Irregular galaxy#dwarf irregular anchor

File:Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope).jpg, 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.Butz, Stephen D. (2002). Science of Earth Systems. Cengage Learning. p. 107. {{ISBN|978-0-7668-3391-3}}. 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.Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." Publications of the Astronomical Society of the Pacific. 69 (409): 291–303. 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.{{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}}

Collectively they are thought to make up about a quarter of all galaxies.{{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}} 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.{{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 }} This is not necessarily true for dwarf irregulars.Walter, F. et al. Astophys J 661, 102–114, 2007 Irregular galaxies may also be formed in galaxy collisions.{{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}}

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.{{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=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 }}

Types

There are three major types of irregular galaxies: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}}

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.

A dI-galaxy (or dIrr) is a dwarf irregular galaxy.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}}. 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. {{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 }}

Irregular galaxies are considered late-type along with spiral galaxies as opposed to early type elliptical galaxies. {{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}}

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).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] 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

File:Feeling blue ESO 338-4.jpg|Blue compact dwarf galaxy ESO 338-4{{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 }}

File:A frenzy of stars IC 4710.jpg|IC 4710 lies roughly 25 million light-years away in the southern constellation of Pavo.{{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}}

File:Surveying the cosmos.jpg|ESO 486-21 is a spiral galaxy with a somewhat irregular and ill-defined structure.{{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}}

File:Spotlight on IC 3583.jpg|Irregular galaxy IC 3583 has been found to have a bar of stars running through its center.{{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}}

File:A lopsided lynx NGC 2337.jpg|NGC 2337 is an irregular galaxy that resides 25 million light-years away in the Lynx constellation.{{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}}

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.{{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}}

File:Meeting the neighbours.jpg|Dwarf irregular galaxy known as PGC 18431{{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}}

File:A spattering of blue.jpg|IC 559 is classified as a type Sm galaxy.{{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}}

File:A cosmic optical illusion.jpg|Irregular dwarf galaxy PGC 16389 covers its neighboring galaxy APMBGC 252+125-117.{{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}}

References

External links

Category:Galaxy morphological types