Editing Galaxy (section)

== Larger-scale structures ==
{{Main|Observable universe#Large-scale structure|Galaxy filament|Galaxy groups and clusters}}
{{multiple image
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| image1 = Seyfert Sextet full.jpg
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| caption1 = [[Seyfert's Sextet]] is an example of a compact galaxy group.
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| caption2 = [[Millennium Simulation]] showing large-scale structure of the Cosmos. The image spans about 400 million light years across.
}}

Deep-sky surveys show that galaxies are often found in groups and [[Clusters of galaxies|clusters]]. Solitary galaxies that have not significantly interacted with other galaxies of comparable mass in the past few billion years are relatively scarce.<ref>{{Cite journal |last1=Argudo-Fernández |first1=M. |last2=Verley |first2=S. |last3=Bergond |first3=G. |last4=Duarte Puertas |first4=S. |last5=Ramos Carmona |first5=E. |last6=Sabater |first6=J. |last7=Fernández Lorenzo |first7=M. |last8=Espada |first8=D. |last9=Sulentic |first9=J. |last10=Ruiz |first10=J. E. |last11=Leon |first11=S. |date=June 2015 |title=Catalogues of isolated galaxies, isolated pairs, and isolated triplets in the local Universe |url=http://www.aanda.org/10.1051/0004-6361/201526016 |journal=[[Astronomy & Astrophysics]] |volume=578 |pages=A110 |doi=10.1051/0004-6361/201526016 |issn=0004-6361|arxiv=1504.00117 |bibcode=2015A&A...578A.110A }}</ref> Only about 5% of the galaxies surveyed are isolated in this sense.<ref>{{Cite journal |last1=Karachentsev |first1=I. D. |last2=Makarov |first2=D. I. |last3=Karachentseva |first3=V. E. |last4=Melnyk |first4=O. V. |date=January 2011 |title=Catalog of nearby isolated galaxies in the volume z < 0.01 |url=http://link.springer.com/10.1134/S1990341311010019 |journal=Astrophysical Bulletin |language=en |volume=66 |issue=1 |pages=1–27 |arxiv=1103.3990 |doi=10.1134/S1990341311010019 |bibcode=2011AstBu..66....1K |issn=1990-3413}}</ref><ref>{{Cite journal |last1=Melnyk |first1=O. |last2=Karachentseva |first2=V. |last3=Karachentsev |first3=I. |date=2015-08-01 |title=Star formation rates in isolated galaxies selected from the Two-Micron All-Sky Survey |url=http://academic.oup.com/mnras/article/451/2/1482/1750461/Star-formation-rates-in-isolated-galaxies-selected |journal=[[Monthly Notices of the Royal Astronomical Society]] |language=en |volume=451 |issue=2 |pages=1482–1495 |doi=10.1093/mnras/stv950 |doi-access=free |issn=1365-2966|arxiv=1504.07990 }}</ref> However, they may have interacted and even merged with other galaxies in the past,<ref>{{Cite journal |last1=Hirschmann |first1=Michaela |last2=De Lucia |first2=Gabriella |last3=Iovino |first3=Angela |last4=Cucciati |first4=Olga |date=2013-08-01 |title=Isolated galaxies in hierarchical galaxy formation models – present-day properties and environmental histories |url=http://academic.oup.com/mnras/article/433/2/1479/1750237/Isolated-galaxies-in-hierarchical-galaxy-formation |journal=[[Monthly Notices of the Royal Astronomical Society]] |language=en |volume=433 |issue=2 |pages=1479–1491 |arxiv=1302.3616 |bibcode=2013MNRAS.433.1479H |doi=10.1093/mnras/stt827 |doi-access=free |issn=1365-2966}}</ref> and may still be orbited by smaller satellite galaxies.<ref>{{Cite journal |last1=Wang |first1=Wenting |last2=White |first2=Simon D. M. |date=2012-08-21 |title=Satellite abundances around bright isolated galaxies: Satellite abundances |journal=[[Monthly Notices of the Royal Astronomical Society]] |language=en |volume=424 |issue=4 |pages=2574–2598 |arxiv=1203.0009 |bibcode=2012MNRAS.424.2574W |doi=10.1111/j.1365-2966.2012.21256.x|doi-access=free }}</ref>

On the largest scale, the universe is continually expanding, resulting in an average increase in the separation between individual galaxies (see [[Hubble's law]]). Associations of galaxies can overcome this expansion on a local scale through their mutual gravitational attraction. These associations formed early, as clumps of dark matter pulled their respective galaxies together. Nearby groups later merged to form larger-scale clusters. This ongoing merging process, as well as an influx of infalling gas, heats the intergalactic gas in a cluster to very high temperatures of 30–100 [[megakelvin]]s.<ref>{{cite web
 |url=http://chandra.harvard.edu/xray_sources/galaxy_clusters.html
 |title=Groups & Clusters of Galaxies
 |website=[[Chandra X-ray Observatory]]
 |publisher=[[Harvard–Smithsonian Center for Astrophysics]]
 |access-date=January 15, 2007
 |archive-date=February 22, 2024
 |archive-url=https://web.archive.org/web/20240222032114/https://chandra.harvard.edu/xray_sources/galaxy_clusters.html
 |url-status=live
 }}</ref> About 70–80% of a cluster's mass is in the form of dark matter, with 10–30% consisting of this heated gas and the remaining few percent in the form of galaxies.<ref>{{cite web
 |last1=Ricker
 |first1=Paul
 |title=When Galaxy Clusters Collide
 |url=http://www.sdsc.edu/pub/envision/v15.2/ricker.html
 |website=[[San Diego Supercomputer Center]]
 |publisher=[[University of California, San Diego]]
 |access-date=April 21, 2024
 |archive-date=August 2, 2023
 |archive-url=https://web.archive.org/web/20230802142051/https://www.sdsc.edu/pub/envision/v15.2/ricker.html
 |url-status=dead
 }}</ref>

Most galaxies are gravitationally bound to a number of other galaxies. These form a [[fractal]]-like hierarchical distribution of clustered structures, with the smallest such associations being termed groups. A group of galaxies is the most common type of galactic cluster; these formations contain the majority of galaxies (as well as most of the [[baryon]]ic mass) in the universe.<ref>{{cite web
 |last1=Pompei|first1=Emanuela
 |last2=Dahlem|first2=Michael
 |last3=Iovino|first3=Angela
 |editor-last=Dahlem |editor-first=Michael
 |date=November 24, 2006
 |title=Optical and radio survey of Southern Compact Groups of galaxies
 |url=http://www.atnf.csiro.au/people/mdahlem/sci/SCGs.html
 |publisher=[[University of Birmingham]] Astrophysics and Space Research Group
 |access-date=January 15, 2007
 |archive-url=https://web.archive.org/web/20070613151936/http://www.atnf.csiro.au/people/mdahlem/sci/SCGs.html
 |archive-date=June 13, 2007
}}</ref><ref>{{cite web
 |last1=Ponman |first1=Trevor
 |date=February 25, 2005
 |title=Galaxy Systems: Groups
 |url=http://www.sr.bham.ac.uk/research/groups.html
 |archive-url=https://web.archive.org/web/20090215023446/http://www.sr.bham.ac.uk/research/groups.html
 |archive-date=2009-02-15
 |publisher=[[University of Birmingham]] Astrophysics and Space Research Group
 |access-date=January 15, 2007
}}</ref> To remain gravitationally bound to such a group, each member galaxy must have a sufficiently low velocity to prevent it from escaping (see [[Virial theorem]]). If there is insufficient [[kinetic energy]], however, the group may evolve into a smaller number of galaxies through mergers.<ref>{{cite journal
 |last1=Girardi |first1=Marisa
 |last2=Giuricin |first2=G.
 |s2cid=14059401
 |date=2000
 |title=The Observational Mass Function of Loose Galaxy Groups
 |journal=[[The Astrophysical Journal]]
 |volume=540 |issue=1 |pages=45–56
 |bibcode=2000ApJ...540...45G
 |doi=10.1086/309314
|arxiv = astro-ph/0004149 }}</ref>

<!---{{unsolved|physics|The [[List of largest cosmic structures|largest structures]] in the universe are larger than expected. Are these actual structures or random density fluctuations?}}--->
Clusters of galaxies consist of hundreds to thousands of galaxies bound together by gravity.<ref name="Hubble protocluster">{{cite press release|title=Hubble Pinpoints Furthest Protocluster of Galaxies Ever Seen|url=http://www.spacetelescope.org/news/heic1201/|access-date=January 22, 2015|publisher=[[NASA]], [[European Space Agency]]|archive-date=June 12, 2018|archive-url=https://web.archive.org/web/20180612140011/http://www.spacetelescope.org/news/heic1201/|url-status=live}}</ref> Clusters of galaxies are often dominated by a single giant elliptical galaxy, known as the [[brightest cluster galaxy]], which, over time, [[tidal force|tidally]] destroys its satellite galaxies and adds their mass to its own.<ref>{{cite journal
 |last         = Dubinski
 |first        = John
 |s2cid = 3137328
 |date         = 1998
 |title        = The Origin of the Brightest Cluster Galaxies
 |url          = http://www.cita.utoronto.ca/~dubinski/bcg/
 |journal      = [[The Astrophysical Journal]]
 |volume       = 502
 |issue        = 2
 |pages        = 141–149
 |doi          = 10.1086/305901
 |bibcode      = 1998ApJ...502..141D
 |arxiv        = astro-ph/9709102
 |archive-url = https://web.archive.org/web/20110514155953/http://www.cita.utoronto.ca/~dubinski/bcg/
 |archive-date = May 14, 2011
 |df           = mdy-all
}}</ref>
[[File:The southern plane of the Milky Way from the ATLASGAL survey.jpg|right|thumb|Southern plane of the Milky Way from submillimeter wavelengths<ref>{{cite web|title=ATLASGAL Survey of Milky Way Completed|publisher=[[European Southern Observatory]]|url=http://www.eso.org/public/news/eso1606/|access-date=7 March 2016|archive-date=March 24, 2021|archive-url=https://web.archive.org/web/20210324074529/https://www.eso.org/public/news/eso1606/|url-status=live}}</ref>]]
[[Supercluster]]s contain tens of thousands of galaxies, which are found in clusters, groups and sometimes individually. At the [[large-scale structure of the Cosmos|supercluster scale]], galaxies are arranged into sheets and filaments surrounding vast empty voids.<ref>{{cite journal
 |last1=Bahcall |first1=Neta A.
 |date=1988
 |title=Large-scale structure in the Universe indicated by galaxy clusters
 |journal=[[Annual Review of Astronomy and Astrophysics]]
 |volume=26
 |issue=1 |pages=631–686
 |bibcode=1988ARA&A..26..631B
 |doi=10.1146/annurev.aa.26.090188.003215
}}</ref> Above this scale, the universe appears to be the same in all directions ([[isotropy|isotropic]] and [[wikt:Homogeneity|homogeneous]]),<ref>{{cite journal
 |last1=Mandolesi |first1=Nazzareno
 |s2cid=4349689
 |display-authors=etal
 |date=1986
 |title=Large-scale homogeneity of the Universe measured by the microwave background
 |journal=[[Letters to Nature]]
 |volume=319
 |issue=6056 |pages=751–753
 |doi=10.1038/319751a0
|bibcode = 1986Natur.319..751M }}</ref> though this notion has been challenged in recent years by numerous findings of large-scale structures that appear to be exceeding this scale. The [[Hercules–Corona Borealis Great Wall]], currently the [[List of largest cosmic structures|largest structure]] in the universe found so far, is 10 billion [[light-year]]s (three gigaparsecs) in length.<ref name=HBHT2>{{cite journal|last1=Horváth|first1=István|last2=Bagoly|first2=Zsolt|last3=Hakkila|first3=Jon|last4=Tóth|first4=L. Viktor|s2cid=56073380|title=New data support the existence of the Hercules–Corona Borealis Great Wall|journal=[[Astronomy & Astrophysics]]|volume = 584|pages = A48|arxiv=1510.01933|year = 2015|doi = 10.1051/0004-6361/201424829|bibcode = 2015A&A...584A..48H }}</ref><ref name=HBHT>{{cite journal|last1=Horváth|first1=István|last2=Bagoly|first2=Zsolt|last3=Hakkila|first3=Jon|last4=Tóth|first4=L. Viktor|title=Anomalies in the GRB spatial distribution|journal=Proceedings of Science|page=78|arxiv=1507.05528|bibcode = 2014styd.confE..78H |year=2014|doi=10.22323/1.233.0078 |doi-access=free }}</ref><ref name=cookie>{{Cite journal|arxiv =1507.00675 |last1 = Balazs|first1 = L. G.|title = A giant ring-like structure at 0.78<z<0.86 displayed by GRBs|journal = [[Monthly Notices of the Royal Astronomical Society]]|volume = 452|issue = 3|page = 2236|last2 =  Bagoly|first2 = Zsolt|last3 =  Hakkila|first3 = Jon E.|last4 = Horváth|first4 = I.|last5 = Kobori|first5 = J.|last6 = Racz|first6 = I.|last7 = Tóth|first7 = Laszlo V.|s2cid = 109936564|year = 2015|doi = 10.1093/mnras/stv1421| doi-access=free |bibcode = 2015MNRAS.452.2236B }}</ref>

The Milky Way galaxy is a member of an association named the [[Local Group]], a relatively small group of galaxies that has a diameter of approximately one&nbsp;megaparsec. The Milky Way and the Andromeda Galaxy are the two brightest galaxies within the group; many of the other member galaxies are dwarf companions of these two.<ref>{{cite journal
 |last1=van den Bergh |first1=Sidney
 |author-link1=Sidney van den Bergh
 |s2cid=1805423
 |date=2000
 |title=Updated Information on the Local Group
 |journal=[[Publications of the Astronomical Society of the Pacific]]
 |volume=112 |issue=770 |pages=529–536
 |bibcode=2000PASP..112..529V
 |doi=10.1086/316548
|arxiv = astro-ph/0001040 }}</ref> The Local Group itself is a part of a cloud-like structure within the [[Virgo Supercluster]], a large, extended structure of groups and clusters of galaxies centered on the [[Virgo Cluster]].<ref name="tully1982">{{cite journal
 |last1=Tully |first1=Richard Brent
 |author-link1=R. Brent Tully
 |date=1982
 |title=The Local Supercluster
 |journal=[[The Astrophysical Journal]]
 |volume=257 |pages=389–422
 |bibcode=1982ApJ...257..389T
 |doi=10.1086/159999
|doi-access=free
 }}</ref> In turn, the Virgo Supercluster is a portion of the [[Laniakea Supercluster]].<ref>{{Cite journal|last=Tempel|first=Elmo|date=2014-09-01|title=Cosmology: Meet the Laniakea supercluster|journal=[[Nature (journal)|Nature]]|volume=513|issue=7516|pages=41–42|doi=10.1038/513041a|pmid=25186896|bibcode=2014Natur.513...41T|doi-access=free}}</ref>