Type-cD galaxy
Encyclopedia
The type-cD galaxy is a galaxy
morphology classification, a subtype of type-D giant elliptical galaxy
, and has a large halo
of star
s. They can be found near the centres of some rich galaxy cluster
s. They are also known as supergiant ellipticals or central dominant galaxies.
cD galaxies are similar to lenticular galaxies (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. It is currently thought that cD's are the result of galaxy merger
s. Some cD's have multiple galactic nuclei. cD galaxies are one of the types frequently found to be the brightest cluster galaxy
(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, cD's themselves are not found as field galaxies, unlike fossil groups. cD's form around 20% of BCGs.
and dust
. This halo can be up to 3 million light years in diameter.
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
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
, the large galaxy gradually spirals toward the centre of the cluster. Once there, the star
s, gas
, dust
and dark matter
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.
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
morphology classification, a subtype of type-D giant elliptical galaxy
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
, and has a large halo
Galactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
of star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s. They can be found near the centres of some rich galaxy cluster
Galaxy cluster
A galaxy cluster is a compact cluster of galaxies. Basic difference between a galaxy group and a galaxy cluster is that there are many more galaxies in a cluster than in a group. Also, galaxies in a cluster are more compact and have higher velocity dispersion. One of the key features of cluster is...
s. They are also known as supergiant ellipticals or central dominant galaxies.
cD galaxies
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 supergiant, while the "D" refers to the fact that the galaxies appear diffuse. A backformation of "cD" is frequently used to mean central Dominant galaxy. cD's are also frequently considered the largest galaxies around.cD galaxies are similar to lenticular galaxies (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. It is currently thought that cD's are the result of galaxy merger
Galaxy merger
Galaxy mergers can occur when two galaxies collide. They are the most violent type of galaxy interaction. Although galaxy mergers do not involve stars or star systems actually colliding, due to the vast distances between stars in most circumstances, the gravitational interactions between galaxies...
s. Some cD's have multiple galactic nuclei. cD galaxies are one of the types frequently found to be the brightest cluster galaxy
Brightest cluster galaxy
Brightest cluster galaxy 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 of their host galaxy cluster, hence at the bottom of the...
(BCG) of a cluster. Many fossil group
Fossil group
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. Galaxies within a group interact and merge. The physical process behind this galaxy-galaxy merger is dynamical friction...
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, cD's themselves are not found as field galaxies, unlike fossil groups. cD's form around 20% of BCGs.
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" mode of growth leads to the overwhelming diameter and luminosity of the cD's. 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 gasGas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
and dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
. This halo can be up to 3 million light years in diameter.
Dynamical friction
Dynamical frictionDynamical friction
Dynamical friction is a term in astrophysics related to loss of momentum and kinetic energy of moving bodies through a gravitational interaction with surrounding matter in space...
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
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
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
Kinetic energy
The kinetic energy of an object is the energy which 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 acceleration, the body maintains this kinetic energy unless its speed changes...
, the large galaxy gradually spirals toward the centre of the cluster. Once there, the star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s, gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
, dust
Dust
Dust consists of particles in the atmosphere that arise from various sources such as soil dust lifted up by wind , volcanic eruptions, and pollution...
and dark matter
Dark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
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 of cD galaxies
- Perseus A
- NGC 6166
- IC 1101IC 1101IC 1101 is a supergiant lenticular galaxy at the center of the Abell 2029 galaxy cluster. It is 1.07 billion light years away in the constellation of Serpens and is classified as a cD class of galaxy.-Size:...
, the largest known galaxy in terms of diameter (around 5.5 million light years) - Messier 87Messier 87Messier 87 is a supergiant elliptical galaxy. It was discovered in 1781 by the French astronomer Charles Messier, who cataloged it as a nebulous feature. The second brightest galaxy within the northern Virgo Cluster, it is located about 16.4 million parsecs from Earth...
, the central galaxy in the Virgo ClusterVirgo ClusterThe Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly away in the constellation Virgo. Comprising approximately 1300 member galaxies, the cluster forms the heart of the larger Local Supercluster, of which the Local Group is an outlying member... - NGC 1399NGC 1399NGC 1399, a large elliptical galaxy in the Southern constellation Fornax, is the central galaxy in the Fornax cluster....
in the Fornax ClusterFornax ClusterAt a distance of approximately 62.0 Mly , the Fornax Cluster is the second richest cluster of galaxies within 100 million light-years, although it is much smaller than the Virgo Cluster. It lies primarily in the constellation Fornax, and may be associated with the nearby Eridanus Group...
Further reading
- PDF - "A Preliminary Classification of the Forms of Galaxies According to Their Stellar Population", W. W. Morgan, Yerkes Obeservatory, 1958, , (PASP 70)
- PDF - "Intracluster light and the extended stellar envelopes of cD galaxies: an analytical description", Marc S. Seigar, Alister W. Graham, Helmut Jerjen, July 2007 , , (MNRAS 07/2007)
See also
- Giant elliptical galaxy (gE)
- Giant galaxy
- Elliptical galaxyElliptical galaxyAn elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
(E#) - Lenticular galaxyLenticular galaxyA lenticular galaxy is a type of galaxy which is intermediate between an elliptical galaxy and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are disk galaxies which have used up or lost most of their interstellar matter and therefore have very little ongoing...
(S0, SA0, SAB0, SB0, E9) - Type-D galaxy