Irregular galaxy: Difference between revisions

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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>
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. Astrophys 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>
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=https://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. Astrophys 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>
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>
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==Magellanic Clouds==
==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]].
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) [https://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 ==
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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: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: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 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=https://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: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=https://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:Meeting the neighbours.jpg|Dwarf irregular galaxy known as [[PGC 18431]]<ref>{{cite web|title=Meeting the neighbours|url=https://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 spattering of blue.jpg|[[IC 559]] is classified as a type Sm galaxy.<ref>{{cite web|title=A spattering of blue|url=https://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>  
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>
</gallery>

Latest revision as of 18:49, 16 December 2025

Template:Short description

File:Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope).jpg
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 or an elliptical galaxy.[1] 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 nuclear bulge nor any trace of spiral arm structure.[2] 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.[3]

Collectively they are thought to make up about a quarter of all galaxies.[4] 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.[5] This is not necessarily true for dwarf irregulars.[6] Irregular galaxies may also be formed in galaxy collisions.[7]

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 colliding with large galaxies and intergalactic clouds.[8]

Types

There are three major types of irregular galaxies:[9]

  • 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 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.

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  • A dI-galaxy (or dIrr) is a dwarf irregular galaxy.[10] 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 galaxies known to exist in deep field galaxy surveys. These types of irregular galaxies also have high amounts of dark matter.[11]

Irregular galaxies are considered late-type along with spiral galaxies as opposed to early type elliptical galaxies.[12]

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

Magellanic Clouds

The Magellanic Cloud galaxies were once classified as irregular galaxies. The Large Magellanic Cloud has since been re-classified as type SBm (barred Magellanic spiral).[13] The Small Magellanic Cloud remains classified as an irregular galaxy of type Im under current galaxy morphological classification, although it does contain a bar structure.

Gallery

See also

References

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  1. ↑ Butz, Stephen D. (2002). Science of Earth Systems. Cengage Learning. p. 107. Template:ISBN.
  2. ↑ Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." Publications of the Astronomical Society of the Pacific. 69 (409): 291–303.
  3. ↑ Script error: No such module "citation/CS1".
  4. ↑ Script error: No such module "citation/CS1".
  5. ↑ Script error: No such module "citation/CS1".
  6. ↑ Walter, F. et al. Astrophys J 661, 102–114, 2007
  7. ↑ Script error: No such module "citation/CS1".
  8. ↑ Script error: No such module "citation/CS1".
  9. ↑ Gallagher, J. S. & Hunter, D. A. (1984). "Structure and Evolution of Irregular Galaxies." Annual Review of Astronomy and Astrophysics. 22: 37–74. Script error: No such module "CS1 identifiers".
  10. ↑ 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. Template:ISBN.
  11. ↑ Script error: No such module "Citation/CS1".
  12. ↑ Script error: No such module "citation/CS1".
  13. ↑ Corso, G. and Buscombe, W. The Observatory, 90, 229–233 (1970) On the spiral structure of the Large Magellanic Cloud
  14. ↑ Script error: No such module "citation/CS1".
  15. ↑ Script error: No such module "citation/CS1".
  16. ↑ Script error: No such module "citation/CS1".
  17. ↑ Script error: No such module "citation/CS1".
  18. ↑ Script error: No such module "citation/CS1".
  19. ↑ Script error: No such module "citation/CS1".
  20. ↑ Script error: No such module "citation/CS1".
  21. ↑ Script error: No such module "citation/CS1".
  22. ↑ Script error: No such module "citation/CS1".

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External links

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