Editing Galaxy groups and clusters (section)

== Clusters of galaxies ==
{{main|Galaxy cluster}}
[[Image:COMBO-17.jpg|thumb|Rich scattering of galaxies was captured by the [[MPG/ESO telescope]].]]

Clusters are larger than groups, although there is no sharp dividing line between the two. When observed visually, clusters appear to be collections of galaxies held together by mutual gravitational attraction. However, their velocities are too large for them to remain gravitationally bound by their mutual attractions, implying the presence of either an additional invisible mass component, or an additional attractive force besides gravity. X-ray studies have revealed the presence of large amounts of intergalactic gas known as the [[intracluster medium]]. This gas is very hot, between 10<sup>7</sup>K and 10<sup>8</sup>K, and hence emits X-rays in the form of [[bremsstrahlung]] and [[atomic spectral line|atomic line emission]].

[[Image:ACO 3341.jpg|thumb|left|180px|Galaxy cluster ACO 3341 seen by [[Very Large Telescope|VLT]]'s [[Visible Multi Object Spectrograph|VIMOS]]]]

The total mass of the gas is greater than that of the galaxies by roughly a factor of two. However, this is still not enough mass to keep the galaxies in the cluster. Since this gas is in approximate [[hydrostatic equilibrium]] with the overall cluster gravitational field,  the total mass distribution can be determined. It turns out the total mass deduced from this measurement is approximately six times larger than the mass of the galaxies or the hot gas. The missing component is known as [[dark matter]] and its nature is unknown. In a typical cluster perhaps only 5% of the total mass is in the form of galaxies, maybe 10% in the form of hot X-ray emitting gas and the remainder is dark matter. Brownstein and Moffat<ref>{{cite journal |doi=10.1111/j.1365-2966.2006.09996.x |arxiv=astro-ph/0507222 |last1=Brownstein |first1=J. R. |last2=Moffat |first2=J. W. |title=Galaxy Cluster Masses Without Non-Baryonic Dark Matter |journal=Monthly Notices of the Royal Astronomical Society |volume=367 |issue=2 |date=2006 |pages=527–540 |doi-access=free |bibcode=2006MNRAS.367..527B|s2cid=119343858 }}</ref> use a theory of modified gravity to explain X-ray cluster masses without dark matter. Observations of the [[Bullet Cluster]] are the strongest evidence for the existence of dark matter;<ref>{{cite journal |author1=Markevitch |author2=Gonzalez |author3=Clowe |author4=Vikhlinin |author5=David |author6=Forman |author7=Jones |author8=Murray |author9=Tucker |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><ref>{{cite journal | doi=10.1088/0004-637X/723/2/1678 | journal= The Astrophysical Journal | volume=723 | issue= 2 | pages=1678–1702 | date=2010 | author=Coe, Dan | author2=Benítez, Narciso | author3=Broadhurst, Tom | author4=Moustakas, Leonidas A. | title=A High-resolution Mass Map of Galaxy Cluster Substructure: LensPerfect Analysis of A1689 | arxiv= 1005.0398 | bibcode=2010ApJ...723.1678C| s2cid= 119916381 }}</ref><ref>{{cite journal | doi=10.1103/PhysRevD.83.063509 | journal= Physical Review D | volume=83 | issue= 6 | pages=063509 | author=McDermott, Samuel D. | author2=Yu, Hai-Bo | author3=Zurek, Kathryn M. | title= Turning off the lights: How dark is dark matter? | date=2011 | arxiv=1011.2907 | bibcode=2011PhRvD..83f3509M | s2cid= 118538115 }}</ref> however, Brownstein and Moffat<ref>{{cite journal|doi=10.1111/j.1365-2966.2007.12275.x |arxiv=astro-ph/0702146v3 |last1 = Brownstein |first1=J. R. |last2=Moffat |first2=J. W. |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 |date=2007 |pages=29–47|doi-access=free |bibcode = 2007MNRAS.382...29B |s2cid=119084968 }}</ref> have shown that their modified gravity theory can also account for the properties of the cluster.