Type-cD galaxy

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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] cD's 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] It is currently thought that cD's are the result of galaxy mergers.[14] Some cD's 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, cD's themselves are not found as field galaxies, unlike fossil groups.[13] cD's 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 cD's.[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] This halo 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[edit]

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[edit]

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[edit]


  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", 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/0507092Freely accessible. Bibcode:2006ApJ...640..691V. doi:10.1086/500288. Retrieved March 12, 2012. 
  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[edit]