The type-cD galaxy [1] (also cD-type galaxy, [2] cD galaxy [3] ) is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy. Characterized by a large halo of stars, [4] they can be found near the centres of some rich galaxy clusters. [5] They are also known as supergiant ellipticals [6] or central dominant galaxies. [7]
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. [8] 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. [9] A backformation of "cD" is frequently used to indicate "central Dominant galaxy". [7] cDs are also frequently considered the largest galaxies. [10] [11]
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. [12] They appear elliptical-like, with large low surface brightness envelopes [13] which may belong as much to the galaxy cluster as the cD galaxy. It is currently thought that cDs are the result of galaxy mergers. [14] Some cDs have multiple galactic nuclei. [15] cD galaxies are one of the types frequently found to be the brightest cluster galaxy (BCG) of a cluster. [16] 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. [17] However, cDs themselves are not found as field galaxies, unlike fossil groups. [13] cDs form around 20% of BCGs. [13]
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. [18]
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. [19] This "cannibalistic" mode of growth leads to the large diameter and luminosity of the cDs. [20] The second-brightest galaxy in the cluster is usually under-luminous, a consequence of its having been "eaten". [21] Remains of "eaten" galaxies sometimes appear as a diffuse halo of gas and dust, [20] or tidal streams, 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. [14] It is estimated that the cD galaxy alone contributes 1–7%, depending on the cluster mass, of the total baryon mass within 12.5 virial radii. [22]
Dynamical friction is believed to play an important role in the formation of cD galaxies at the centres of galaxy clusters. [23] 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 stars, 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. [24] A giant or supergiant diffuse or elliptical galaxy will result from this accumulation. [25] The centers of merged or merging galaxies can remain recognizable for long times, appearing as multiple "nuclei" of the cD galaxy. [26]
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". [27]
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 Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.
The Centaurus Cluster (A3526) is a cluster of hundreds of galaxies, located approximately 170 million light-years away in the Centaurus constellation. The brightest member galaxy is the elliptical galaxy NGC 4696 (~11m). The Centaurus cluster shares its supercluster, the Hydra–Centaurus Supercluster, with IC4329 Cluster and Hydra Cluster.
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.
Messier 86 is an elliptical or lenticular galaxy in the constellation Virgo. It was discovered by Charles Messier in 1781. M86 lies in the heart of the Virgo Cluster of galaxies and forms a most conspicuous group with another large galaxy known as Messier 84. It displays the highest blue shift of all Messier objects, as it is, net of its other vectors of travel, approaching the Milky Way at 244 km/s. This is due to both galaxies falling roughly towards the center of the Virgo cluster from opposing ends.
Messier 90 is an intermediate spiral galaxy exhibiting a weak inner ring structure about 60 million light-years away[a] in the constellation Virgo. It was discovered by Charles Messier in 1781.
The Coma Cluster is a large cluster of galaxies that contains over 1,000 identified galaxies. Along with the Leo Cluster, it is one of the two major clusters comprising the Coma Supercluster. It is located in and takes its name from the constellation Coma Berenices.
NGC 1316 is a lenticular galaxy about 60 million light-years away in the constellation Fornax. It is a radio galaxy and at 1400 MHz is the fourth-brightest radio source in the sky.
The Eyes Galaxies are a pair of galaxies about 52 million light-years away in the constellation Virgo. The pair are members of the string of galaxies known as Markarian's Chain.
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 123.65 to 169.61 kiloparsecs. It possesses a diffuse core which is the largest known core of any galaxy to date, and contains a supermassive black hole, one of the largest discovered. The galaxy is located at 354.0 megaparsecs from Earth. The galaxy was discovered on 19 June 1790, by the British astronomer William Herschel.
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.
Abell 1413 is a massive and rich type I galaxy cluster straddling the border between the constellations Leo and Coma Berenices, with the projected comoving distance of approximately 640 Mpc (2.1 billion ly). The cluster is especially notable due to the presence of its very large brightest cluster galaxy (BCG), one of the most extreme examples of its type, as well as one of the largest galaxies known. The cluster was first noted by George O. Abell in 1958.
NGC 6166 is an elliptical galaxy in the Abell 2199 cluster. It lies 490 million light years away in the constellation Hercules. The primary galaxy in the cluster, it is one of the most luminous galaxies known in terms of X-ray emissions.
Abell 1201 BCG (short for Abell 1201 Brightest Cluster Galaxy) is a type-cD massive elliptical galaxy residing as the brightest cluster galaxy (BCG) of the Abell 1201 galaxy cluster. At a redshift of 0.169, this system is around 2.7 billion light-years from Earth, and offset about 11 kiloparsecs from the X-ray peak of the intracluster gas. With an ellipticity of 0.32±0.02, the stellar distribution is far from spherical. In solar units, the total stellar luminosity is 4×1011 L☉ in SDSS r-band, and 1.6×1012 L☉ in 2MASS K-band. Half the stars orbit within an effective radius of 15 kpc, and their central velocity dispersion is about 285 km s−1 within 5 kpc rising to 360 km s−1 at 20 kpc distance.
NGC 4457 is an intermediate spiral galaxy located about 55 million light-years away in the constellation of Virgo. It is also classified as a LINER galaxy, a class of active galaxy defined by their spectral line emissions. NGC 4457 Is inclined by about 33°. It was discovered by astronomer William Herschel on February 23, 1784. Despite being listed in the Virgo Cluster Catalog as VCC 1145, NGC 4457 is a member of the Virgo II Groups which form an extension of the Virgo cluster.
ESO 444-46 is a class E4 supergiant elliptical galaxy; the dominant and brightest member of the Abell 3558 galaxy cluster around 640 million light-years away in the constellation Centaurus. It lies within the core of the massive Shapley Supercluster, one of the closest neighboring superclusters. It is one of the largest galaxies in the local universe, and possibly contains one of the most massive black holes known. The black hole's mass is very uncertain, with estimates ranging from as low as 501 million M☉, to as high as 77.6 billion M☉.
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.
NGC 708 is an elliptical galaxy located 240 million light-years away in the constellation Andromeda and was discovered by astronomer William Herschel on September 21, 1786. It is classified as a cD galaxy and is the brightest member of Abell 262. NGC 708 is a weak FR I radio galaxy and is also classified as a type 2 Seyfert galaxy.
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 about 55 million light years from Earth, which, given its apparent dimensions, means that NGC 4636 is about 105,000 light years across.
NGC 547 is an elliptical galaxy and radio galaxy located in the constellation Cetus. It is located at a distance of about 220 million light years from Earth, which, given its apparent dimensions, means that NGC 547 is about 120,000 light years across. It was discovered by William Herschel on October 1, 1785. It is a member of the Abell 194 galaxy cluster and is included along with NGC 547 in the Atlas of Peculiar Galaxies.
NGC 7720 is an elliptical galaxy located in the constellation Pegasus. It is located at a distance of about 380 million light years from Earth, which, given its apparent dimensions, means that NGC 7720 is about 180,000 light years across. NGC 7720 is the main galaxy of Abell 2634 galaxy cluster and is a radio galaxy. It was discovered by William Herschel on September 10, 1784.
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