View source for Astronomy:Type-cD galaxy

{{Short description|Galaxy morphology classification}}
[[File:ESO_383-76_Legacy_DR10.png|thumb|right|ESO 383-76 within the Abell 3571 cluster, imaged by Dark Energy Spectroscopic Instrument in 2019]]

The '''type-cD galaxy'''<ref>Sidereal Times, June 2002, page 3</ref> (also '''cD-type galaxy''',<ref>Proceedings of PATRAS 2008, page 59</ref> '''cD galaxy'''<ref>Galaxy Clusters, Jan Hartlap, page 3</ref>) is a [[Astronomy:Galaxy|galaxy]] morphology classification, a subtype of type-D [[Astronomy:Elliptical galaxy#Sizes and shapes|giant]] [[Astronomy:Elliptical galaxy|elliptical galaxy]]. Characterized by a large [[Astronomy:Galactic halo|halo]] of [[Astronomy:Star|star]]s,<ref>Surface Photometry and the Structure of Elliptical Galaxies, [http://nedwww.ipac.caltech.edu/level5/Sept01/Kormendy/Kormendy11.html "Chapter 11. cD and Brightest Cluster Galaxies"], John Kormendy, S. Djorgovski, 1989</ref> they can be found near the centres of some rich [[Astronomy:Galaxy cluster|galaxy cluster]]s.<ref>A Dictionary of Astronomy, [http://www.highbeam.com/doc/1O80-cDgalaxy.html "cD galaxy"]{{dead link|date=February 2019|bot=medic}}{{cbignore|bot=medic}} (accessed 14 April 2010)</ref> They are also known as '''supergiant ellipticals'''<ref>encyclopedia.com [http://www.encyclopedia.com/doc/1O80-supergiantelliptical.html "supergiant elliptical"]</ref> or '''central dominant galaxies'''.<ref name=UoCoGd/>

==Characteristics==
The cD-type is a classification in the [[Astronomy:Galaxy morphological classification|Yerkes galaxy classification scheme]], one of two Yerkes classifications still in common use, along with D-type.<ref>An Atlas of DRAGNs, [http://www.jb.man.ac.uk/atlas/gloss.html#cD "Glossary"], J. P. Leahy, 15 March 1997 (accessed 14 April 2010)</ref> 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.<ref>Global Telescope Network, [http://gtn.sonoma.edu/resources/normal_galaxies/types.php "Types of Galaxies"] {{Webarchive|url=https://web.archive.org/web/20100716001648/http://gtn.sonoma.edu/resources/normal_galaxies/types.php |date=16 July 2010 }}, Kevin McLin, 14 April 2010 (accessed 14 April 2010)</ref> A backformation of "cD" is frequently used to indicate "central Dominant galaxy".<ref name=UoCoGd>"Uncertainties on Clusters of Galaxies Distances", C. Adami, M.P. Ulmer, 18 July 2000, {{arxiv|astro-ph/0007265}} (accessed 14 April 2010)</ref> cDs are also frequently considered the largest galaxies.<ref>Universe Today, [http://www.universetoday.com/guide-to-space/galaxies/what-is-the-largest-galaxy/ "What is the Largest Galaxy?"], Fraser Cain (accessed 14 April 2010)</ref><ref>EurekAlert, [http://www.eurekalert.org/pub_releases/2005-01/ou-sol010505.php "Scientists observe largest explosion in space"], Andrea Gibson, 5 January 2005 (accessed 15 April 2010)</ref>

cD galaxies are similar to lenticular galaxies (S0) or elliptical galaxies (E#), but many times larger, some having envelopes that exceed one million [[Light year|light year]]s in radius.<ref>''Encyclopædia Britannica'', [http://www.britannica.com/EBchecked/topic/100915/cD-galaxy "cD-galaxy"] (accessed 14 April 2010)</ref> They appear elliptical-like, with large low surface brightness envelopes<ref name="ICyese-cD"/> which may belong as much to the galaxy cluster as the cD galaxy. It is currently thought that cDs are the result of [[Astronomy:Galaxy merger|galaxy merger]]s.<ref name="cosmos-sao-CDgalaxies">COSMOS—The SAO Encyclopedia of Astronomy, [http://astronomy.swin.edu.au/cosmos/C/CD+Galaxies "CD Galaxies"], Swinburne University of Technology (accessed 14 April 2010)</ref> Some cDs have multiple galactic nuclei.<ref>Internet Encyclopedia of Science, [http://www.daviddarling.info/encyclopedia/D/D_galaxy.html "D galaxy"], David Darling (accessed 14 April 2010)</ref> cD galaxies are one of the types frequently found to be the [[Astronomy:Brightest cluster galaxy|brightest cluster galaxy]] (BCG) of a cluster.<ref>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)</ref> 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.<ref>Universe Today, [http://www.universetoday.com/2006/04/27/how-do-fossil-galaxy-clusters-form-so-quickly/ "How Do Fossil Galaxy Clusters Form so Quickly?"], Fraser Cain, 27 April 2006 (accessed 15 April 2010)</ref> However, cDs themselves are not found as field galaxies, unlike fossil groups.<ref name="ICyese-cD">[[Organization:Monthly Notices of the Royal Astronomical Society|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}}, {{bibcode|2007MNRAS.378.1575S}}, {{arxiv|astro-ph/0612229v2}} (accessed 15 April 2010)</ref> cDs form around 20% of BCGs.<ref name="ICyese-cD"/>

==Importance==
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.<ref name="ProgenitorBarbosa">{{cite journal |last1=Barbosa |first1=C.E. |last2=Spiniello |first2=C. |last3=Arnaboldi |first3=M. |last4=Coccato |first4=L. |last5=Hilker |first5=M. |last6=Richtler |first6=T. |title=A preserved high-z compact progenitor in the heart of NGC 3311 revealed with MUSE 2D stellar population analysis |journal=Astronomy & Astrophysics |date=May 2021 |volume=649 |pages=19 |doi=10.1051/0004-6361/202039809 |arxiv=2012.11609 |bibcode=2021A&A...649A..93B |s2cid=232157305 |doi-access=free }}</ref>

==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.<ref>{{Cite thesis |degree=PhD |title=The Dynamics of the Coma Cluster of Galaxies |bibcode=1965PhDT.........3R |last=Rood |first=Herbert J.|date=1965 |publisher=The [[Organization:University of Michigan|University of Michigan]]}}</ref> This "[[Astronomy:Interacting galaxy#Galactic cannibalism|cannibalistic]]" mode of growth leads to the large diameter and luminosity of the cDs.<ref name="curious">{{Cite web|url=http://curious.astro.cornell.edu/|title=Curious About Astronomy?|access-date=28 March 2007}}</ref> The second-brightest galaxy in the cluster is usually under-luminous, a consequence of its having been "eaten".<ref>{{Cite journal
  | doi = 10.1086/182554
  | last1 = Hausman
  | first1 = M. J.
  | last2 = Ostriker
  | first2 = J. P.
  | title = Cannibalism among the galaxies—Dynamically produced evolution of cluster luminosity functions
  | journal = The Astrophysical Journal Letters
  | volume = 217
  | pages = L125–L128
  | date = November 1977
  | bibcode = 1977ApJ...217L.125O| doi-access = free
  }}</ref> Remains of "eaten" galaxies sometimes appear as a diffuse halo of [[Physics:Gas|gas]] and dust,<ref name="curious" /> 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&nbsp;million light years in diameter.<ref name="cosmos-sao-CDgalaxies"/> It is estimated that the cD galaxy alone contributes 1–7%, depending on the cluster mass, of the total [[Physics:Baryon|baryon]] mass within 12.5 [[Astronomy:Virial mass#Virial radius|virial radii]].<ref name="Vikhlinin">Section 8 of {{cite journal
 | last1 = Vikhlinin
 | first1 = A.
 | last2 = Kravtsov
 | first2 = A.
 | last3 = Forman, W.; Jones, C.; Markevitch, M.; Murray, S. S.; Van Speybroeck, L.
 |date=April 2006
 | title = Chandra Sample of Nearby Relaxed Galaxy Clusters: Mass, Gas Fraction, and Mass-Temperature Relation
 | journal = The Astrophysical Journal
 | volume = 640
 | issue = 2
 | pages = 691–709
 | location = Chicago, Illinois, USA
 | arxiv = astro-ph/0507092
 | bibcode = 2006ApJ...640..691V
 | doi = 10.1086/500288
 | s2cid = 18940822
 }}</ref>

===Dynamical friction===
[[Astronomy:Dynamical friction|Dynamical friction]] is believed to play an important role in the formation of cD galaxies at the centres of galaxy clusters.<ref name="DM1983">
{{Cite journal
  | doi = 10.1086/160571
  | last = Merritt
  | first = David
  | title = Relaxation and tidal stripping in rich clusters of galaxies. I. Evolution of the mass distribution
  | journal = The Astrophysical Journal
  | volume = 264
  | pages = 24–48
  | date = January 1983
  | bibcode = 1983ApJ...264...24M}}</ref> This process begins when the motion of a large galaxy in a cluster attracts smaller galaxies and [[Astronomy:Dark matter|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 [[Physics:Kinetic energy|kinetic energy]], the large galaxy gradually spirals toward the centre of the cluster. Once there, the [[Astronomy:Star|star]]s, [[Physics:Gas|gas]], dust and [[Astronomy:Dark matter|dark matter]] of the large galaxy and its trailing galaxies will join with those of other galaxies who preceded them in the same fate.<ref name="DM1984">
{{Cite journal
  | doi = 10.1086/161590
  | last = Merritt
  | first = David
  | title = Relaxation and tidal stripping in rich clusters of galaxies. II. Evolution of the luminosity distribution
  | journal = The Astrophysical Journal
  | volume = 276
  | pages = 26–37
  | date = January 1984
  | bibcode = 1984ApJ...276...26M| doi-access = free
  }}</ref> A giant or supergiant diffuse or elliptical galaxy will result from this accumulation.<ref name="DM1985">
{{Cite journal
  | doi = 10.1086/162860
  | last = Merritt
  | first = David
  | title = Relaxation and tidal stripping in rich clusters of galaxies. III. Growth of a massive central galaxy
  | journal = The Astrophysical Journal
  | volume = 289
  | pages = 18–32
  | date = February 1985
  | bibcode = 1985ApJ...289...18M| doi-access = free
  }}</ref> The centers of merged or merging galaxies can remain recognizable for long times, appearing as multiple "nuclei" of the cD galaxy.<ref>{{Cite journal
  | doi = 10.1086/184257
  | last = Merritt
  | first = David
  | title = The nature of multiple-nucleus cluster galaxies
  | journal = The Astrophysical Journal
  | volume = 280
  | pages = L5–8
  | date = May 1984
  | bibcode = 1984ApJ...280L...5M| doi-access = free
  }}</ref>

==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".<ref>[http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1971PASP...83..313R&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf PDF], "'Tuning Fork' Classification of Rich Clusters of Galaxies", Herbert J.Rood, Gummuluru N. Sastry, June 1971, {{doi|10.1086/129128}}, {{bibcode|1971PASP...83..313R}} (accessed 14 April 2010)</ref>

==Examples ==
* [[Astronomy:3C 401|3C 401]]
* [[Astronomy:Abell 1201 BCG|Abell 1201 BCG]]
* [[Astronomy:Abell 1413#Abell 1413 BCG|Abell 1413 BCG]]
* [[Astronomy:ESO 383-76|ESO 383-76]], the large central galaxy of Abell 3571
* [[Astronomy:Holmberg 15A|Holmberg 15A]] (currently home to one of the largest black holes)
* [[Astronomy:IC 1101|IC 1101]], the large central galaxy of the massive cluster [[Astronomy:Abell 2029|Abell 2029]].<ref>Science, [http://www.sciencemag.org/cgi/content/abstract/250/4980/539 "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}}</ref><ref>"Galaxy Found", ''Ellensburg Daily Record'', United Press International, 27 October 1990, p. 16</ref><ref>Lodi News-Sentinel, "Giant Galaxy Discovered", UPI, 26 October 1990, p. 9</ref>
* [[Astronomy:Messier 87|Messier 87]], the central galaxy in the [[Astronomy:Virgo Cluster|Virgo Cluster]]
* [[Astronomy:NeVe 1|NeVe 1]], the host galaxy of the [[Astronomy:Ophiuchus Supercluster|Ophiuchus Supercluster]] eruption event, the most energetic outburst known.
* [[Astronomy:NGC 1399|NGC 1399]] in the [[Astronomy:Fornax Cluster|Fornax Cluster]]
* [[Astronomy:NGC 4889|NGC 4889]], is also known as the Caldwell 35, a supergiant [[Astronomy:Galaxy|galaxy]], a class-4 elliptical galaxy, it is the brightest within Caldwell Objects in the constellation [[Astronomy:Coma Berenices|Coma Berenices]]
* [[Astronomy:NGC 6086|NGC 6086]]
* [[Astronomy:NGC 6166|NGC 6166]]<ref>''Nature'', [http://www.nature.com/nature/journal/v460/n7252/fig_tab/nature08135_F3.html "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)</ref>
* Perseus A<ref>''Nature'', [http://www.nature.com/nature/journal/v460/n7252/fig_tab/nature08135_F4.html "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)</ref>
* QSO 0957, the first identified gravitationally lensed object

== See also ==
* [[Astronomy:Elliptical galaxy|Elliptical galaxy]] (E#)
* Giant elliptical galaxy (gE)
* [[Astronomy:Lenticular galaxy|Lenticular galaxy]] (S0, SA0, SAB0, SB0, E9)
* [[Astronomy:Seyfert galaxy|Seyfert galaxy]]
* Type-D galaxy
* [[Astronomy:List of largest galaxies|List of largest galaxies]] - a broader list containing type-cD galaxies, arranged by size.

==References==
{{Reflist}}

==Further reading==
* [http://nedwww.ipac.caltech.edu/level5/Morgan2/paper.pdf PDF]—[http://nedwww.ipac.caltech.edu/level5/Morgan2/frames.html "A Preliminary Classification of the Forms of Galaxies According to Their Stellar Population"], W. W. Morgan, Yerkes Obeservatory, 1958, {{doi|10.1086/127415}}, {{bibcode|1959PASP...71..394M}} (PASP 70)
* [https://arxiv.org/abs/astro-ph/0612229 "Intracluster light and the extended stellar envelopes of cD galaxies: an analytical description"], Marc S. Seigar, Alister W. Graham, Helmut Jerjen, July 2007 {{doi|10.1111/j.1365-2966.2007.11899.x}}, {{bibcode|2007MNRAS.378.1575S}}, {{arxiv|astro-ph/0612229v2}} (MNRAS 07/2007)

{{Galaxy}}

{{DEFAULTSORT:Type-Cd Galaxy}}
[[Category:Galaxy morphological types]]

{{Sourceattribution|Type-cD galaxy}}