Editing Dwarf galaxy (section)

==Ultra-compact dwarfs==
[[File:M59-UCD3 M85-HCCI.png|thumb|[[M59-UCD3]] and [[M85-HCC1|M85-HCCI]] are the densest galaxies discovered.]]
'''Ultra-compact dwarf''' galaxies (UCD) are a class of very compact galaxies with very high stellar densities,
discovered<ref name="hilker1999">{{cite journal
   |author1=Hilker, M.
   |author2=Infante, L.
   |author3=Vieira, G.
   |author4=Kissler-Patig, M.
   |author5=Richtler, T.
   |title=The central region of the Fornax cluster. II. Spectroscopy and radial velocities of member and background galaxies
   |journal=[[Astronomy and Astrophysics Supplement]]
   |volume=134
   |date=1999
   |pages=75–86
   |doi=10.1051/aas:1999434
   |bibcode=1999A&AS..134...75H
   |arxiv=astro-ph/9807144
|s2cid=17710039
 }}</ref><ref name="drinkwater2000">{{cite journal
   |author1=Drinkwater, M. J.
   |author2=Jones, J. B.
   |author3=Gregg, M. D.
   |author4=Phillipps, S.
   |title=Compact Stellar Systems in the Fornax Cluster: Super-massive Star Clusters or Extremely Compact Dwarf Galaxies?
   |journal=[[Publications of the Astronomical Society of Australia]]
   |volume=17
   |issue=3
 |date=2000
   |pages=227–233
   |doi=10.1071/AS00034
   |bibcode=2000PASA...17..227D
   |arxiv=astro-ph/0002003
|s2cid=13161406
 }}</ref><ref name="smh2003">
{{cite news|last1=Smith|first1=Deborah|title=Star search finds millions masquerading as one|work=[[Sydney Morning Herald]]|date=29 May 2003|issn=0312-6315|page=5}}
</ref> in the 2000s.
They are thought to be on the order of 200 light years across, containing about 100 million stars.<ref>Anglo-Australian Observatory [https://www.aao.gov.au/public/mediarelease/astronomers-discover-dozen-of-mini-galaxies Astronomers discover dozens of mini-galaxies] {{Webarchive|url=https://web.archive.org/web/20180427015808/https://www.aao.gov.au/public/mediarelease/astronomers-discover-dozen-of-mini-galaxies |date=27 April 2018 }} 0100 AEST Friday 2 April 2004.</ref> It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by [[Galactic tide|tidal interactions]], travelling through the hearts of rich clusters.<ref name="kazantzidis2003">
{{cite journal
   |arxiv=astro-ph/0307362
   |author1=Stelios Kazantzidis
   |author2=Ben Moore
   |author3=Lucio Mayer
   |title=Galaxies and Overmerging: What Does it Take to Destroy a Satellite Galaxy?
   |date=2004
   |journal=[[ASP Conference Series]]
   |volume=327
   |page=155
   |url=http://aspbooks.org/custom/publications/paper/327-0155.html
   |bibcode=2004ASPC..327..155K
}}
</ref> UCDs have been found in the [[Virgo Cluster]], [[Fornax Cluster]], [[Abell 1689]], and the [[Coma Cluster]], amongst others.<ref>
{{cite journal
   |author1=Mieske
   |author2=Infante
   |author3=Benitez
   |author4=Coe
   |author5=Blakeslee
   |author6=Zekser
   |author7=Ford
   |author8=Broadhurst
   |author9=Illingworth
   |title=Ultra Compact Dwarf galaxies in Abell 1689: a photometric study with the ACS
   |doi=10.1086/423701
   |date=2004
   |journal=[[The Astronomical Journal]]
   |volume=128
   |issue=4
   |pages=1529–1540
   |arxiv=astro-ph/0406613
   |bibcode=2004AJ....128.1529M
   |display-authors=8
   |last10=Hartig|first10=G. F.
   |last11=Clampin|first11=M.
   |last12=Ardila|first12=D. R.
   |last13=Bartko|first13=F.
   |last14=Bouwens|first14=R. J.
   |last15=Brown|first15=R. A.
   |last16=Burrows|first16=C. J.
   |last17=Cheng|first17=E. S.
   |last18=Cross|first18=N. J. G.
   |last19=Feldman|first19=P. D.
   |last20=Franx|first20=M.
   |last21=Golimowski|first21=D. A.
   |last22=Goto|first22=T.
   |last23=Gronwall|first23=C.
   |last24=Holden|first24=B.
   |last25=Homeier|first25=N.
   |last26=Kimble|first26=R. A.
   |last27=Krist|first27=J. E.
   |last28=Lesser|first28=M. P.
   |last29=Martel|first29=A. R.
   |last30=Menanteau|first30=F.
|s2cid=15575071
 }}</ref>
In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team.<ref>{{cite web|url=https://www.eso.org/sci/meetings/2012/surveys2012/Presentations/Day3-Wednesday/Ferrarese.pdf|access-date=2023-08-20|title=The Next Generation Virgo Cluster Survey}}</ref> The first ever relatively robust studies of the global properties of Virgo UCDs suggest that
UCDs have distinct dynamical<ref name="zhang2015">{{cite journal
   |title= The Next Generation Virgo Cluster Survey. VI: The Kinematics of Ultracompact Dwarfs and Globular Clusters in M87
   |journal=[[Astrophysical Journal]]
   |date=March 2015
   |last1=Zhang|first1=Hong-Xin|display-authors=etal|doi=10.1088/0004-637X/802/1/30
   |arxiv=1501.03167
   |bibcode=2015ApJ...802...30Z
   |volume=802
   |issue=1
 |page=30
|s2cid=73517961
 }}</ref>
and structural<ref name="liu2015">
{{cite journal
   |title=The Next Generation Virgo Cluster Survey. X: Properties of Ultracompact Dwarfs in The M87, M49 and M60 Regions
   |journal=[[Astrophysical Journal]]
   |date=November 2015
   |last1=Liu|first1=Chengze|display-authors=etal|doi=10.1088/0004-637X/812/1/34
   |arxiv=1508.07334
   |bibcode=2015ApJ...812...34L
   |volume=812
   |issue=1
 |page=34
|s2cid=35610312
 }}</ref> properties from normal globular clusters. An extreme  example of UCD is [[M60-UCD1]], about 54 million light years away, which contains approximately 200 million solar masses within a 160 light year radius; the stars in its central region are packed 25 times more densely than stars in Earth's region in the Milky Way.<ref>
{{cite journal
   |title=The Densest Galaxy
   |journal=[[Astrophysical Journal Letters]]
   |date=August 2013
   |volume=775
   |issue=1
   |pages=L6
   |doi=10.1088/2041-8205/775/1/L6
   |last1=Strader|first1=Jay
   |display-authors=4
   |last2=Seth|first2=Anil C.
   |last3=Forbes|first3=Duncan A.
   |last4=Fabbiano |first4=Giuseppina
   |last5=Romanowsky|first5=Aaron J.
   |last6=Brodie|first6=Jean P.
   |last7=Conroy |first7=Charlie
   |last8=Caldwell|first8=Nelson
   |last9=Pota |first9=Vincenzo
   |last10=Usher|first10=Christopher
   |last11=Arnold|first11=Jacob A.
   |arxiv = 1307.7707
   |bibcode = 2013ApJ...775L...6S
|s2cid=52207639
 }}
</ref><ref>
{{cite web|title=Evidence for densest galaxy in nearby universe|url=http://phys.org/news/2013-09-evidence-densest-galaxy-nearby-universe.html|publisher=Phys.org (Omicron Technology Ltd)|access-date=25 September 2013|date=24 September 2013|quote=What makes M60-UCD1 so remarkable is that about half of this mass is found within a radius of only about 80 light years. The density of stars is about 15,000 times greater—meaning the stars are about 25 times closer to each other—than in Earth's region of the [[Milky Way]] galaxy.
}}
</ref>
[[M59-UCD3]] is approximately the same size as M60-UCD1 with a [[Effective radius|half-light radius]], r<sub>h</sub>, of approximately 20 [[parsecs]] but is 40% more luminous with an [[absolute visual magnitude]] of approximately −14.6.  This makes M59-UCD3 the second densest known galaxy.<ref name=m59-ucd3>{{cite journal
   |last1=Sandoval|first1=Michael A.
   |last2=Vo|first2=Richard P.
   |last3=Romanowsky|first3=Aaron J.
   |last4=Strader|first4=Jay
   |last5=Choi|first5=Jieun
   |last6=Jennings|first6=Zachary G.
   |last7=Conroy|first7=Charlie
   |last8=Brodie|first8=Jean P.
   |last9=Foster|first9=Caroline
   |last10=Villaume|first10=Alexa
   |last11=Norris|first11=Mark A.
   |last12=Janz|first12=Joachim
   |last13=Forbes|first13=Duncan A.
   |title=Hiding in Plain Sight: Record-breaking Compact Stellar Systems in the Sloan Digital Sky Survey
   |journal=[[The Astrophysical Journal]]
   |date=23 July 2015
   |volume=808
   |issue=1
   |pages=L32
   |doi=10.1088/2041-8205/808/1/L32
   |arxiv=1506.08828
   |bibcode=2015ApJ...808L..32S
|s2cid=55254708
 }}</ref>
Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have [[supermassive black holes]] weighing 13% and 18% of the galaxies' masses.<ref name="ahn2017">{{cite journal
   |author1=Ahn, C. P.
   |author2=Seth, A. C.
   |author3=den Brok, M.
   |author4=Strader, J.
   |author5=Baumgardt, H.
   |author6=van den Bosch, R.
   |author7=Chilingarian, I.
   |author8=Frank, M.
   |author9=Hilker, M.
   |author10=McDermid, R.
   |author11=Mieske, S.
   |author12=Romanowsky, A. J.
   |author13=Spitler, L.
   |author14=Brodie, J.
   |author15=Neumayer, N.
   |author16=Walsh, J. L.
   |title=Detection of Supermassive Black Holes in Two Virgo Ultracompact Dwarf Galaxies
   |journal=[[Astrophysical Journal]]
   |volume=839
   |issue=2
 |date=2017
   |page=72
   |doi=10.3847/1538-4357/aa6972
   |bibcode=2017ApJ...839...72A
   |arxiv=1703.09221
|s2cid=55131811
 |doi-access=free
 }}</ref>