Irregular galaxy

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]

Collectively they are thought to make up about a quarter of all 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.[3] This is not necessarily true for dwarf irregulars.[4]

Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy. Due to their small sizes, they are prone to environmental effects like crashing with large galaxies and intergalactic clouds.[5]

Types

There are three major types of irregular galaxies:[6]

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.[7] 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.

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 [8] a type of barred spiral galaxy, the barred Magellanic spiral type. 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

Blue compact dwarf galaxy ESO 338-4.[9]

IC 4710 lies roughly 25 million light-years away in the southern constellation of Pavo.[10]

ESO 486-21 is a spiral galaxy with a somewhat irregular and ill-defined structure.[11]

Irregular galaxy IC 3583 has been found to have a bar of stars running through its center.[12]

NGC 2337 is an irregular galaxy that resides 25 million light-years away in the constellation of Lynx.[13]

UGC 4459 is an irregular dwarf galaxy located approximately 11 million light-years away in the constellation of Ursa Major.[14]

Dwarf irregular galaxy known as PGC 18431.[15]

IC 559 is classified as a type Sm galaxy.[16]

Irregular dwarf galaxy PGC 16389 covers its neighboring galaxy APMBGC 252+125-117.[17]

See also

Dwarf galaxy
Dwarf elliptical galaxy
Peculiar galaxy
Galaxy morphological classification

References

Butz, Stephen D. (2002). Science of Earth Systems. Cengage Learning. p. 107. ISBN 978-0-7668-3391-3.
Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." Publications of the Astronomical Society of the Pacific. 69 (409): 291–303.
Faulkes Telescope Educational Guide - Galaxies - Irregulars
Walter, F. et al. Astophys J 661, 102 - 114, 2007
Elmegreen, Debra Meloy, and Bruce G. Elmegreen. "Galaxies." Space Sciences, edited by Pat Dasch, vol. 2: Planetary Science and Astronomy, Macmillan Reference USA, 2002, pp. 50-56. Gale Virtual Reference Library, go.galegroup.com/ Accessed 25 September 2017.
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
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.
Corso, G. and Buscombe, W. The Observatory, 90, 229 - 233 (1970) On the spiral structure of the Large Magellanic Cloud
"Feeling blue". www.spacetelescope.org. Retrieved 5 November 2018.
"A frenzy of stars". www.spacetelescope.org. Retrieved 26 February 2018.
"Surveying the cosmos". www.spacetelescope.org. Retrieved 19 June 2017.
"Spotlight on IC 3583". www.spacetelescope.org. Retrieved 28 November 2016.
"A lopsided lynx". www.spacetelescope.org. Retrieved 9 August 2016.
"A distinctly disorganised dwarf". Retrieved 29 March 2016.
"Meeting the neighbours". www.spacetelescope.org. ESA/Hubble. Retrieved 8 June 2015.
"A spattering of blue". www.spacetelescope.org. ESA/Hubble. Retrieved 8 September 2014.

"A cosmic optical illusion". ESA/Hubble Picture of the Week. Retrieved 20 August 2013.

External links

Irregular galaxy at the Encyclopædia Britannica

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Galaxies
Morphology

Disc Lenticular
barred unbarred Spiral
anemic barred flocculent grand design intermediate Magellanic unbarred Dwarf galaxy
elliptical irregular spheroidal spiral Elliptical galaxy
cD Irregular
barred Peculiar Ring
Polar

Structure

Active galactic nucleus Bar Bulge Dark matter halo Disc
Disc galaxy Halo
corona Galactic center Galactic plane Galactic ridge Interstellar medium Protogalaxy Spiral arm Supermassive black hole

Active nuclei

Blazar LINER Markarian Quasar Radio
X-shaped Relativistic jet Seyfert

Energetic galaxies

Lyman-alpha emitter Luminous infrared Starburst
blue compact dwarf pea faint blue Hot dust-obscured

Low activity

Low surface brightness Ultra diffuse Dark galaxy

Interaction

Field Galactic tide Cloud Groups and clusters
group cluster Brightest cluster galaxy fossil group Interacting
merger Jellyfish Satellite Stellar stream Superclusters Walls Voids and supervoids
void galaxy

Lists

Galaxies
Galaxies named after people Largest Nearest Polar-ring Ring Spiral Groups and clusters Large quasar groups Quasars

Superclusters Voids