Galaxy group

Last updated • 3 min readFrom Wikipedia, The Free Encyclopedia
Four of the seven members of galaxy group HCG 16 Hubble views bizarre cosmic quartet HCG 16.jpg
Four of the seven members of galaxy group HCG 16

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.

Contents

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

Characteristics

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, accounting for 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]

Types

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] This hypothesis is supported by studies of computer simulations of cosmological volumes. [13]

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. [14]

Proto-groups

Proto-groups are groups that are in the process of formation. They are the smaller form of protoclusters. [15] 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. [16]

List

Notable groups
GroupNotes
Local Group The group where the Milky Way, including the Earth, is located
Stephan's Quintet One of the most photogenic groups
Robert's Quartet Another very notable group
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

Notes

  1. see 1022 m for distance comparisons

Related Research Articles

<span class="mw-page-title-main">Galaxy</span> Large gravitationally bound system of stars and interstellar matter

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

<span class="mw-page-title-main">Galaxy groups and clusters</span> Largest known gravitationally bound object in universe; aggregation of galaxies

Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large-scale structure of the Universe. In models for the gravitational formation of structure with cold dark matter, the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called superclusters.

<span class="mw-page-title-main">Globular cluster</span> Spherical collection of stars

A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".

<span class="mw-page-title-main">Andromeda Galaxy</span> Barred spiral galaxy in the Local Group

The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a D25 isophotal diameter of about 46.56 kiloparsecs (152,000 light-years) and is approximately 765 kpc (2.5 million light-years) from Earth. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology.

<span class="mw-page-title-main">Virgo Supercluster</span> Galactic supercluster containing the Virgo Cluster

The Local Supercluster is a defined supercluster containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At least 100 galaxy groups and clusters are located within its diameter of 33 megaparsecs. The Virgo SC is one of about 10 million superclusters in the observable universe and is in the Pisces–Cetus Supercluster Complex, a galaxy filament.

In cosmology and physics, cold dark matter (CDM) is a hypothetical type of dark matter. According to the current standard model of cosmology, Lambda-CDM model, approximately 27% of the universe is dark matter and 68% is dark energy, with only a small fraction being the ordinary baryonic matter that composes stars, planets, and living organisms. Cold refers to the fact that the dark matter moves slowly compared to the speed of light, giving it a vanishing equation of state. Dark indicates that it interacts very weakly with ordinary matter and electromagnetic radiation. Proposed candidates for CDM include weakly interacting massive particles, primordial black holes, and axions.

<span class="mw-page-title-main">Stephan's Quintet</span> Visual grouping of five galaxies

Stephan's Quintet is a visual grouping of five galaxies of which four form the first compact galaxy group ever discovered. The group, visible in the constellation Pegasus, was discovered by Édouard Stephan in 1877 at the Marseille Observatory. The group is the most studied of all the compact galaxy groups. The brightest member of the visual grouping is NGC 7320, which has extensive H II regions, identified as red blobs, where active star formation is occurring.

<span class="mw-page-title-main">Sombrero Galaxy</span> Galaxy in the constellation Virgo

The Sombrero Galaxy is a peculiar galaxy of unclear classification in the constellation borders of Virgo and Corvus, being about 9.55 megaparsecs from the Milky Way galaxy. It is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster. It has an isophotal diameter of approximately 29.09 to 32.32 kiloparsecs, making it slightly bigger in size than the Milky Way.

<span class="mw-page-title-main">Dwarf galaxy</span> Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

A Hickson Compact Group is a collection of galaxies designated as published by Paul Hickson in 1982.

<span class="mw-page-title-main">NGC 5170</span> Edge-on spiral galaxy in the constellation Virgo

NGC 5170 is a large, nearby, edge-on spiral galaxy in the equatorial constellation of Virgo. It was discovered on February 7, 1785 by William Herschel. This galaxy is located at a distance of 83.5 million light years and is receding at a heliocentric radial velocity of 1,502 km/s. It is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster.

<span class="mw-page-title-main">NGC 7319</span> Galaxy in the constellation Pegasus

NGC 7319 is a highly distorted barred spiral galaxy that is a member of the compact Stephan's Quintet group located in the constellation Pegasus, some 311 megalight-years distant from the Milky Way. The galaxy's arms, dust and gas have been highly disturbed as a result of the interaction with the other members of the Quintet. Nearly all of the neutral hydrogen has been stripped from this galaxy, most likely as a result of a collision with NGC 7320c some 100 million years ago. A pair of long, parallel tidal tails extend southward from NGC 7319 in the direction of NGC 7320c, and is undergoing star formation.

<span class="mw-page-title-main">NGC 4449</span> Galaxy in the constellation Canes Venatici

NGC 4449, also known as Caldwell 21, is an irregular Magellanic type galaxy in the constellation Canes Venatici, being located about 13 million light-years away. It is part of the M94 Group or Canes Venatici I Group that is relatively close to the Local Group hosting our Milky Way galaxy.

The Eridanus Group, sometimes called the Eridanus Cloud, is a nearby loose grouping of galaxies at a mean distance of approximately 75 Mly in the constellation Eridanus. Redshift values show that there are approximately 200 galaxies associated with the group, approximately 70% of which are spiral and irregular type galaxies while the remaining 30% are elliptical and lenticular types.

<span class="mw-page-title-main">Type-cD galaxy</span> Galaxy morphology classification

The type-cD galaxy is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy. Characterized by a large halo of stars, they can be found near the centres of some rich galaxy clusters. They are also known as supergiant ellipticals or central dominant galaxies.

<span class="mw-page-title-main">NGC 6388</span> Globular cluster in the constellation Scorpius

NGC 6388 is a globular cluster of stars located in the southern constellation of Scorpius. The cluster was discovered by Scottish astronomer James Dunlop on May 13, 1826 using a 20 cm (9 in) reflector telescope. It was later determined to be a globular cluster by English astronomer John Herschel, who was able to resolve it into individual stars. NGC 6388 is located at a distance of approximately 35,600 light-years (10.90 kpc) from the Sun. Due to its apparent visual magnitude of +6.8, binoculars or a small telescope are required to view it.

<span class="mw-page-title-main">NGC 3311</span> Galaxy in the constellation Hydra

NGC 3311 is a super-giant elliptical galaxy located about 190 million light-years away in the constellation Hydra. The galaxy was discovered by astronomer John Herschel on March 30, 1835. NGC 3311 is the brightest member of the Hydra Cluster and forms a pair with NGC 3309 which along with NGC 3311, dominate the central region of the Hydra Cluster.

<span class="mw-page-title-main">Copeland Septet</span> A group of galaxies in the constellation Leo

The Copeland Septet is a group of galaxies in the constellation Leo that includes NGC 3748, NGC 3754, NGC 3750, NGC 3751, NGC 3745, NGC 3753 and NGC 3746. The group was discovered by British astronomer Ralph Copeland in 1874. The location of Copeland's Septet is right ascension 11h 37m 50s / declination +21° 59′ (2000.0), about three degrees northwest of third magnitude star 93 Leonis.

<span class="mw-page-title-main">NGC 720</span> Galaxy in the constellation Cetus

NGC 720 is an elliptical galaxy located in the constellation Cetus. It is located at a distance of circa 80 million light years from Earth, which, given its apparent dimensions, means that NGC 720 is about 110,000 light years across. It was discovered by William Herschel on October 3, 1785. The galaxy is included in the Herschel 400 Catalogue. It lies about three and a half degrees south and slightly east from zeta Ceti.

<span class="mw-page-title-main">NGC 536</span> Galaxy in the constellation Andromeda

NGC 536 is a barred spiral galaxy located in the constellation Andromeda. It is located at a distance of circa 200 million light-years from Earth, which, given its apparent dimensions, means that NGC 536 is about 180,000 light years across. It was discovered by William Herschel on September 13, 1784. It is a member of Hickson Compact Group 10, which also includes the galaxies NGC 529, NGC 531, and NGC 542. It belongs to the Perseus–Pisces Supercluster.

References

  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. Sparke, L. S. & Gallagher, J. S. (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, Knoxville. 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. 552 (published April 2013): 18. arXiv: 1212.2624 . Bibcode:2013A&A...552A..80M. doi:10.1051/0004-6361/201118513. S2CID   17865754. A80.
  9. 1 2 Mulchaey, John S. (22 September 2000). "X-ray Properties of Groups of Galaxies". Annual Review of Astronomy and Astrophysics. 38 (published 2000): 289–335. arXiv: astro-ph/0009379 . Bibcode:2000ARA&A..38..289M. doi:10.1146/annurev.astro.38.1.289. S2CID   1427929.
  10. 1 2 3 4 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. S2CID   18870169.
  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. Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations
  14. An old galaxy group: Chandra X-ray observations of the nearby fossil group NGC 6482
  15. Yujin Yang (2008). Testing Both Modes of Galaxy Formation: A Closer Look at Galaxy Mergers and Gas Accretion. University of Arizona. p. 205. ISBN   9780549692300.
  16. 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. 765 (2) (published March 2013): 11. arXiv: 1210.2723 . Bibcode:2013ApJ...765..109D. doi:10.1088/0004-637X/765/2/109. S2CID   17700696. 109.{{cite journal}}: CS1 maint: multiple names: authors list (link)