Type-cD galaxy

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]


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.[18] This "cannibalistic" mode of growth leads to the large diameter and luminosity of the cDs.[19] The second-brightest galaxy in the cluster is usually under-luminous, a consequence of its having been "eaten".[20] Remains of "eaten" galaxies sometimes appear as a diffuse halo of gas and dust[19], 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.[21]

Dynamical friction

Dynamical friction is believed to play an important role in the formation of cD galaxies at the centres of galaxy clusters.[22] 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.[23] A giant or supergiant diffuse or elliptical galaxy will result from this accumulation.[24] The centers of merged or merging galaxies can remain recognizable for long times, appearing as multiple "nuclei" of the cD galaxy.[25]

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".[26]


See also


  1. ^ Sidereal Times, June 2002, page 3
  2. ^ Proceedings of PATRAS 2008, page 59
  3. ^ Galaxy Clusters, Jan Hartlap, page 3
  4. ^ Surface Photometry and the Structure of Elliptical Galaxies, "Chapter 11. cD and Brightest Cluster Galaxies", John Kormendy, S. Djorgovski, 1989
  5. ^ A Dictionary of Astronomy, "cD galaxy" (accessed 14 April 2010)
  6. ^ encyclopedia.com "supergiant elliptical"
  7. ^ a b "Uncertainties on Clusters of Galaxies Distances", C. Adami, M.P. Ulmer, 18 July 2000, arXiv:astro-ph/0007265 (accessed 14 April 2010)
  8. ^ An Atlas of DRAGNs, "Glossary", J. P. Leahy, 15 March 1997 (accessed 14 April 2010)
  9. ^ Global Telescope Network, "Types of Galaxies" Archived 16 July 2010 at the Wayback Machine, Kevin McLin, 14 April 2010 (accessed 14 April 2010)
  10. ^ Universe Today, "What is the Largest Galaxy?", Fraser Cain (accessed 14 April 2010)
  11. ^ EurekAlert, "Scientists observe largest explosion in space", Andrea Gibson, 5 January 2005 (accessed 15 April 2010)
  12. ^ Encyclopædia Britannica, "cD-galaxy" (accessed 14 April 2010)
  13. ^ a b c Monthly Notices of the Royal Astronomical Society, "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, doi:10.1111/j.1365-2966.2007.11899.x, Bibcode2007MNRAS.378.1575S, arXiv:astro-ph/0612229v2 (accessed 15 April 2010)
  14. ^ a b COSMOS - The SAO Encyclopedia of Astronomy, "CD Galaxies", Swinburne University of Technology (accessed 14 April 2010)
  15. ^ Internet Encyclopedia of Science, "D galaxy", David Darling (accessed 14 April 2010)
  16. ^ IAU Symposium 245, "Star Formation in Bulges from GALEX", Sukyoung K. Yi, 5 September 2007, doi:10.1017/S174392130801819X, arXiv:0709.0177 (accessed 14 April 2010)
  17. ^ Universe Today, "How Do Fossil Galaxy Clusters Form so Quickly?", Fraser Cain, 27 April 2006 (accessed 15 April 2010)
  18. ^ Rood, Herbert J. (1965). The Dynamics of the Coma Cluster of Galaxies (PhD thesis). The University of Michigan. Bibcode:1965PhDT.........3R.
  19. ^ a b "Curious About Astronomy?". Retrieved 28 March 2007.
  20. ^ Hausman, M. J.; Ostriker, J. P. (November 1977). "Cannibalism among the galaxies - Dynamically produced evolution of cluster luminosity functions". The Astrophysical Journal Letters. 217: L125–L128. Bibcode:1977ApJ...217L.125O. doi:10.1086/182554.
  21. ^ Section 8 of Vikhlinin, A.; Kravtsov, A.; Forman, W.; Jones, C.; Markevitch, M.; Murray, S. S.; Van Speybroeck, L. (April 2006). "Chandra Sample of Nearby Relaxed Galaxy Clusters: Mass, Gas Fraction, and Mass-Temperature Relation" (PDF). The Astrophysical Journal. Chicago, Illinois, USA: University of Chicago Press. 640 (2): 691–709. arXiv:astro-ph/0507092. Bibcode:2006ApJ...640..691V. doi:10.1086/500288. Retrieved March 12, 2012.CS1 maint: Multiple names: authors list (link)
  22. ^ Merritt, David (January 1983). "Relaxation and tidal stripping in rich clusters of galaxies. I. Evolution of the mass distribution". The Astrophysical Journal. 264: 24–48. Bibcode:1983ApJ...264...24M. doi:10.1086/160571.
  23. ^ Merritt, David (January 1984). "Relaxation and tidal stripping in rich clusters of galaxies. II. Evolution of the luminosity distribution". The Astrophysical Journal. 276: 26–37. Bibcode:1984ApJ...276...26M. doi:10.1086/161590.
  24. ^ Merritt, David (February 1985). "Relaxation and tidal stripping in rich clusters of galaxies. III. Growth of a massive central galaxy". The Astrophysical Journal. 289: 18–32. Bibcode:1985ApJ...289...18M. doi:10.1086/162860.
  25. ^ Merritt, David (May 1984). "The nature of multiple-nucleus cluster galaxies". The Astrophysical Journal. 280: L5–8. Bibcode:1984ApJ...280L...5M. doi:10.1086/184257.
  26. ^ PDF, "'Tuning Fork' Classification of Rich Clusters of Galaxies", Herbert J.Rood, Gummuluru N. Sastry, June 1971, doi:10.1086/129128, Bibcode1971PASP...83..313R (accessed 14 April 2010)
  27. ^ Nature, "FIGURE 4. Optical, radio and X-ray images of the Perseus cluster." 9 July 2009, ISSN 0028-0836 ; E-ISSN 1476-4687 ; (accessed 15 April 2010)
  28. ^ Nature, "FIGURE 3. The entropy of the intracluster medium in spherical shells of radius r." 9 July 2009, ISSN 0028-0836 ; E-ISSN 1476-4687 ; (accessed 15 April 2010)
  29. ^ Science, "The Central Galaxy in Abell 2029: An Old Supergiant", Juan M. Uson, Stephen P. Boughn, and Jeffrey R. Kuhn, 26 October 1990, Vol. 250, no. 4980, pp.539-540, doi:10.1126/science.250.4980.539
  30. ^ Ellensburg Daily Record, "Galaxy Found", United Press International, 27 October 1990, p.16
  31. ^ Lodi News-Sentinel, "Giant Galaxy Discovered", UPI, 26 October 1990, pg.9

Further reading

3C 401

3C 401 is a powerful radio galaxy located in the constellation Draco. It is near the center of a rich cluster of galaxies and dominates the cluster. That is, it is the type-cD galaxy of its cluster. It has a double nucleus, indicating that it is merging with another galaxy.3C 401 is classified as a Fanaroff and Riley class II radio source (FR II), but has characteristics of both types of sources. FR II radio sources are brightest at the ends of their radio lobes while FR I sources are brightest toward their centers. 3C 401 has hot spots at the ends of its two extended radio lobes, but also has a bright one-sided jet like a FR I source. The spectra of this jet is also intermediate between the spectra of jets in the two types of sources.

Abell 2162

Abell 2162 is a galaxy cluster in the Abell catalogue located in the constellation Corona Borealis. It is a member of the Hercules Superclusters, the redshifts of the member galaxies of which lie between 0.0304 and 0.0414. The cluster hosts a massive Type-cD galaxy called NGC 6086.

Caldwell catalogue

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 catalogue is used by amateur astronomers as a list of deep-sky objects for observation, Moore noted that Messier's list was not compiled for that purpose and excluded many of the sky's brightest deep-sky objects, such as the Hyades, the Double Cluster (NGC 869 and NGC 884), and the Sculptor Galaxy (NGC 253). The Messier catalogue was actually compiled as a list of known objects that might be confused with comets. Moore also observed that since Messier compiled his list from observations in Paris, it did not include bright deep-sky objects visible in the Southern Hemisphere, such as Omega Centauri, Centaurus A, the Jewel Box, and 47 Tucanae. Moore compiled a list of 109 objects to match the commonly accepted number of Messier objects (he excluded M110), and the list was published in Sky & Telescope in December 1995.Moore used his other surname – Caldwell – to name the list, since the initial of "Moore" is already used for the Messier catalogue. Entries in the catalogue are designated with a "C" and the catalogue number (1 to 109).

Unlike objects in the Messier catalogue, which are listed roughly in the order of discovery by Messier and his colleagues, the Caldwell catalogue is ordered by declination, with C1 being the most northerly and C109 being the most southerly, although two objects (NGC 4244 and the Hyades) are listed out of sequence. Other errors in the original list have since been corrected: it incorrectly identified the S Norma Cluster (NGC 6087) as NGC 6067 and incorrectly labelled the Lambda Centauri Cluster (IC 2944) as the Gamma Centauri Cluster.

Dragonfly 44

Dragonfly 44 is an ultra diffuse galaxy in the Coma Cluster. Observations of the velocity dispersion suggest a mass of about one trillion solar masses, about the same as the mass of the Milky Way; the galaxy shows no evidence of rotation. This is also consistent with about 90 globular clusters observed around Dragonfly 44. However, the galaxy emits only 1% of the light emitted by the Milky Way. The galaxy was discovered with the Dragonfly Telephoto Array.To determine the amount of dark matter in this galaxy, they used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass.

The scientists then used the Gemini Multi-Object Spectrograph on the 8-m Gemini North telescope to reveal a halo of spherical clusters of stars around the galaxy’s core.In August 2016, astronomers reported that this galaxy might be made almost entirely of dark matter.

Messier 87

Messier 87 (also known as Virgo A or NGC 4486, generally abbreviated to M87) is a supergiant elliptical galaxy in the constellation Virgo. One of the most massive galaxies in the local Universe, it is notable for its large population of globular clusters—about 12,000 compared to the 150–200 orbiting the Milky Way—and its jet of energetic plasma that originates at the core and extends at least 1,500 parsecs (4,900 light-years), traveling at relativistic speed. It is one of the brightest radio sources in the sky, and a popular target for both amateur and professional astronomers.

The French astronomer Charles Messier discovered M87 in 1781, and catalogued it as a nebulous feature while searching for objects that would otherwise confuse comet hunters. M87 is located about 16.4 million parsecs (53 million light-years) from Earth and is the second-brightest galaxy within the northern Virgo Cluster, having many satellite galaxies. Unlike a disk-shaped spiral galaxy, M87 has no distinctive dust lanes. Instead, it has an almost featureless, ellipsoidal shape typical of most giant elliptical galaxies, diminishing in luminosity with distance from the center. Forming around one sixth of its mass, M87's stars have a nearly spherically symmetric distribution. Their population density decreases with increasing distance from the core. It has an active supermassive black hole at its core, which forms the primary component of an active galactic nucleus.

The galaxy is a strong source of multiwavelength radiation, particularly radio waves. Its galactic envelope extends to a radius of about 150 kiloparsecs (490 thousand light-years), where it is truncated—possibly by an encounter with another galaxy. Its interstellar medium consists of diffuse gas enriched by elements emitted from evolved stars.

NGC 1275

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. NGC 1275 corresponds to the radio galaxy Perseus A and is situated near the center of the large Perseus Cluster of galaxies.

NGC 1399

NGC 1399 is a large elliptical galaxy in the Southern constellation Fornax, the central galaxy in the Fornax cluster.

NGC 3311

NGC 3311 is a supergiant elliptical galaxy (a type-cD 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 3311 is surrounded by a rich and extensive globular cluster system rivaling that of Messier 87 in the Virgo Cluster.

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 NGC 6086.

NGC 6166

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.

Supergiant (disambiguation)

A supergiant is a massive and luminous star, including:

Blue supergiant star, a hot supergiant

Yellow supergiant star, a supergiant with a temperature similar to the sun

Red supergiant star, a cool supergiantSupergiant may also refer to:

Supergiant Games, a video game development company

Super Giant, a Japanese superhero

Alicella gigantea, the supergiant Amphipod

Rising Pune Supergiant, a cricket team in the Indian Premier League

Type-cD galaxy, a supergiant elliptical galaxy

Supergiant, a fictional character in the Marvel Universe

Ultra diffuse galaxy

An ultra diffuse galaxy (UDG) is an extremely low luminosity galaxy, the first example of which was discovered in the nearby Virgo Cluster by Allan Sandage and Bruno Binggeli in 1984. Such a galaxy may have the same size and mass as the Milky Way but a visible star count of only 1%. Their lack of luminosity is due to the lack of star-forming gas in the galaxy. This results in old stellar populations.Some ultra diffuse galaxies found in the Coma Cluster, about 330 million light years from Earth, have diameters of 60 kly (18 kpc) (more than half the size of our galaxy) with 1% of the stars of the Milky Way Galaxy. The distribution of ultra diffuse galaxies in the Coma Cluster is the same as luminous galaxies; this suggests that the cluster environment strips the gas from the galaxies, while allowing them to populate the cluster the same as more luminous galaxies. The similar distribution in the higher tidal force zones suggests a larger dark matter fraction to hold the galaxies together under the higher stress.Dragonfly 44, a ultra diffuse galaxy in the Coma Cluster, is one example. Observations of the rotational speed suggest a mass of about one trillion solar masses, about the same as the mass of the Milky Way. This is also consistent with about 90 globular clusters observed around Dragonfly 44. However, the galaxy emits only 1% of the light emitted by the Milky Way. On 25 August 2016, astronomers reported that Dragonfly 44 may be made almost entirely of dark matter. In 2018 the same authors reported the discovery of a dark matter-free UDG (NGC 1052-DF2, which was already identified on photoplates by Igor Karachentsev) based on velocity measurements of ~10 globular cluster system. The authors concluded that this may rule out modified gravity theories like MOND, but other theories such as the External Field Effect are also possibilities.

Active nuclei
Energetic galaxies
Low activity
See also

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