of z=1.9 at right ascension 2h 26m 44s declination −04° 41′ 37″ (J2000.0). There are at least 19 members in the cluster.

Local Group

The Local Group is the galaxy group that includes the Milky Way. The Local Group comprises more than 54 galaxies, most of them dwarf galaxies. Its gravitational center is located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group has a diameter of 10 Mly (3.1 Mpc) (about 1023 meters) and has a binary (dumbbell)

distribution. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster.

The three largest members of the group (in descending order) are the Andromeda Galaxy, the Milky Way and the significantly smaller Triangulum Galaxy. The larger two of these spiral galaxies each have their own system of satellite galaxies.

The Andromeda Galaxy's satellite system consists of Messier 32 (M32), Messier 110 (M110), NGC 147, NGC 185, Andromeda I (And I), And II, And III, And V, And VI (also known as Pegasus Dwarf Spheroidal Galaxy, or Pegasus DSph), And VII (also known as Cassiopeia Dwarf Galaxy), And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.

The Milky Way's satellite galaxies system comprises Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy (disputed, considered by some not a galaxy), Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I (a dwarf galaxy), Leo II (a dwarf galaxy), and Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies.

The Triangulum Galaxy may or may not be a companion to the Andromeda Galaxy. Pisces Dwarf Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.

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.

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.

NGC 1090

NGC 1090 is a barred spiral galaxy located in the constellation Cetus.

NGC 1090 has a pseudo inner ring. The disc has a very low surface brightness.

This galaxy has been the site of two known supernovae (in 1962 and 1971).

NGC 1090 is not part of a galaxy group, even though it appears close to NGC 1087, M-77 (NGC 1068), NGC 1055, NGC 1073, and five other small irregular galaxies.

The distance to NGC 1090 is approximately 124 million light years and its diameter is about 144,000 light years.

NGC 125

NGC 125 (also known as PGC 1772) is a lenticular galaxy located in the constellation Pisces. It is designated as subclass Sa Ring in the galaxy morphological classification scheme. It lies approximately 235 million light-years away.

NGC 128

NGC 128 is a lenticular galaxy in the constellation Pisces. It is approximately 190 million light-years from earth and has a diameter of about 165,000 light years.

NGC 2775

NGC 2775 (also known as Caldwell 48) is an unbarred spiral galaxy with a prominent ring structure in the constellation Cancer. This galaxy has a bulge and multiple spiral arms, on which few HII regions can be detected, implying recent star formation. It was discovered by William Herschel in 1783.

NGC 2775 is the most prominent galaxy in a small galaxy group known as NGC 2775 group, part of the Virgo Supercluster, along with the Local Group. Other members of the NGC 2775 group include NGC 2777 and UGC 4781.SN1993z is the only supernova known to have occurred in NGC 2775 and was a Type Ia on Sept. 23rd that year at a magnitude of 13.9. By the 25th, spectra that was obtained showed that it had peaked four weeks earlier.

NGC 4027

NGC 4027 (also known as Arp 22) is a barred spiral galaxy approximately 83 million light-years away in the constellation Corvus. It is also a peculiar galaxy because one of its spiral arms goes out more than the other. This is probably due to a galactic collision in NGC 4027's past.

NGC 4244

NGC 4244, also Caldwell 26, is an edge-on loose Spiral galaxy and Caldwell object in the constellation Canes Venatici. It is part of the M94 Group (the Canes Venatici I Group), a galaxy group relatively close to the Local Group containing the Milky Way. It shines at magnitude +10.2/+10.6. Its celestial cooridinates are RA 12h 17.5m, dec +37° 49′. It is located near a naked-eye G-class star Beta Canum Venaticorum, barred spiral galaxy NGC 4151, and irregular galaxy NGC 4214. The galaxy lies approximately 6.5 million/14 million light years away, with a redshift of +243/493 km/s. A nuclear star cluster and halo is located at the centre of this galaxy.

NGC 4261

NGC 4261 is an elliptical galaxy located around 100 million light-years away in the constellation Virgo. The galaxy is a member of its own galaxy group known as the NGC 4261 group.The active galactic nucleus (AGN) contains a 400-million-solar mass supermassive black hole (SMBH) with an 800-light-year-wide spiral-shaped disk of dust fueling it.The galaxy is estimated to be about 60 thousand light-years across, and a jet eminating from it is estimated to span about 88 thousand light-years.

NGC 4631

NGC 4631 (also known as the Whale Galaxy or Caldwell 32) is a barred spiral galaxy in the constellation Canes Venatici. This galaxy's slightly distorted wedge shape gives it the appearance of a herring or a whale, hence its nickname. Because this nearby galaxy is seen edge-on from Earth, professional astronomers observe this galaxy to better understand the gas and stars located outside the plane of the galaxy.

NGC 4945

NGC 4945 is a barred spiral galaxy in the constellation Centaurus, visible near the star Xi Centauri. The galaxy was discovered by James Dunlop in 1826 and is thought to be similar to the Milky Way Galaxy, although X-ray observations show that NGC 4945 has an unusual energetic Seyfert 2 nucleus that might house a supermassive black hole. This object has an estimated mass of 1.4+1.4−0.7×106 M☉.

NGC 5371

NGC 5371 is a face-on spiral galaxy in the constellation Canes Venatici. NGC 5371 (which also seems to be known as NGC 5390) is a symmetrical face-on Sbc barred spiral galaxy at a distance of 100 million light years. This galaxy with Hickson Galaxy Group 68 makes up the Big Lick Galaxy Group.

NGC 584

NGC 584 is an elliptical galaxy in the constellation Cetus. The galaxy was discovered on 10 September 1785 by the German-British astronomer William Herschel.

It is about 20 megaparsecs (60 million light-years) distant. NGC 584 belongs at the NGC 584 galaxy group, which also includes the galaxies NGC 596, NGC 600, NGC 615 and NGC 636.

NGC 596

NGC 596 is an elliptical galaxy in the constellation Cetus. The galaxy lies 65 million light years away from Earth, which means, given its apparent dimensions, that NGC 596 is approximately 60,000 light years across. The galaxy shows an outer envelope and is a merger remnant. The surface brightness profil is smooth and featureless. The galaxy hosts a supermassive black hole, whose mass is estimated to be 170 millions (108.24) M ⊙ {\displaystyle {\begin{smallmatrix}M_{\odot }\end{smallmatrix}}} .

NGC 596 belongs at the NGC 584 galaxy group, which also includes the galaxies NGC 584, which lies 25 minutes to the northwest, NGC 600, NGC 615 and NGC 636.

The galaxy is included in the Herschel 400 Catalogue. It lies about 2 and half degrees northeast from theta Ceti.

NGC 6215

NGC 6215 (also known as PGC 59112) is a spiral galaxy located in the constellation Ara. It is designated as SA(s)c in the galaxy morphological classification scheme. It was discovered by astronomer John Herschel on 9 July 1836.

NGC 6221

NGC 6221 (also known as PGC 59175) is a barred spiral galaxy located in the constellation Ara. It is designated as SB(s)bc in the galaxy morphological classification scheme and was discovered by British astronomer John Herschel on 3 May 1835. NGC 6221 is located at about 69 million light years from earth.

NGC 7320

NGC 7320 is a spiral galaxy in the Stephan's Quintet. However, it is not an actual member of the galaxy group, but a much closer line-of-sight galaxy at a distance of about 40 million light years. Other galaxies of Stephan's Quintet are some 300 million light-year distant.

NGC 80

NGC 80 is a lenticular galaxy located in the constellation Andromeda. It is interacting with NGC 47 and NGC 68, and is the brightest cluster galaxy of the NGC 80 group, a galaxy group named after it.

Active nuclei
Energetic galaxies
Low activity
See also

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