Galaxy group

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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

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure.

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

A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars. 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, also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about 46.56 kiloparsecs approximately 765 kpc from Earth and the nearest large galaxy to the Milky Way. 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 Virgo Supercluster or the Local Supercluster is a mass concentration of galaxies 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.

<span class="mw-page-title-main">Messier 87</span> Elliptical galaxy in the Virgo Galaxy Cluster

Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters — about 15,000 compared with the 150–200 orbiting the Milky Way — and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.

<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> Peculiar 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 a diameter of approximately 15 kiloparsecs, three-tenths the size of the Milky Way.

<span class="mw-page-title-main">Messier 59</span> Elliptical galaxy in the constellation Virgo

Messier 59 or M59, also known as NGC 4621, is an elliptical galaxy in the equatorial constellation of Virgo. It is a member of the Virgo Cluster, with the nearest fellow member 8′ away and around 5 magnitudes fainter. The nearest cluster member of comparable brightness is the lenticular galaxy NGC 4638, which is around 17′ away. It and the angularly nearby elliptical galaxy Messier 60 were both discovered by Johann Gottfried Koehler in April 1779 when observing comet seeming close by. Charles Messier listed both in the Messier Catalogue about three days after Koehler's discovery.

<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.

<span class="mw-page-title-main">Dark matter halo</span> Theoretical cosmological structure

According to modern models of physical cosmology, a dark matter halo is a basic unit of cosmological structure. It is a hypothetical region that has decoupled from cosmic expansion and contains gravitationally bound matter. A single dark matter halo may contain multiple virialized clumps of dark matter bound together by gravity, known as subhalos. Modern cosmological models, such as ΛCDM, propose that dark matter halos and subhalos may contain galaxies. The dark matter halo of a galaxy envelops the galactic disc and extends well beyond the edge of the visible galaxy. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through observations of their effects on the motions of stars and gas in galaxies and gravitational lensing. Dark matter halos play a key role in current models of galaxy formation and evolution. Theories that attempt to explain the nature of dark matter halos with varying degrees of success include cold dark matter (CDM), warm dark matter, and massive compact halo objects (MACHOs).

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> Highly distorted spiral 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> Irregular Magellanic type 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 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 3311</span> Elliptical galaxy in the constellation Hydra

NGC 3311 is a supergiant 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">NGC 720</span> Elliptical 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> Barred spiral 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.

<span class="mw-page-title-main">NGC 4636</span> Elliptical galaxy in the constellation Virgo

NGC 4636 is an elliptical galaxy located in the constellation Virgo. It is a member of the NGC 4753 Group of galaxies, which 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 is located at a distance of circa 55 million light years from Earth, which, given its apparent dimensions, means that NGC 4636 is about 105,000 light years across.

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 (published April 2013). 552: 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 (published 2000). 38: 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 (published March 2013). 765 (2): 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)
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