Galaxy groups and clusters: Difference between revisions

'''Galaxy groups and clusters''' are the largest [[Observable universe|known]] [[Gravitational binding energy|gravitationally bound]] objects to have arisen thus far in the process of cosmic structure formation.<ref>{{cite journal|doi=10.1103/revmodphys.77.207|title=Tracing cosmic evolution with clusters of galaxies|journal=Reviews of Modern Physics|volume=77|issue=1|pages=207–258|year=2005|last1=Voit|first1=G. Mark|bibcode=2005RvMP...77..207V|arxiv=astro-ph/0410173|s2cid=119465596}}</ref> They form the densest part of the [[Observable universe#Large-scale structure|large-scale structure of the Universe]]. In models for the gravitational formation of structure with [[cold dark matter]], the smallest structures collapse first and eventually build the largest structures, clusters of galaxies. Clusters are then formed relatively recently between 10 billion years ago and now. Groups and clusters may contain ten to thousands of individual galaxies. The clusters themselves are often associated with larger, non-gravitationally bound, groups called [[supercluster]]s.

Our own galaxy, the [[Milky Way]], is contained in the [[Local Group]] of more than 54 galaxies.<ref>{{cite web|title=The Local Group|url=http://www.ast.cam.ac.uk/~mike/local_more.html|author=Mike Irwin|access-date=2009-11-07|archive-date=2019-06-19|archive-url=https://web.archive.org/web/20190619131606/https://www.ast.cam.ac.uk/~mike/local_more.html|url-status=dead}}</ref>

In July 2017 S. Paul, R. S. John et al. defined clear distinguishing parameters for classifying galaxy aggregations as ‘galaxy groups’ and ‘clusters’ on the basis of scaling laws that they followed.<ref>{{cite journal|title=Understanding 'galaxy groups' as a unique structure in the universe|journal = Monthly Notices of the Royal Astronomical Society|volume = 471|issue = 1|pages = 2–11|author=S. Paul|author2=R. S. John|author3=P. Gupta|author4= H. Kumar|doi = 10.1093/mnras/stx1488|year = 2017| doi-access=free |bibcode = 2017MNRAS.471....2P|arxiv = 1706.01916}}</ref> According to this paper, galaxy aggregations less massive than 8 × 10¹³ [[solar mass]]es are classified as galaxy groups.

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.

Clusters of galaxies are the most recent and most massive objects to have arisen in the hierarchical structure formation of the Universe and the study of clusters tells one about the way galaxies form and evolve. Clusters have two important properties: their masses are large enough to retain any energetic gas ejected from member galaxies and the thermal energy of the gas within the cluster is observable within the X-Ray bandpass. The observed state of gas within a cluster is determined by a combination of [[shock heating]] during accretion, radiative cooling, and thermal feedback triggered by that cooling. The [[density]], [[temperature]], and substructure of the intracluster X-Ray gas therefore represents the entire thermal history of cluster formation. To better understand this thermal history one needs to study the entropy of the gas because entropy is the quantity most directly changed by increasing or decreasing the thermal energy of intracluster gas.<ref>{{cite book|url=https://books.google.com/books?id=cAVS0GFTZlgC10|title=Galaxies|publisher=Wikimedia Foundation|page=55}}{{Dead link|date=May 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>