View source for Galaxy cluster

{{short description|Structure made up of a gravitationally-bound aggregation of hundreds of galaxies}}[[File:BoRG-58.jpg|thumb|right|300px|Composite image of [[BoRG-58]], a group of 5 galaxies clustered together just 600 million years after the Universe's birth<ref name="Hubble protocluster">{{cite news|title=Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen|url=http://www.spacetelescope.org/news/heic1201/|access-date=13 January 2015|newspaper=ESA/Hubble Press Release}}</ref>]]

A '''galaxy cluster''', or a '''cluster of galaxies''', is a structure that consists of anywhere from hundreds to thousands of [[galaxy|galaxies]] that are bound together by [[gravity]],<ref name="Hubble protocluster" /> with typical masses ranging from 10<sup>14</sup> to 10<sup>15</sup> [[solar mass]]es. Clusters consist of galaxies, heated gas, and dark matter.<ref name=":0" /> They are the biggest known [[gravitational binding energy|gravitationally bound]] structures in the [[universe]]. They were believed to be the [[Observable universe#Large-scale structure|largest known structures]] in the universe until the 1980s, when [[supercluster]]s were discovered.<ref name="Kravtsov2012">{{Cite journal |last1=Kravtsov|first1=A. V. |last2=Borgani|first2=S. |doi=10.1146/annurev-astro-081811-125502 |title=Formation of Galaxy Clusters | journal=[[Annual Review of Astronomy and Astrophysics]] |volume=50 |pages=353–409 |year=2012 |arxiv=1205.5556 |bibcode=2012ARA&A..50..353K |s2cid=119115331}}</ref> Small aggregates of galaxies are referred to as [[galaxy group]]s rather than clusters of galaxies. Together, [[galaxy groups and clusters]] form superclusters.

==Basic properties==
[[File:Galaxy cluster IDCS J1426.jpg|thumb|Galaxy cluster [[IDCS J1426.5+3508|IDCS J1426]] is located 10 billion light-years from Earth and has the mass of almost 500 trillion suns (multi-wavelength image: X-rays in blue, visible light in green, and infrared light in red).<ref>{{cite web|title=Galaxy cluster IDCS J1426|url=http://www.spacetelescope.org/images/opo1602a/|access-date=11 January 2016}}</ref>]]

Galaxy clusters typically have the following properties:
* They contain 100 to 1,000 galaxies, hot [[X-ray]] emitting gas and large amounts of [[dark matter]].<ref name=":0">{{Cite web | url=http://chandra.harvard.edu/xray_sources/galaxy_clusters.html | title=Groups & Clusters of Galaxies |website=Chandra X-ray Observatory}}</ref> Details are described in the "Composition" section.
* They have total masses of 10<sup>14</sup> to 10<sup>15</sup> [[Solar mass|solar masses]].
* They typically have diameters from 1 to 5 [[Megaparsec|Mpc]] (see [[1 E23 m|10<sup>23</sup> m]] for distance comparisons).
* The spread of velocities for the individual galaxies is about 800–1000&nbsp;km/s.

==Composition==
Galaxy clusters have three main components. Galaxies themselves only make up a small fraction of clusters, although they are the only component we can detect in the [[visible spectrum]]. The heated gas of the intracluster medium (ICM) has a peak temperature between 30 and 100 million degrees Celsius.<ref name=":0" /> Dark matter makes up the majority of the mass of galaxy clusters, but cannot be detected optically.<ref name="Kravtsov2012" /> 
{| class="wikitable"
|-
! Component
! Mass fraction
! Description
|-
| Galaxies
| 1%
| In optical observations, only galaxies are visible
|-
| Intergalactic gas in [[intracluster medium]]
| 9%
| Plasma between the galaxies at high temperature and emit x-ray radiation by thermal [[bremsstrahlung]]
|-
| Dark matter
| 90%
| Most massive component but cannot be detected optically and is inferred through gravitational interactions
|}

== Cluster formation and evolution ==
As galaxy clusters form, massive amounts of energy are released due to shock waves, the heating of gas, and galaxy interactions.<ref name="Kravtsov2012" /> Gas collides with existing material which generates shock waves, heating it to tens of millions of degrees and producing X-ray emissions.<ref>{{Cite journal |last=Sunyaev |first=R.A. |date=February 25, 1972 |title=Formation of Clusters of Galaxies; Protocluster Fragmentation and Intergalactic Heating |url=https://adsabs.harvard.edu/full/1972A%26A....20..189S |journal=Astron. Astrophys. |volume=20 |pages=189 |bibcode=1972A&A....20..189S }}</ref> Galaxy evolution within the cluster is governed by interactions between galaxies, such as [[Galaxy merger|galaxy mergers]], and gas stripping. 

==Classification==
There are many classification systems for galaxy clusters, based on characteristics such as shape symmetry, X-ray [[luminosity]], and dominant galaxy type.<ref>{{Cite web |title=Clusters and Superclusters of Galaxies |url=https://ned.ipac.caltech.edu/level5/Sept01/Bahcall2/Bahcall2_11.html |access-date=2025-02-17 |website=ned.ipac.caltech.edu}}</ref> The [[Bautz–Morgan classification|Bautz-Morgan classification]] sorts clusters into types I, II, and III based on the relative brightness of their galaxies–type I with greatest contrast and type III with the least.<ref>{{Cite journal |last1=Bautz |first1=L. P. |last2=Morgan |first2=W. W. |date=1970-12-01 |title=On the Classification of the Forms of Clusters of Galaxies |url=https://ui.adsabs.harvard.edu/abs/1970ApJ...162L.149B/abstract |journal=The Astrophysical Journal |volume=162 |pages=L149 |doi=10.1086/180643 |bibcode=1970ApJ...162L.149B |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Dressler |first1=A. |title=  Bautz-Morgan classes and the luminosity function for clusters of galaxies|url=https://adsabs.harvard.edu/full/1978ApJ...222...23D |access-date=2025-02-17 |journal=The Astrophysical Journal |date=1978 |volume=222 |page=23 |doi=10.1086/156117 |bibcode=1978ApJ...222...23D }}</ref>

== Galaxy clusters as measuring instruments ==

=== Gravitational redshift ===
Galaxy clusters have been used by Radek Wojtak from the Niels Bohr Institute at the University of Copenhagen to test predictions of [[general relativity]]: energy loss from light escaping a gravitational field. Photons emitted from the center of a galaxy cluster should lose more energy than photons coming from the edge of the cluster because gravity is stronger in the center. Light emitted from the center of a cluster has a longer wavelength than light coming from the edge. This effect is known as [[gravitational redshift]]. Using the data collected from 8000 galaxy clusters, Wojtak was able to study the properties of gravitational redshift for the distribution of galaxies in clusters. He found that the light from the clusters was redshifted in proportion to the distance from the center of the cluster as predicted by general relativity. The result also strongly supports the [[Lambda-CDM model|Lambda-Cold Dark Matter]] model of the Universe, according to which most of the cosmos is made up of [[Dark matter|Dark Matter]] that does not interact with matter.<ref>{{cite magazine |last1=Yudhijit |first1=Bhattacharjee |title=Galaxy Clusters Back Up Einstein's Theory of Relativity |url=https://www.wired.com/2011/09/galaxies-einstein-relativity/ |magazine=Wired |access-date=2022-04-04}}</ref>

=== Gravitational lensing ===
Galaxy clusters are also used for their strong gravitational potential as [[gravitational lens]]es to boost the reach of telescopes.<ref>{{Cite journal |last1=Walker |first1=Stephen |last2=Simionescu |first2=Aurora |last3=Nagai |first3=Daisuke |last4=Okabe |first4=Nobuhiro |last5=Eckert |first5=Dominique |last6=Mroczkowski |first6=Tony |last7=Akamatsu |first7=Hiroki |last8=Ettori |first8=Stefano |last9=Ghirardini |first9=Vittorio |date=2019-01-02 |title=The Physics of Galaxy Cluster Outskirts |url=https://link.springer.com/article/10.1007/s11214-018-0572-8 |journal=Space Science Reviews |language=en |volume=215 |issue=1 |page=7 |doi=10.1007/s11214-018-0572-8 |issn=1572-9672|arxiv=1810.00890 |bibcode=2019SSRv..215....7W }}</ref> The gravitational distortion of space-time occurs near massive galaxy clusters and bends the path of photons to create a cosmic magnifying glass. This can be done with photons of any wavelength from the optical to the X-ray band. The latter is more difficult, because galaxy clusters emit a lot of X-rays.<ref>{{Cite journal |last1=Reiprich |first1=Thomas H. |last2=Basu |first2=Kaustuv |last3=Ettori |first3=Stefano |last4=Israel |first4=Holger |last5=Lovisari |first5=Lorenzo |last6=Molendi |first6=Silvano |last7=Pointecouteau |first7=Etienne |last8=Roncarelli |first8=Mauro |date=2013-08-01 |title=Outskirts of Galaxy Clusters |url=https://link.springer.com/article/10.1007/s11214-013-9983-8 |journal=Space Science Reviews |language=en |volume=177 |issue=1 |pages=195–245 |doi=10.1007/s11214-013-9983-8 |issn=1572-9672|arxiv=1303.3286 |bibcode=2013SSRv..177..195R }}</ref> However, X-ray emission may still be detected when combining X-ray data to optical data. One particular case is the use of the Phoenix galaxy cluster to observe a dwarf galaxy in its early high energy stages of star formation.<ref>{{cite web |last1=Chu |first1=Jennifer |title=Astronomers use giant galaxy cluster as X-ray magnifying lens |url=https://news.uchicago.edu/story/astronomers-use-giant-galaxy-cluster-x-ray-magnifying-lens |website=MIT News |date=15 October 2019 |access-date=2022-04-04}}</ref>

==Notable galaxy clusters==
[[File:07-Laniakea_(LofE07240).png|thumb|The [[Laniakea Supercluster]] with many [[galaxy clusters]]]]

{{Main|List of galaxy groups and clusters}}Notable galaxy clusters in the relatively nearby universe include the [[Virgo Cluster]], [[Fornax Cluster]], [[Hercules Cluster]], and the [[Coma Cluster]]. A very large aggregation of galaxies known as the [[Great Attractor]], dominated by the [[Norma Cluster]], is massive enough to affect the [[Hubble's law|local expansion of the Universe]]. Notable galaxy clusters in the distant, high-redshift universe include [[SPT-CL J0546-5345]] and [[SPT-CL J2106-5844]], the most massive galaxy clusters found in the early Universe. In the last few decades, they are also found to be relevant sites of particle acceleration, a feature that has been discovered by observing non-thermal diffuse radio emissions, such as [[radio halo]]s and [[radio relics]]. Using the [[Chandra X-ray Observatory]], structures such as cold fronts and [[shock waves in astrophysics|shock waves]] have also been found in many galaxy clusters.
{|class=wikitable
|+
|- <!-- this serves as a summary style presentation of the list article; and to prevent an empty section from appearing -->
!| Cluster
!| Notes
|-
|| [[Virgo Cluster]]
|| The nearest massive galaxy cluster
|-
|| [[Norma Cluster]]
|| The cluster at the heart of the [[Great Attractor]]
|-
|| [[Bullet Cluster]]
|| A cluster merger with the first observed separation between dark matter and normal matter
|-class=sortbottom
|colspan=99| {{small| This lists some of the most notable clusters; for more clusters, see the list article. }}
|-
|}

==Gallery==
{{multiple images |total_width=800 |direction=horizontal |align=center |header=Left: Image by the [[Hubble Space Telescope]] (2017) Right: Image by the [[James Webb Space Telescope]] (2022)<ref name="NBC-20220712">{{cite news |last1=Chow |first1=Denise |last2=Wu |first2=Jiachuan |title=Photos: How pictures from the Webb telescope compare to Hubble's - NASA's $10 billion telescope peers deeper into space than ever, revealing previously undetectable details in the cosmos.|url=https://www.nbcnews.com/data-graphics/compare-photos-nasas-james-webb-space-telescope-hubble-space-telescope-rcna37875 |date=12 July 2022 |work=[[NBC News]] |access-date=16 July 2022 }}</ref>
 |image1=NASA-HubbleSpaceTelescope-DeepField-2017.jpg |image2=Webb's First Deep Field (adjusted).jpg
 |footer=[[Webb's First Deep Field|Deep Field]] – Galaxy cluster [[SMACS J0723.3-7327]].<ref name="NASA-20220711">{{cite news |last=Garner |first=Rob |title=NASA's Webb Delivers Deepest Infrared Image of Universe Yet |url=https://www.nasa.gov/image-feature/goddard/2022/nasa-s-webb-delivers-deepest-infrared-image-of-universe-yet |date=11 July 2022 |work=[[NASA]] |access-date=12 July 2022 |archive-date=12 July 2022 |archive-url=https://web.archive.org/web/20220712000119/https://www.nasa.gov/image-feature/goddard/2022/nasa-s-webb-delivers-deepest-infrared-image-of-universe-yet/ |url-status=live }}</ref><ref name="NYT-20220711">{{cite news |last1=Overbye |first1=Dennis |last2=Chang |first2=Kenneth |last3=Tankersley |first3=Jim |title=Biden and NASA Share First Webb Space Telescope Image – From the White House on Monday, humanity got its first glimpse of what the observatory in space has been seeing: a cluster of early galaxies. |url=https://www.nytimes.com/2022/07/11/science/nasa-webb-telescope-images-livestream.html |date=11 July 2022 |work=[[The New York Times]] |access-date=12 July 2022 |archive-date=12 July 2022 |archive-url=https://web.archive.org/web/20220712005736/https://www.nytimes.com/2022/07/11/science/nasa-webb-telescope-images-livestream.html |url-status=live }}</ref><ref name="SA-202207">{{cite news |last=Pacucci |first=Fabio |title=How Taking Pictures of 'Nothing' Changed Astronomy - Deep-field images of "empty" regions of the sky from Webb and other space telescopes are revealing more of the universe than we ever thought possible |url=https://www.scientificamerican.com/article/how-taking-pictures-of-nothing-changed-astronomy1/ |date=15 July 2022|work=[[Scientific American]] |access-date=16 July 2022 }}</ref><ref name="ABC-20220714">{{cite news |last1=Deliso |first1=Meredith |last2=Longo |first2=Meredith |last3=Rothenberg |first3=Nicolas |title=Hubble vs. James Webb telescope images: See the difference |url=https://abcnews.go.com/Technology/hubble-james-webb-telescope-images-difference/story?id=86763039 |date=14 July 2022 |work=[[ABC News (United States)|ABC News]] |access-date=15 July 2022 }}</ref><ref name="CNET-20120713">{{cite news |last=Kooser |first=Amanda |title=Hubble and James Webb Space Telescope Images Compared: See the Difference - The James Webb Space Telescope builds on Hubble's legacy with stunning new views of the cosmos. |url=https://www.cnet.com/pictures/hubble-and-james-webb-space-telescope-images-compared-see-the-difference/ |date=13 July 2012 |work=[[CNET]] |access-date=16 July 2022 }}</ref><ref name="UT-20220502">{{cite news |last=Atkinson |first=Nancy |title=Now, We can Finally Compare Webb to Other Infrared Observatories |url=https://www.universetoday.com/155686/now-we-can-finally-compare-webb-to-other-infrared-observatories/ |date=2 May 2022 |work=[[Universe Today]] |access-date=12 May 2022 |archive-date=10 May 2022 |archive-url=https://web.archive.org/web/20220510035557/https://www.universetoday.com/155686/now-we-can-finally-compare-webb-to-other-infrared-observatories/ |url-status=live }}</ref>}}
{{wide image|14-283-Abell2744-DistantGalaxies-20141016.jpg|800px|align-cap=center|[[Abell 2744]] galaxy cluster – extremely distant [[galaxies]] revealed by [[gravitational lensing]] (16 October 2014).<ref name="NASA-20140107">{{cite news |last1=Clavin |first1=Whitney |last2=Jenkins |first2=Ann |last3=Villard |first3=Ray |title=NASA's Hubble and Spitzer Team up to Probe Faraway Galaxies |url=http://www.jpl.nasa.gov/news/news.php?release=2014-007 |date=7 January 2014 |work=[[NASA]] |access-date=8 January 2014 }}</ref><ref name="NASA-20141016">{{cite web |last1=Chou |first1=Felecia |last2=Weaver |first2=Donna |title=RELEASE 14-283 – NASA's Hubble Finds Extremely Distant Galaxy through Cosmic Magnifying Glass |url=http://www.nasa.gov/press/2014/october/nasa-s-hubble-finds-extremely-distant-galaxy-through-cosmic-magnifying-glass/ |date=16 October 2014 |work=[[NASA]] |access-date=17 October 2014 }}</ref>}}

===Images===
<gallery>
File:Distant_and_ancient_SPT0615-JD.jpg|Galaxy cluster [[SPT-CL J0615-5746]].<ref>{{cite web |title=Distant and ancient |url=https://www.spacetelescope.org/images/potw1918a/ |website=www.spacetelescope.org |access-date=6 May 2019 |language=en}}</ref>
File:Strings of homeless stars RXC J0232.2-4420.jpg| Galaxy cluster [[RXC J0232.2-4420]].<ref>{{cite web |title=Strings of homeless stars |url=http://www.spacetelescope.org/images/potw1824a/ |website=www.spacetelescope.org |access-date=11 June 2018}}</ref>
File:From toddlers to babies RXC J0032.1+1808.jpg|Galaxy cluster RXC J0032.1+1808 as part of the [https://relics.stsci.edu/ RELICS] program.<ref>{{cite web|title=From toddlers to babies|url=https://www.spacetelescope.org/images/potw1819a/|website=www.spacetelescope.org|access-date=7 May 2018}}</ref>
File:Approaching the Universe's origins PSZ2 G138.61-10.84.jpg|Massive galaxy cluster [[PSZ2 G138.61-10.84]] is about six billion light-years away.<ref>{{cite web|title=Approaching the Universe's origins|url=http://www.spacetelescope.org/images/potw1816a/|website=www.spacetelescope.org|access-date=16 April 2018}}</ref>
File:HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns.jpg|[[Very Large Telescope#Instruments|HAWK-I]] and [[Hubble Space Telescope|Hubble]] explore [[RCS2 J2327]] cluster with the mass of two quadrillion Suns.<ref>{{cite web|title=HAWK-I and Hubble Explore a Cluster with the Mass of two Quadrillion Suns|url=https://www.eso.org/public/images/potw1752a/|website=www.eso.org|access-date=25 December 2017}}</ref>
File:Streaks and stripes Abell 2537.jpg|[[Abell 2537]] is useful in probing cosmic phenomena like dark matter and dark energy.<ref>{{cite web|title=Streaks and stripes|url=http://www.spacetelescope.org/images/potw1748a/|website=www.spacetelescope.org|access-date=27 November 2017}}</ref>
File:Cosmic RELICS Abell 1300.jpg|[[Abell 1300]] acts like a lens, bending the very fabric of space around it.<ref>{{cite web|title=Cosmic RELICS|url=http://www.spacetelescope.org/images/potw1745a/|website=www.spacetelescope.org|access-date=6 November 2017}}</ref>
File:Cosmic archaeology WHL J24.3324-8.477.jpg|Galaxy cluster WHL J24.3324-8.477.<ref>{{cite web|title=Cosmic archaeology|url=https://www.spacetelescope.org/images/potw1743a/|website=www.spacetelescope.org|access-date=24 October 2017}}</ref>
File:Hubble pushed beyond limits to spot clumps of new stars in distant galaxy.jpg|Background galaxy has been gravitationally lensed by the intervening galaxy cluster.<ref>{{cite web|title=Hubble pushed beyond limits to spot clumps of new stars in distant galaxy|url=https://www.spacetelescope.org/images/opo1727a/|website=www.spacetelescope.org|access-date=12 July 2017}}</ref>
File:HST-Smiling-GalaxyClusterSDSS-J1038+4849-20150210.jpg|"[[Smiley]]" image – galaxy cluster (SDSS J1038+4849) & [[gravitational lensing]] (an [[Einstein ring]]) ([[Hubble Space Telescope|HST]]).<ref name="NASA-20150210">{{cite web|last1=Loff|first1=Sarah|last2=Dunbar|first2=Brian|title=Hubble Sees A Smiling Lens|url=http://www.nasa.gov/content/hubble-sees-a-smiling-lens/|date=10 February 2015|work=[[NASA]]|access-date=10 February 2015 }}</ref>
File:Image of the galaxy cluster SpARCS1049.jpg|Galaxy cluster [[SpARCS1049+56|SpARCS1049]] taken by Spitzer and the Hubble Space Telescope.<ref>{{cite web|title=Image of the galaxy cluster SpARCS1049|url=http://www.spacetelescope.org/images/heic1519a/|access-date=11 September 2015}}</ref>
File:PIA20052-GalaxyCluster-MOO-J1142+1527-20151103.jpg|Galaxy cluster [[MOO J1142+1527]] discovered by the [[MaDCoWS]] survey
File:Heic1401a-Abell2744-20140107.jpg|[[Abell 2744]] galaxy cluster ([[Hubble Space Telescope#Frontier Fields program|HST]]).<ref name="NASA-20140107" />
File:Magnifying the distant Universe.jpg|Magnifying the distant universe through [[MACS J0454.1-0300]].<ref>{{cite news|title=Magnifying the distant Universe|url=http://www.spacetelescope.org/images/potw1412a/|access-date=10 April 2014|newspaper=ESA/Hubble Picture of the Week }}</ref>
File:14-296-GalaxyClusters-PerseusVirgo-ChandraXRay-20141027.jpg|[[Turbulence]] may prevent galaxy clusters from cooling; illustrated: [[Perseus Cluster]] and [[Virgo Cluster]] ([[Chandra X-ray Observatory|Chandra X-ray]]).
File:Color image of galaxy cluster MCS J0416.1–2403.jpg|[[MACS0416.1-2403]] imaged by the [[Hubble Space Telescope|HST]]
File:Light_Bends_from_the_Beyond.jpg| The galaxy cluster Abell 2813 (also known as ACO 2813) image from the NASA/ESA [[Hubble Space Telescope]]
File:A_Menagerie_of_Galaxies.jpg| A menagerie of galaxies — the galaxy cluster [[ACO S 295]]
File:Cosmic_Lens_Flare.jpg| Cosmic lens flare
File:Hubble spots three images of a distant supernova.jpg|Hubble spots three images of a distant supernova
File:Galaxy cluster WHL0137-08 (sunrisearc1).jpg|A massive galaxy cluster called WHL0137-08 
File:El Gordo (NIRCam Image) (2023-119).png|Galaxy cluster known as "El Gordo"
File:Seeing Triple (potm2302a).jpeg|Observation from the [[James Webb Space Telescope]] the massive galaxy cluster RX J2129.<ref>{{cite news |url=https://esawebb.org/images/potm2302a/|title=Seeing Triple |date=October 18, 2023}}</ref>
</gallery>

===Videos===
<gallery>
File:Artist’s impression of a protocluster forming in the early Universe.ogv|Video: Formation of galaxy cluster [[MRC 1138-262]] (artist's concept).
</gallery>

==See also==
{{Commons category|Galaxy clusters}}
* [[Abell catalogue]]
* [[Intracluster medium]]
* [[List of Abell clusters]]
{{Clear}}

==References==
{{reflist}}

{{Galaxy}}
{{Portal bar|Astronomy|Stars|Outer space}}

[[Category:Galaxy clusters| Cluster]]
[[Category:Articles containing video clips]]
[[Category:Types of groupings]]