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Type-cD galaxy
The type-cD galaxy〔Sidereal Times, June 2002, page 3〕 (also cD-type galaxy,〔Proceedings of PATRAS 2008, page 59〕 cD galaxy〔Galaxy Clusters, Jan Hartlap, page 3〕) is a galaxy morphology classification, a subtype of type-D giant elliptical galaxy, and has a large halo of stars.〔Surface Photometry and the Structure of Elliptical Galaxies, ("Chapter 11. cD and Brightest Cluster Galaxies" ), John Kormendy, S. Djorgovski, ''1989''〕 They can be found near the centres of some rich galaxy clusters.〔A Dictionary of Astronomy, ("cD galaxy" ) (accessed 14 April 2010)〕 They are also known as supergiant ellipticals〔encyclopedia.com ("supergiant elliptical" )〕 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.〔An Atlas of DRAGNs, ("Glossary" ), J. P. Leahy, ''15 March 1997'' (accessed 14 April 2010)〕 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.〔Global Telescope Network, ("Types of Galaxies" ), Kevin McLin, ''14 April 2010'' (accessed 14 April 2010)〕 A backformation of "cD" is frequently used to mean ''central Dominant galaxy''.〔"Uncertainties on Clusters of Galaxies Distances", C. Adami, M.P. Ulmer, ''18 July 2000'', (accessed 14 April 2010)〕 cD's are also frequently considered the largest galaxies.〔Universe Today, ("What is the Largest Galaxy?" ), Fraser Cain (accessed 14 April 2010)〕〔EurekAlert, ("Scientists observe largest explosion in space" ), Andrea Gibson, ''5 January 2005'' (accessed 15 April 2010)〕
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.〔Encyclopædia Britannica, ("cD-galaxy" ) (accessed 14 April 2010)〕 They appear elliptical-like, with large low surface brightness envelopes.〔 It is currently thought that cD's are the result of galaxy mergers.〔COSMOS - The SAO Encyclopedia of Astronomy, ("CD Galaxies" ), Swinburne University of Technology (accessed 14 April 2010)〕 Some cD's have multiple galactic nuclei.〔Internet Encyclopedia of Science, ("D galaxy" ), David Darling (accessed 14 April 2010)〕 cD galaxies are one of the types frequently found to be the brightest cluster galaxy (BCG) of a cluster.〔IAU Symposium 245, "Star Formation in Bulges from GALEX", Sukyoung K. Yi, ''5 September 2007'', , (accessed 14 April 2010)〕 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.〔Universe Today, ("How Do Fossil Galaxy Clusters Form so Quickly?" ), Fraser Cain, ''27 April 2006'' (accessed 15 April 2010)〕 However, cD's themselves are not found as field galaxies, unlike fossil groups.〔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, , , (accessed 15 April 2010)〕 cD's form around 20% of BCGs.〔
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