Editing Irregular galaxy

{{Short description|Class of galaxy}}
[[File:Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope).jpg|thumb|[[NGC 1427A]], an example of an irregular galaxy. It is an Irr-I category galaxy about 52 Mly distant.]]

An '''irregular galaxy''' is a [[galaxy]] that does not have a distinct regular shape, unlike a [[Spiral galaxy|spiral]] or an [[elliptical galaxy]].<ref>Butz, Stephen D. (2002). ''Science of Earth Systems.'' Cengage Learning. p. 107. {{ISBN|978-0-7668-3391-3}}.</ref> Irregular galaxies do not fall into any of the regular classes of the [[Hubble sequence]], and they are often chaotic in appearance, with neither a [[bulge (astronomy)|nuclear bulge]] nor any trace of [[spiral arm]] structure.<ref>Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." ''Publications of the Astronomical Society of the Pacific.'' '''69''' (409): 291&ndash;303.</ref> This absence of structure in an irregular galaxy leads to little density waves in these galaxies. This makes irregular galaxies prime areas to study [[star formation]] without the effects of density waves.<ref>{{Cite web |title=Star Formation in Irregular Galaxies |url=https://ned.ipac.caltech.edu/level5/Hunter/Hunter1.html#:~:text=Without%20spiral%20density%20waves%20to,from%20the%20spiral%20density%20wave. |access-date=2025-01-27 |website=ned.ipac.caltech.edu}}</ref>

Collectively they are thought to make up about a quarter of all galaxies.<ref>{{Cite web |url=https://www.mssl.ucl.ac.uk/www_astro/outreach/School/Galaxies/irreg.html |access-date=2025-01-27 |website=www.mssl.ucl.ac.uk |title=Irregular galaxies}}</ref> Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of gas and dust.<ref>{{cite web |url=http://www.le.ac.uk/ph/faulkes/web/galaxies/r_ga_irregular.html |title=Faulkes Telescope Educational Guide – Irregular Galaxies |url-status=dead |archive-date=17 October 2022 |archive-url=https://web.archive.org/web/20221017082000/https://www.le.ac.uk/ph/faulkes/web/galaxies/r_ga_irregular.html }}</ref> This is not necessarily true for dwarf irregulars.<ref>Walter, F. et al. Astophys J '''661''', 102–114, 2007</ref> Irregular galaxies may also be formed in galaxy collisions.<ref>{{Cite web |date=2020-09-21 |title=Types – NASA Science |url=https://science.nasa.gov/universe/galaxies/types/ |access-date=2025-01-27 |language=en-US}}</ref>

Irregular galaxies are commonly small, about one tenth the mass of the [[Milky Way]] galaxy, though there are also unusual cases of large irregulars like [[UGC 6697]]. Due to their small sizes, they are prone to environmental effects like [[Interacting galaxy|colliding]] with large galaxies and intergalactic clouds.<ref>{{cite book |author-link1=Debra Elmegreen |last1=Elmegreen |first1=Debra Meloy |author-link2=Bruce Elmegreen |first2=Bruce G. |last2=Elmegreen |chapter=Galaxies |title=Space Sciences |editor-first=Pat |editor-last=Dasch |volume=2 |series=Planetary Science and Astronomy |publisher=[[Gale (publisher)|Macmillan Reference USA]] |year=2002 |pages=50–56 |via=Gale Virtual Reference Library |chapter-url-access=subscription |chapter-url=https://go.galegroup.com/ps/i.do?p=GVRL&sw=w&u=mcc_pv&v=2.1&it=r&id=GALE%7CCX3408800138&asid=f1f899f2eb1f500bc3341c4f13abb896 |access-date=25 September 2017 }}</ref>

==Types==
There are three major types of irregular galaxies:<ref>Gallagher, J. S. & Hunter, D. A. (1984). "Structure and Evolution of Irregular Galaxies." ''Annual Review of Astronomy and Astrophysics.'' '''22''': 37–74. {{doi|10.1146/annurev.aa.22.090184.000345}}</ref>

* An Irr-I galaxy ('''Irr I''') is an irregular galaxy that features some [[structure]] but not enough to place it cleanly into the Hubble sequence. 
**Subtypes with some spiral structure are called [[Magellanic spiral|Sm]] galaxies
**Subtypes without spiral structure are called '''Im''' galaxies.

* An Irr-II galaxy ('''Irr II''') is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence.

{{anchor|dwarf_irregular_anchor}}
* A dI-galaxy (or '''dIrr''') is a [[dwarf galaxy|dwarf]] irregular galaxy.<ref>Grebel, Eva K. (2004). ''The evolutionary history of Local Group irregular galaxies.'' in McWilliam, Andrew; Rauch, Michael (eds) ''Origin and evolution of the elements.'' Cambridge University Press. p. 234–254. {{ISBN|978-0-521-75578-8}}.</ref> This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of [[metallicity]] and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local (and therefore more recent) version of the [[Faint blue galaxy|faint blue galaxies]] known to exist in [[Hubble Ultra-Deep Field|deep field galaxy surveys]]. These types of irregular galaxies also have high amounts of [[dark matter]]. <ref>{{Cite journal |last1=Rivas |first1=Jonathan |last2=Flores-Velazquez |first2=Jeysen |last3=Wheeler |first3=Coral |date=2024-02-01 |title=Major Merger between a Classical Dwarf Irregular Galaxy and its Massive Ultra-Faint Satellite |journal=American Astronomical Society Meeting Abstracts |url=https://ui.adsabs.harvard.edu/abs/2024AAS...24340418R/exportcitation |volume=243 |pages=404.18|bibcode=2024AAS...24340418R }}</ref>

Irregular galaxies are considered late-type along with [[Spiral galaxy|spiral galaxies]] as opposed to early type [[elliptical galaxies]]. <ref>{{Cite web |date=2023-12-15 |title=Hubble Looks at a Late-type Galaxy – NASA Science |url=https://science.nasa.gov/missions/hubble/hubble-looks-at-a-late-type-galaxy/#:~:text=The%20terminology%20%E2%80%9Clate-type%E2%80%9D,type%E2%80%9D%20refers%20to%20elliptical%20galaxies. |access-date=2025-01-27 |language=en-US}}</ref>

Some of the irregular galaxies, especially of the [[Magellanic spiral|Magellanic type]], are small [[spiral galaxies]] that are being distorted by the gravity of a larger neighbor.

==Magellanic Clouds==

The [[Magellanic Clouds|Magellanic Cloud galaxies]] were once classified as irregular galaxies. The [[Large Magellanic Cloud]] has since been re-classified as type SBm (barred [[Magellanic spiral]]).<ref>Corso, G. and Buscombe, W. The Observatory, '''90''', 229–233 (1970) [http://adsabs.harvard.edu/full/1970Obs....90..229C On the spiral structure of the Large Magellanic Cloud]</ref> The [[Small Magellanic Cloud]] remains classified as an irregular galaxy of type Im under current [[galaxy morphological classification]], although it does contain a [[Barred spiral galaxy|bar structure]].

== Gallery ==
<gallery>
File:Feeling blue ESO 338-4.jpg|[[Blue compact dwarf galaxy]] ESO 338-4<ref>{{cite web |title=Feeling blue |url=https://www.spacetelescope.org/images/potw1845a/ |website=www.spacetelescope.org |access-date=5 November 2018 |archive-date=5 November 2018 |archive-url=https://web.archive.org/web/20181105094515/https://www.spacetelescope.org/images/potw1845a/ |url-status=dead }}</ref>
File:A frenzy of stars IC 4710.jpg|[[IC 4710]] lies roughly 25 million light-years away in the southern constellation of [[Pavo (constellation)|Pavo]].<ref>{{cite web|title=A frenzy of stars|url=https://www.spacetelescope.org/images/potw1809a/|website=www.spacetelescope.org|access-date=26 February 2018|archive-date=25 October 2020|archive-url=https://web.archive.org/web/20201025055918/https://www.spacetelescope.org/images/potw1809a/|url-status=dead}}</ref>
File:Surveying the cosmos.jpg|[[ESO 486-21]] is a spiral galaxy with a somewhat irregular and ill-defined structure.<ref>{{cite web|title=Surveying the cosmos|url=https://www.spacetelescope.org/images/potw1725a/|website=www.spacetelescope.org|access-date=19 June 2017|archive-date=18 July 2020|archive-url=https://web.archive.org/web/20200718145537/https://www.spacetelescope.org/images/potw1725a/|url-status=dead}}</ref>
File:Spotlight on IC 3583.jpg|Irregular galaxy [[IC 3583]] has been found to have a bar of stars running through its center.<ref>{{cite web|title=Spotlight on IC 3583|url=https://www.spacetelescope.org/images/potw1648a/|website=www.spacetelescope.org|access-date=28 November 2016|archive-date=17 August 2019|archive-url=https://web.archive.org/web/20190817191823/https://www.spacetelescope.org/images/potw1648a/|url-status=dead}}</ref>
File:A lopsided lynx NGC 2337.jpg|[[NGC 2337]] is an irregular galaxy that resides 25 million light-years away in the [[Lynx (constellation)|Lynx constellation]].<ref>{{cite web|title=A lopsided lynx|url=http://www.spacetelescope.org/images/potw1632a/|website=www.spacetelescope.org|access-date=9 August 2016|archive-date=21 November 2020|archive-url=https://web.archive.org/web/20201121154809/https://www.spacetelescope.org/images/potw1632a/|url-status=dead}}</ref>
File:A distinctly disorganised dwarf.jpg|[[UGC 4459]] is an irregular dwarf galaxy located approximately 11 million light-years away in the constellation of [[Ursa Major]].<ref>{{cite web|title=A distinctly disorganised dwarf|url=http://www.spacetelescope.org/images/potw1613a/|access-date=29 March 2016|archive-date=25 October 2020|archive-url=https://web.archive.org/web/20201025054728/https://www.spacetelescope.org/images/potw1613a/|url-status=dead}}</ref>
File:Meeting the neighbours.jpg|Dwarf irregular galaxy known as [[PGC 18431]]<ref>{{cite web|title=Meeting the neighbours|url=http://www.spacetelescope.org/images/potw1523a/|website=www.spacetelescope.org|publisher=ESA/Hubble|access-date=8 June 2015|archive-date=27 November 2020|archive-url=https://web.archive.org/web/20201127231709/https://www.spacetelescope.org/images/potw1523a/|url-status=dead}}</ref>
File:A spattering of blue.jpg|[[IC 559]] is classified as a type Sm galaxy.<ref>{{cite web|title=A spattering of blue|url=http://www.spacetelescope.org/images/potw1436a/|website=www.spacetelescope.org|publisher=ESA/Hubble|access-date=8 September 2014|archive-date=5 June 2019|archive-url=https://web.archive.org/web/20190605094619/https://spacetelescope.org/images/potw1436a/|url-status=dead}}</ref>
File:A cosmic optical illusion.jpg|Irregular dwarf galaxy [[PGC 16389]] covers its neighboring galaxy [[APMBGC 252+125-117]].<ref>{{cite news|title=A cosmic optical illusion|url=http://www.spacetelescope.org/images/potw1333a/|access-date=20 August 2013|newspaper=ESA/Hubble Picture of the Week|archive-date=7 January 2018|archive-url=https://web.archive.org/web/20180107061724/http://www.spacetelescope.org/images/potw1333a/|url-status=dead}}</ref> 
</gallery>

== See also ==
* [[Blueberry galaxy]] – Small and very active galaxies.
* [[Dwarf galaxy]]
* [[Dwarf elliptical galaxy]]
* [[Peculiar galaxy]]
* [[Galaxy morphological classification]]

==References==
{{Reflist|2}}

==External links==
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[[Category:Irregular galaxies| ]]
[[Category:Galaxy morphological types]]