The type-cD galaxy (also cD-type galaxy, cD galaxy) is a

galaxy A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek ' (), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. ...

morphology classification, a subtype of type-D

giant In folklore, giants (from Ancient Greek: '' gigas'', cognate giga-) are beings of human-like appearance, but are at times prodigious in size and strength or bear an otherwise notable appearance. The word ''giant'' is first attested in 1297 fr ...

elliptical galaxy An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work ''The R ...

. Characterized by a large

halo Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * ''Halo'' (franch ...

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s, they can be found near the centres of some rich

galaxy cluster A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-l ...

s. They are also known as supergiant ellipticals or central dominant galaxies.

Characteristics

The cD-type is a classification in the Yerkes galaxy classification scheme, one of two Yerkes classifications still in common use, along with D-type. The "c" in "cD" refers to the fact that the galaxies are very large, hence the adjective supergiant, while the "D" refers to the fact that the galaxies appear diffuse. A backformation of "cD" is frequently used to indicate "central Dominant galaxy"."Uncertainties on Clusters of Galaxies Distances", C. Adami, M.P. Ulmer, 18 July 2000, (accessed 14 April 2010) cDs are also frequently considered the largest galaxies. cD galaxies are similar to

lenticular galaxies A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. ...

(S0) or elliptical galaxies (E#), but many times larger, some having envelopes that exceed one million light years in radius. They appear elliptical-like, with large low surface brightness envelopes which may belong as much to the galaxy cluster as the cD galaxy. It is currently thought that cDs are the result of

galaxy merger Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies ...

s.COSMOS—The SAO Encyclopedia of Astronomy
"CD Galaxies"
Swinburne University of Technology (accessed 14 April 2010) Some cDs have multiple

galactic nuclei An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not pr ...

. cD galaxies are one of the types frequently found to be the

brightest cluster galaxy A brightest cluster galaxy (BCG) is defined as the brightest galaxy in a cluster of galaxies. BCGs include the most massive galaxies in the universe. They are generally elliptical galaxies which lie close to the geometric and kinematical center ...

(BCG) of a cluster. Many fossil group galaxies are similar to cD BCG galaxies, leading some to theorize that the cD results from the creation of a fossil group, and then the new cluster accumulating around the fossil group. However, cDs themselves are not found as

field galaxies A field galaxy is a galaxy that does not belong to a larger galaxy group or cluster and hence is gravitationally alone. Roughly 80% of all galaxies located within of the Milky Way The Milky Way is the galaxy that includes our Solar Sys ...

, unlike fossil groups.

Monthly Notices of the Royal Astronomical Society ''Monthly Notices of the Royal Astronomical Society'' (MNRAS) is a peer-reviewed scientific journal covering research in astronomy and astrophysics. It has been in continuous existence since 1827 and publishes letters and papers reporting orig ...

, "Intracluster light and the extended stellar envelopes of cD galaxies: an analytical description", Marc S. Seigar, Alister W. Graham, Helmut Jerjen, July 2007, Volume 378, Issue 4, pp. 1575–1588, , , (accessed 15 April 2010) cDs form around 20% of BCGs.

Importance

Massive galaxies such as supergiant elliptical galaxies are important to understanding the evolution of the Universe, because they, along with other large-early type galaxies, account for half of the Universe's stellar mass, contribute a lot to its chemical enrichment and provide clues to the star formation history of the Universe.

Growth

cD galaxies are believed to grow via mergers of galaxies that spiral in to the center of a galaxy cluster, a theory first proposed by Herbert J. Rood in 1965. This "

cannibalistic Cannibalism is the act of consuming another individual of the same species as food. Cannibalism is a common ecological interaction in the animal kingdom and has been recorded in more than 1,500 species. Human cannibalism is well documented, bo ...

" mode of growth leads to the large diameter and luminosity of the cDs. The second-brightest galaxy in the cluster is usually under-luminous, a consequence of its having been "eaten". Remains of "eaten" galaxies sometimes appear as a diffuse halo of

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

and

dust Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in homes ...

, or

tidal streams Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...

, or undigested off-center nuclei in the cD galaxy. The envelope or halo may also consist of the "intra-cluster light", originating from stars stripped away from their original galaxy, and it can be up to 3 million light years in diameter. It is estimated that the cD galaxy alone contributes 1–7%, depending on the cluster mass, of the total

baryon In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classified ...

mass within 12.5 virial radii.Section 8 of

Dynamical friction

Dynamical friction In astrophysics, dynamical friction or Chandrasekhar friction, sometimes called ''gravitational drag'', is loss of momentum and kinetic energy of moving bodies through gravitational interactions with surrounding matter in space. It was first d ...

is believed to play an important role in the formation of cD galaxies at the centres of galaxy clusters. This process begins when the motion of a large galaxy in a cluster attracts smaller galaxies and

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...

into a wake behind it. This over-density follows behind the larger galaxy and exerts a constant gravitational force on it, causing it to slow down. As it loses

kinetic energy In physics, the kinetic energy of an object is the energy that it possesses due to its motion. It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...

, the large galaxy gradually spirals toward the centre of the cluster. Once there, the

and

of the large galaxy and its trailing galaxies will join with those of other galaxies who preceded them in the same fate. A giant or supergiant diffuse or elliptical galaxy will result from this accumulation. The centers of merged or merging galaxies can remain recognizable for long times, appearing as multiple "nuclei" of the cD galaxy.

cD clusters

Type-cD galaxies are also used to define clusters. A galaxy cluster with a cD at its centre is termed a "cD cluster" or "cD galaxy cluster" or "cD cluster of galaxies".

Examples

* 3C 401 * Holmberg 15A (currently home to one of the largest black holes) *

IC 1101 IC 1101 is a class S0 supergiant ( cD) lenticular galaxy at the center of the Abell 2029 galaxy cluster. It has an isophotal diameter at about . It possesses a diffuse core which is the largest known core of any galaxy to date, and also hosts a ...

, the large central galaxy of the massive cluster Abell 2029. *

Messier 87 Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy with several trillion stars in the constellation Virgo. One of the largest and most massive galaxies in the local ...

, the central galaxy in the

Virgo Cluster The Virgo Cluster is a large cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the constellation Virgo. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the lar ...

* NeVe 1, the host galaxy of the Ophiuchus Supercluster eruption event, the most energetic outburst known. * NGC 1399 in the

Fornax Cluster The Fornax Cluster is a cluster of galaxies lying at a distance of 19 megaparsecs (62 million light-years). It has an estimated mass of solar masses, making it the second richest galaxy cluster within 100 million light-years, after the consider ...

NGC 4889 NGC 4889 (also known as Caldwell 35) is an E4 supergiant elliptical galaxy. It was discovered in 1785 by the British astronomer William Herschel, Frederick William Herschel I, who catalogued it as a bright, nebulous patch. The brightest galaxy wi ...

, is also known as the Caldwell 35, a supergiant

, a class-4 elliptical galaxy, it is the brightest within

Caldwell Objects The Caldwell catalogue is an astronomical catalogue of 109 star clusters, nebulae, and galaxies for observation by amateur astronomers. The list was compiled by Patrick Moore as a complement to the Messier catalogue. While the Messier catal ...

in the constellation

Coma Berenices Coma Berenices is an ancient asterism in the northern sky, which has been defined as one of the 88 modern constellations. It is in the direction of the fourth galactic quadrant, between Leo and Boötes, and it is visible in both hemisphe ...

NGC 6086 NGC 6086 is an elliptical galaxy in the constellation of Corona Borealis. It has an apparent magnitude of 12.7. A Type-cD galaxy, it is the brightest cluster galaxy in the cluster Abell 2162. In 2010, a supermassive black hole was discovered in NG ...

NGC 6166 NGC 6166 is an elliptical galaxy in the Abell 2199 cluster. It lies 490 million light years away in the constellation Hercules constellation, Hercules. The primary galaxy in the cluster, it is one of the most luminous galaxies known in terms of ...

Perseus A NGC 1275 (also known as Perseus A or Caldwell 24) is a type 1.5 Seyfert galaxy located around 237 million light-years away in the direction of the constellation Perseus (constellation), Perseus. NGC 1275 corresponds to the radio galaxy Perseus A ...

''Nature''
"FIGURE 4. Optical, radio and X-ray images of the Perseus cluster."
9 July 2009, ; E- ; (accessed 15 April 2010) * QSO 0957, the first identified gravitationally lensed object

Characteristics

Importance

Growth

Dynamical friction

cD clusters

Examples

See also

References

Further reading