View source for Bullet Cluster

{{Short description|Two colliding clusters of galaxies in constellation Carina}}
{{distinguish|Bullet Group|Bullet Galaxy}}
{{Infobox galaxy cluster
| name = Bullet Cluster
| image = Bullet cluster.jpg
| caption = X-ray photo by [[Chandra X-ray Observatory]]. Exposure time was 140 hours. The scale is shown in mega[[parsec]]s. [[Redshift]] (''z'') = 0.3, meaning its light has wavelengths stretched by a factor of 1.3.
| credit =
| epoch = [[Epoch (astronomy)#Julian years and J2000|J2000]]
| ra = {{RA|06|58|37.9}}
| dec = {{DEC|-55|57|0}}
| constellation = [[Carina (constellation)|Carina]]
| member_no = ~40
| brightest_member =
| richness =
| redshift = 0.296<ref name="Tucker1998">{{cite journal
 | last1 = Tucker
 | first1 = W.
 | author-link1 = Wallace Hampton Tucker
 | last2 = Blanco
 | first2 = P.
 | last3 = Rappoport, S. |others=David, L.; Fabricant, D.; Falco, E. E.; Forman, W.; Dressler, A.; Ramella, M.
 |date=March 1998
 | title = 1E 0657-56: A Contender for the Hottest Known Cluster of Galaxies
 | journal = Astrophysical Journal Letters
 | volume = 496
 | issue = 1
 | pages = L5
 | bibcode = 1998ApJ...496L...5T
 | doi = 10.1086/311234
 |arxiv = astro-ph/9801120 | s2cid = 16140198
 }}</ref>
| distance = 1.141 Gpc (3.7 billion light-years).<ref name="NED">{{cite web
 | url = http://ned.ipac.caltech.edu/cgi-bin/nph-objsearch?objname=Bullet+Cluster
 | title = NED results for object Bullet Cluster
 | publisher = NASA Extragalactic Database
 | access-date = March 4, 2012
 }}</ref>
| temperature = 17.4 ± 2.5 [[electron volt|keV]]
| mass =
| luminosity = 1.4 ± 0.3 × 10<sup>39</sup> ''h''<sub>50</sub><sup>−2</sup> joule/s ([[Bolometric magnitude#Bolometric magnitude|bolometric]])<ref name="Tucker1998"/>
| flux = 5.6 ± 0.6 × 10<sup>−19</sup> watt/cm<sup>2</sup>  (0.1–2.4 keV)<ref name="Tucker1998"/>
| other_names = 1E 0657-56, 1E 0657-558}}
The '''Bullet Cluster''' ('''1E 0657-56''') consists of two colliding [[Galaxy cluster|clusters of galaxies]]. Strictly speaking, the name ''Bullet Cluster'' refers to the smaller subcluster, moving away from the larger one. It is at a [[Comoving and proper distances|comoving radial distance]] of {{convert|1.141|Gpc|e9ly|abbr=in|lk=on}}.<ref name="NED"/>

The object is of a particular note for astrophysicists, because [[gravitational lens]]ing studies of the Bullet Cluster are claimed to provide strong evidence for the existence of [[dark matter]].<ref>{{cite journal
| first1 = Douglas | last1=Clowe | first2 = Anthony | last2=Gonzalez
| first3 = Maxim | last3=Markevich
| doi = 10.1086/381970
| journal = Astrophys. J.
| volume = 604
| title = Weak lensing mass reconstruction of the interacting cluster 1E0657-558: Direct evidence for the existence of dark matter
| issue = 2
| pages = 596–603
| year = 2004
| arxiv = astro-ph/0312273|bibcode = 2004ApJ...604..596C | s2cid=12184057 }}</ref><ref name="Markevitch1">{{cite journal
| author = M. Markevitch| author2 = A. H. Gonzalez
| author3 = D. Clowe| author4 = A. Vikhlinin
| author5 = L. David| author6 = W. Forman
| author7 = C. Jones| author8 = S. Murray
| author9 = W. Tucker
| name-list-style = amp
| doi = 10.1086/383178
| journal = Astrophys. J.
| volume = 606
| title = Direct constraints on the dark matter self-interaction cross-section from the merging galaxy cluster 1E0657-56
| issue = 2
| pages = 819–824
| year = 2004
| arxiv = astro-ph/0309303 |bibcode=2004ApJ...606..819M| s2cid = 119334056
}}</ref>
Observations of other galaxy cluster collisions, such as [[MACS J0025.4-1222]], similarly support the existence of dark matter.<ref name=Brada2008>{{Cite journal|title=Revealing the Properties of Dark Matter in the Merging Cluster MACS J0025.4-1222|date = 2008 |last1=Brada|first1= M |last2=Allen|first2=S. W |last3=Ebeling|first3=H |last4=Massey|first4=R |last5=Morris|first5=R. G|last6=von der Linden|first6=A|last7=Applegate|first7=D| journal = The Astrophysical Journal|volume = 687 |issue = 2 |page = 959 |arxiv = 0806.2320|doi = 10.1086/591246 |bibcode = 2008ApJ...687..959B |s2cid = 14563896 }}</ref>

== Overview ==
The major components of the cluster pair—[[star]]s, [[gas]] and the putative dark matter—behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible [[light]], were not greatly affected by the collision, and most passed right through, [[gravitationally]] slowed but not otherwise altered. The hot gas of the two colliding components, seen in [[X-ray]]s, represents most of the [[baryon]]ic, or "ordinary", matter in the cluster pair. The gases of the [[intracluster medium]] interact electromagnetically, causing the gases of both clusters to slow much more than the stars. The third component, the dark matter, was detected indirectly by the [[gravitational lens]]ing of background objects, as calculated using the best available theory of gravity [[general relativity]]. This provides support for the idea that most of the gravitation in the cluster pair is in the form of two regions of collisionless dark matter, which bypassed the gas regions during the collision.<ref name="Markevitch1"/><ref>{{cite conference
 |author1=Markevitch, M.
 |author2=Randall, S.
 |author3=Clowe, D.
 |author4=Gonzalez, A.
 |author5=Bradac, M.
 |name-list-style=amp
 |title=Dark Matter and the Bullet Cluster
 |conference=36th COSPAR Scientific Assembly
 |date=16–23 July 2006
 |location=Beijing, China
 |type=abstract
 |url=http://cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf
}}</ref>
[[File:1e0657 scale.jpg|thumb|right|300px| X-ray image (pink) superimposed over a visible light image (galaxies), with matter distribution calculated from gravitational lensing (blue)]]

The Bullet Cluster is one of the [[Temperature|hottest-known]] [[galaxy groups and clusters|clusters of galaxies]]. It provides an observable constraint for cosmological models, which may diverge at temperatures beyond their predicted critical cluster temperature.<ref name="Tucker1998"/> Observed from [[Earth]], the subcluster passed through the cluster center 150&nbsp;million years ago, creating a "bow-shaped [[shock wave]] located near the right side of the cluster" formed as "70&nbsp;million kelvin gas in the sub-cluster plowed through [[1 E6 K|100&nbsp;million]] kelvin gas in the main cluster at a speed of about nearly 10&nbsp;million&nbsp;km/h (6&nbsp;million miles per hour)".<ref>{{cite AV media |author=Chandra X-ray Observatory |author-link=Chandra X-ray Observatory |year=2002 |title=1E 0657-56: A bow shock in a merging galaxy cluster |medium=image and description |series=Chandra photo album |publisher=Harvard University |url=http://chandra.harvard.edu/photo/2002/0066/}}</ref><ref>{{cite news |title=The dynamical status of the cluster of galaxies 1E0657-56 |year=2002 |website=edpsciences-usa.org |url=http://edpsciences-usa.org/articles/aa/abs/2002/18/aah3368/aah3368.html |access-date=2007-09-02 |archive-date=2015-04-18 |archive-url=https://web.archive.org/web/20150418151134/http://www.edpsciences-usa.org/articles/aa/abs/2002/18/aah3368/aah3368.html |url-status=dead }}</ref> The bow shock radiation output is equivalent to the energy of 10&nbsp;typical [[quasar]]s.<ref name="Tucker1998"/>

According to Greg Madejski:
{{blockquote|Particularly compelling results were inferred from the [[Chandra X-ray Observatory|Chandra]] observations of the 'bullet cluster' (1E0657-56; Fig.&nbsp;2) by Markevitch et al. (2004) and Clowe et al. (2004). Those authors report that the cluster is undergoing a high-velocity (around 4,500&nbsp;km/s) merger, evident from the spatial distribution of the hot, [[X-ray]]-emitting gas, but this gas lags behind the subcluster galaxies. Furthermore, the dark matter clump, revealed by the weak lensing map, is coincident with the collisionless galaxies, but lies ahead of the collisional gas. This—and other similar observations—allow good limits on the cross-section of the self-interaction of dark matter.<ref>{{cite journal
 |last1 = Tucker    |first1 = W.
 |last2 = Blanco    |first2 = P.
 |last3 = Rappoport |first3 = S.
 |last4 = David     |first4 = L.
 |last5 = Fabricant |first5 = D.
 |last6 = Falco     |first6 = E.E.
 |last7 = Forman    |first7 = W.
 |last8 = Dressler  |first8 = A.
 |last9 = Ramella   |first9 = M.
 |year  = 2006
 |title = Recent and Future Observations in the X-ray and Gamma-ray Bands: Chandra, Suzaku, GLAST, and NuSTAR
 |journal= AIP Conference Proceedings
 |volume = 801  |pages = 21–30
 |doi = 10.1063/1.2141828 |arxiv = astro-ph/0512012
 |bibcode = 2005AIPC..801...21M
 |s2cid = 14601312
 |url = https://digital.library.unt.edu/ark:/67531/metadc877627/
}}</ref>}}

According to Eric Hayashi:
{{blockquote|The [[velocity]] of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured [[Lambda-CDM model]] [[cosmology]]."<ref>{{cite journal
 |author1=Hayashi, Eric
 |author2=White, ?
 |year=2006
 |title=How rare is the Bullet Cluster?
 |journal=Monthly Notices of the Royal Astronomical Society Letters
 |volume=370 |issue=1 |pages=L38–L41
 |arxiv=astro-ph/0604443 |bibcode = 2006MNRAS.370L..38H
 |doi=10.1111/j.1745-3933.2006.00184.x
|doi-access=free
 |s2cid=16684392
 }}{{full citation needed|date=September 2020|reason=2nd author first name}}</ref>}}

A 2010 study claimed that the velocities of the collision are "incompatible with the prediction of a LCDM model".<ref>{{cite journal
 |author1=Lee, Jounghun
 |author2=Komatsu, Eiichiro
 |year=2010
 |title=Bullet Cluster: A Challenge to LCDM Cosmology
 |journal=Astrophysical Journal
 |volume=718 |issue=1 |pages=60–65
 |arxiv=1003.0939 |bibcode = 2010ApJ...718...60L
 |doi = 10.1088/0004-637X/718/1/60
|s2cid=119250064
 }}</ref> However, subsequent work has found the collision to be consistent with LCDM simulations,<ref name=":0">{{cite journal
 |last1=Thompson |first1=Robert
 |last2=Davé     |first2=Romeel
 |last3=Nagamine |first3=Kentaro
 |date=2015-09-01
 |title=The rise and fall of a challenger: The Bullet Cluster in Lambda cold dark matter simulations
 |journal=Monthly Notices of the Royal Astronomical Society
 |volume=452 |issue=3 |pages=3030–3037
 |doi=10.1093/mnras/stv1433 |doi-access=free
 |bibcode=2015MNRAS.452.3030T
 |issn=0035-8711 |arxiv=1410.7438
}}</ref> with the previous discrepancy stemming from small simulations and the methodology of identifying pairs. Earlier work claiming the Bullet Cluster was inconsistent with standard cosmology was based on an erroneous estimate of the in-fall velocity based on the speed of the shock in the X-ray-emitting gas.<ref name=":0"/> Based on the analysis of the shock driven by the merger, it was recently argued that a lower merger velocity ~3,950&nbsp;km/s is consistent with the [[Sunyaev–Zeldovich effect]] and X-ray data, provided that the equilibration of the electron and ion downstream temperatures is not instantaneous.<ref>{{cite journal
 | author1 = Di Mascolo, L.
 | author2 = Mroczkowski
 | author3 = Churazov, E.
 | author4 = Markevitch, M.
 | author5 = Basu, K.
 | author6 = Clarke, T.E.
 | author7 = Devlin, M.
 | author8 = Mason, B.
 | author9 = Randall, S.W.
 | author10 = Reese, E.D.
 | author11 = Sunyaev, R.
 | author12 = Wik, D.R.
 | display-authors = 6
 | year = 2019
 | title = An ALMA+ACA measurement of the shock in the Bullet Cluster
 | journal = Astronomy and Astrophysics
 | volume = 628 | pages = A100
 | doi = 10.1051/0004-6361/201936184
 | arxiv = 1907.07680 |bibcode=2019A&A...628A.100D
| s2cid = 197545195
 }}</ref>

== As evidence against modified gravity ==
[[File:Bullet_Cluster_with_gravitational_potential_in_MOND.png|thumb|right|300px|Despite modifying gravity MOND still requires dark matter to fit the observations. The white lines trace the gravitational potential measured by lensing, the pink clouds show hot X-ray emitting gas, the full color dots are galaxies and some foreground stars, the blue is the inferred dark matter distribution. Image based on data from Angus et al. 2006.<ref name="AngusBC" />]]

The Bullet Cluster has been claimed as a significant challenge for all theories proposing a modified gravity solution to the missing mass problem, including [[modified Newtonian dynamics]] (MOND).<ref name="CloweBC">{{Cite journal |last1=Clowe |first1=Douglas |last2=Bradač |first2=Maruša |last3=Gonzalez |first3=Anthony H. |last4=Markevitch |first4=Maxim |last5=Randall |first5=Scott W. |last6=Jones |first6=Christine |last7=Zaritsky |first7=Dennis |name-list-style=amp |date=2006 |title=A Direct Empirical Proof of the Existence of Dark Matter |journal=The Astrophysical Journal Letters |volume=648 |issue=2 |pages=L109–L113 |doi=10.1086/508162 |arxiv=astro-ph/0608407 |bibcode=2006ApJ...648L.109C|s2cid=2897407 }}</ref> Astronomers measured the distribution of stellar and gas mass in the clusters using [[visible light|visible]] and [[X-ray]] light, respectively, and also mapped the gravitational potential using gravitational lensing. As shown in the images on the right, the X-ray gas is in the center, while the galaxies are on the outskirts. During the collision, the X-ray gas interacted and slowed down, remaining in the center, while the galaxies largely passed by one another, as the distances between them were vast. The gravitational potential reveals two large concentrations centered on the galaxies, not on the X-ray gas, where most of the normal matter is located. In ΛCDM one would also expect the clusters to each have a dark matter halo that would pass through each other during the collision (assuming, as is conventional, that dark matter is collisionless). This expectation for the dark matter is a clear explanation for the offset between the peaks of the gravitational potential and the X-ray gas which was detected at a [[statistical significance]] of 8{{mvar|σ}}.

It is this offset between the gravitational potential and normal matter that was claimed by Clowe et al. as ''"A Direct Empirical Proof of the Existence of Dark Matter"'' arguing that modified gravity theories fail to account for it.<ref name="CloweBC"/> However, this study by Clowe et al. made no attempt to analyze the Bullet Cluster using MOND or any other modified gravity theory. Furthermore, in the same year, Angus et al. demonstrated that MOND does indeed reproduce the offset between the gravitational potential and the X-ray gas in this highly non-spherically symmetric system.<ref>{{cite journal| author=G.W. Angus |author2=B. Famaey |author3=H. Zhao  |name-list-style=amp | title=Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry| journal=Mon. Not. R. Astron. Soc.| volume=371| issue=1| pages=138–146 |date=September 2006| doi= 10.1111/j.1365-2966.2006.10668.x|doi-access=free | arxiv=astro-ph/0606216v1|bibcode = 2006MNRAS.371..138A |s2cid=15025801 }}</ref> In MOND, one would expect the "missing mass" to be centred on regions which experience accelerations lower than a<sub>0</sub>, which, in the case of the Bullet Cluster, correspond to the areas containing the galaxies, not the X-ray gas. Nevertheless, MOND still fails to fully explain this cluster, as it does with all other galaxy clusters, due to the remaining mass residuals in several core regions of the Bullet Cluster.<ref name="AngusBC">{{cite journal | last1=Angus | first1=Garry W. | last2=Shan | first2=Huan Yuan | last3=Zhao | first3=Hong Sheng | last4=Famaey | first4=Benoit | title=On the Proof of Dark Matter, the Law of Gravity, and the Mass of Neutrinos | journal=The Astrophysical Journal | volume=654 | issue=1 | date=2007-01-01 | issn=0004-637X | doi=10.1086/510738 | doi-access=free | pages=L13–L16 | arxiv=astro-ph/0609125 | bibcode=2007ApJ...654L..13A }}</ref> [[Mordehai Milgrom]], the original proposer of MOND, has posted an online rebuttal<ref>{{citation |first=Moti |last=Milgrom |author-link=Mordehai Milgrom |title=Milgrom's perspective on the Bullet Cluster |url=http://www.astro.umd.edu/~ssm/mond/moti_bullet.html |archive-url=https://web.archive.org/web/20160721044735/http://www.astro.umd.edu/~ssm/mond/moti_bullet.html |archive-date=July 21, 2016 |work=The MOND Pages |access-date=December 27, 2016}}</ref> of claims that the Bullet Cluster proves the existence of dark matter. He contends that the observed characteristics of the Bullet Cluster could just as well be caused by undetected standard matter. He has argued that all galaxy clusters could host cold dense hydrogen gas clouds of roughly equal to the mass of the visible baryons which could explain the failures of MOND in galaxy clusters.<ref name="j070">{{cite journal | last=Milgrom | first=Mordehai | title=Ultra-diffuse cluster galaxies as key to the MOND cluster conundrum | journal=Monthly Notices of the Royal Astronomical Society | volume=454 | issue=4 | date=2015-12-21 | issn=0035-8711 | doi=10.1093/mnras/stv2202 | doi-access=free | pages=3810–3815 | url=https://academic.oup.com/mnras/article-pdf/454/4/3810/18509089/stv2202.pdf | access-date=2025-03-09}}</ref> Such cold dense hydrogen clouds are unlikely to exist however due to feedback from AGNs which prevent hydrogen gas from cooling.<ref name="b913">{{cite journal | last=Fabian | first=A. C. | title=Cooling Flows in Clusters of Galaxies | journal=Annual Review of Astronomy and Astrophysics | volume=32 | issue=1 | date=1994 | issn=0066-4146 | doi=10.1146/annurev.aa.32.090194.001425 | pages=277–318| bibcode=1994ARA&A..32..277F }}</ref><ref name="z146">{{cite journal | last1=McDonald | first1=M. | last2=Gaspari | first2=M. | last3=McNamara | first3=B. R. | last4=Tremblay | first4=G. R. | title=Revisiting the Cooling Flow Problem in Galaxies, Groups, and Clusters of Galaxies | journal=The Astrophysical Journal | volume=858 | issue=1 | date=2018-05-01 | issn=0004-637X | doi=10.3847/1538-4357/aabace | doi-access=free | page=45| arxiv=1803.04972 | bibcode=2018ApJ...858...45M }}</ref>

There are other alternate theories of gravity like the MOG<ref>{{Cite journal |last1=Brownstein |first1=J.R. |last2=Moffat |first2=J.W. |year=2007 |title=The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter |journal=Monthly Notices of the Royal Astronomical Society |volume=382 |issue=1 |pages=29–47 |doi=10.1111/j.1365-2966.2007.12275.x|doi-access=free |arxiv=astro-ph/0702146 |bibcode=2007MNRAS.382...29B }}</ref> and Many-body gravity (MBG),<ref>{{Cite journal |last=Ganesh |first=S |year=2025 |title=Many body gravity and the bullet cluster |url=https://www.sciencedirect.com/science/article/pii/S0927650525000039 |journal=Astroparticle Physics |volume=167 |pages=103080 |arxiv=2501.05126 |doi=10.1016/j.astropartphys.2025.103080 |bibcode=2025APh...16703080G |issn=0927-6505 |via=Elsevier Science Direct}}</ref> which claim to be able to explain the bullet cluster's weak gravitational lensing.

==See also==
* [[Galaxy group]]
* [[Galaxy cluster]]
* [[Abell 520]] – a similar galaxy cluster whose dark and luminous matter may have been separated during a major collision
* [[NGC 1052-DF2]]
* [[List of galaxy groups and clusters]]

== References ==
{{reflist|25em}}

==Further reading==
{{Commons category}}
* {{cite journal |arxiv=astro-ph/0608408 |doi=10.1086/508601 |bibcode=2006ApJ...652..937B |title=Strong and Weak Lensing United. III. Measuring the Mass Distribution of the Merging Galaxy Cluster 1ES 0657−558 |year=2006 |last1=Bradač |first1=Maruša |last2=Clowe |first2=Douglas |last3=Gonzalez |first3=Anthony H. |last4=Marshall |first4=Phil |last5=Forman |first5=William |last6=Jones |first6=Christine |last7=Markevitch |first7=Maxim |last8=Randall |first8=Scott |last9=Schrabback |first9=Tim |last10=Zaritsky |first10=Dennis |journal=The Astrophysical Journal |volume=652 |issue=2 |pages=937–947 |s2cid=119388532}}
* CXO: [http://chandra.harvard.edu/chronicle/0306/devil/ Bedeviling Devil's Advocate Cosmology] (The Chandra Chronicles) August 21, 2006
* CXO: [http://chandra.harvard.edu/photo/2006/1e0657/ 1E 0657-56: NASA Finds Direct Proof of Dark Matter] Combined image of x-rays, visual and DM
* Harvard [http://chandra.harvard.edu/photo/2006/1e0657/media/bullet.mpg animation of the collision] showing how the dark matter and normal matter become separated.
* Harvard [http://cxc.harvard.edu/symposium_2005/proceedings/files/markevitch_maxim.pdf Harvard Symposium: Markevitch PDF] 36 color images and text slides modelling the existence of Dark Matter from Bullet cluster data
* NASA: [http://www.nasa.gov/centers/marshall/news/news/releases/2006/06-096.html NASA Finds Direct Proof of Dark Matter] {{Webarchive|url=https://web.archive.org/web/20170625013145/https://www.nasa.gov/centers/marshall/news/news/releases/2006/06-096.html |date=2017-06-25 }} (NASA Press Release 06-096) Aug. 21, 2006
* Scientific American [https://web.archive.org/web/20071015185726/http://www.sciam.com/article.cfm?chanID=sa003&articleID=000EB9FD-823F-14EB-823F83414B7F0000 Scientific American] article ''Science News August 22, 2006 Colliding Clusters Shed Light on Dark Matter'' that includes a movie of a simulation of the collision

{{Sky|06|58|37.9|-|55|57|00|3721000000}}
{{Carina (constellation)|state=collapsed}}
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{{DEFAULTSORT:Bullet Cluster}}
[[Category:Carina (constellation)]]
[[Category:Dark matter]]
[[Category:Galaxy clusters]]