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Irregular galaxy

From Wikipedia, the free encyclopedia
Class of galaxy
NGC 1427A, an example of an irregular galaxy. It is an Irr-I category galaxy about 52 Mly distant.

Irregular galaxy is agalaxy that does not have a distinct regular shape, unlike aspiral or anelliptical galaxy.[1] Irregular galaxies do not fall into any of the regular classes of theHubble sequence, and they are often chaotic in appearance, with neither anuclear bulge nor any trace ofspiral arm structure.[2] Other galaxies have an axis of symmetry for their blue-light distribution, which cannot be found in the sporadic organization of irregular galaxies.[3] This absence of structure in an irregular galaxy leads to little density waves in these galaxies. This makes irregular galaxies prime areas to studystar formation without the effects of density waves.[4]

Collectively they are thought to make up about a quarter of all galaxies.[5] A large chunk of our own Local Group is made up of irregular galaxies, most being distant and classified as dwarf irregulars.[6] Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force, or may be young galaxies between spiral and elliptical status.[7] Irregular galaxies may contain abundant amounts of gas and dust.[8] This is not necessarily true for dwarf irregulars.[9] Irregular galaxies may also be formed in galaxy collisions.[10]

Irregular galaxies are commonly small, about one tenth the mass of theMilky Way galaxy, though there are also unusual cases of large irregulars likeUGC 6697. Due to their small sizes, they are prone to environmental effects likecolliding with large galaxies and intergalactic clouds.[11]

Types

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There are three major types of irregular galaxies:[12]

  • An Irr-I galaxy (Irr I) is an irregular galaxy that features somestructure but not enough to place it cleanly into the Hubble sequence.
    • Subtypes with some spiral structure are calledSm galaxies
    • Subtypes without spiral structure are calledIm 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, and is often shaped via collision or merger.[7]

  • A dI-galaxy (ordIrr) is adwarf irregular galaxy.[13] Irregular galaxies with luminosities below 10 to the power of 8 solar luminosities fall into the dwarf irregular galaxy category.[14] 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 ofmetallicity 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 thefaint blue galaxies known to exist indeep field galaxy surveys. These types of irregular galaxies also have high amounts ofdark matter.[15]

Irregular galaxies are considered late-type along withspiral galaxies as opposed to early typeelliptical galaxies.[16]

Some of the irregular galaxies, especially of theMagellanic type, are smallspiral galaxies that are being distorted by the gravity of a larger neighbor.

Magellanic Clouds

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TheMagellanic Cloud galaxies were once classified as irregular galaxies. TheLarge Magellanic Cloud has since been re-classified as type SBm (barredMagellanic spiral).[17] TheSmall Magellanic Cloud remains classified as an irregular galaxy of type Im under currentgalaxy morphological classification, although it does contain abar structure.

Star formation

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Star formation in irregular galaxies is both common and varied, with some having rates comparable to spiral galaxies. The processes of star formation once in a cloud are the same as in other types of galaxies. However, its origins of star formation are perhaps more influenced by the random movement of gas, as well as the effects of massive stars and the unusually common giantHII regions on theinterstellar medium. This is because they lack the density waves believed to contribute to star formation in spiral galaxies.[18]

Gallery

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See also

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References

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  1. ^Butz, Stephen D. (2002).Science of Earth Systems. Cengage Learning. p. 107.ISBN 978-0-7668-3391-3.
  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. ^Gallagher, John S. (1984)."Structure and Evolution of Irregular Galaxies".Astrophysics Data System.
  4. ^"Star Formation in Irregular Galaxies".ned.ipac.caltech.edu. Retrieved2025-01-27.
  5. ^"Irregular galaxies".www.mssl.ucl.ac.uk. Retrieved2025-01-27.
  6. ^Mo, Houjun; Van den Bosch, Frank; White, S. (2010).Galaxy formation and evolution. Cambridge ; New York: Cambridge University Press.ISBN 978-0-521-85793-2.
  7. ^ab"What is an irregular galaxy? - Technology Org". 2022-09-16. Retrieved2026-02-09.
  8. ^"Faulkes Telescope Educational Guide – Irregular Galaxies". Archived fromthe original on 17 October 2022.
  9. ^Walter, F. et al. Astrophys J661, 102–114, 2007
  10. ^"Types – NASA Science". 2020-09-21. Retrieved2025-01-27.
  11. ^Elmegreen, Debra Meloy;Elmegreen, Bruce G. (2002)."Galaxies". In Dasch, Pat (ed.).Space Sciences. Planetary Science and Astronomy. Vol. 2.Macmillan Reference USA. pp. 50–56. Retrieved25 September 2017 – via Gale Virtual Reference Library.
  12. ^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
  13. ^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.
  14. ^Sparke, Linda Siobhan; Gallagher, John S. (2007).Galaxies in the universe: an introduction (2nd ed.). Cambridge ; New York: Cambridge University Press.ISBN 978-0-521-85593-8.
  15. ^Rivas, Jonathan; Flores-Velazquez, Jeysen; Wheeler, Coral (2024-02-01)."Major Merger between a Classical Dwarf Irregular Galaxy and its Massive Ultra-Faint Satellite".American Astronomical Society Meeting Abstracts.243: 404.18.Bibcode:2024AAS...24340418R.
  16. ^"Hubble Looks at a Late-type Galaxy – NASA Science". 2023-12-15. Retrieved2025-01-27.
  17. ^Corso, G. and Buscombe, W. The Observatory,90, 229–233 (1970)On the spiral structure of the Large Magellanic Cloud
  18. ^"Star Formation in Irregular Galaxies".ned.ipac.caltech.edu. Retrieved2026-02-09.
  19. ^"Feeling blue".www.spacetelescope.org. Archived fromthe original on 5 November 2018. Retrieved5 November 2018.
  20. ^"A frenzy of stars".www.spacetelescope.org. Archived fromthe original on 25 October 2020. Retrieved26 February 2018.
  21. ^"Surveying the cosmos".www.spacetelescope.org. Archived fromthe original on 18 July 2020. Retrieved19 June 2017.
  22. ^"Spotlight on IC 3583".www.spacetelescope.org. Archived fromthe original on 17 August 2019. Retrieved28 November 2016.
  23. ^"A lopsided lynx".www.spacetelescope.org. Archived fromthe original on 21 November 2020. Retrieved9 August 2016.
  24. ^"A distinctly disorganised dwarf". Archived fromthe original on 25 October 2020. Retrieved29 March 2016.
  25. ^"Meeting the neighbours".www.spacetelescope.org. ESA/Hubble. Archived fromthe original on 27 November 2020. Retrieved8 June 2015.
  26. ^"A spattering of blue".www.spacetelescope.org. ESA/Hubble. Archived fromthe original on 5 June 2019. Retrieved8 September 2014.
  27. ^"A cosmic optical illusion".ESA/Hubble Picture of the Week. Archived fromthe original on 7 January 2018. Retrieved20 August 2013.

External links

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