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]

Irregular galaxy NGC 1427A (captured by the Hubble Space Telescope)
NGC 1427A, an example of an irregular galaxy. It is an Irr-I category galaxy about 52 Mly distant.


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.


A frenzy of stars IC 4710

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

Surveying the cosmos

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

Spotlight on IC 3583

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

A lopsided lynx NGC 2337

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

A distinctly disorganised dwarf

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

Meeting the neighbours

Dwarf irregular galaxy known as PGC 18431.[15]

A spattering of blue

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

A cosmic optical illusion

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

See also


  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. ^ Faulkes Telescope Educational Guide - Galaxies - Irregulars
  4. ^ Walter, F. et al. Astophys J 661, 102 - 114, 2007
  5. ^ 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, Accessed 25 September 2017.
  6. ^ 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
  7. ^ 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.
  8. ^ Corso, G. and Buscombe, W. The Observatory, 90, 229 - 233 (1970) On the spiral structure of the Large Magellanic Cloud
  9. ^ "Feeling blue". Retrieved 5 November 2018.
  10. ^ "A frenzy of stars". Retrieved 26 February 2018.
  11. ^ "Surveying the cosmos". Retrieved 19 June 2017.
  12. ^ "Spotlight on IC 3583". Retrieved 28 November 2016.
  13. ^ "A lopsided lynx". Retrieved 9 August 2016.
  14. ^ "A distinctly disorganised dwarf". Retrieved 29 March 2016.
  15. ^ "Meeting the neighbours". ESA/Hubble. Retrieved 8 June 2015.
  16. ^ "A spattering of blue". ESA/Hubble. Retrieved 8 September 2014.
  17. ^ "A cosmic optical illusion". ESA/Hubble Picture of the Week. Retrieved 20 August 2013.

External links

Barred irregular galaxy

A barred irregular galaxy is an irregular version of a barred spiral galaxy. Examples include the Large Magellanic Cloud and NGC 6822. Some barred irregular galaxies (like the Large Magellanic Cloud) may be dwarf spiral galaxies, which have been distorted into an irregular shape by tidal interactions with a more massive neighbor.

Canis Major Overdensity

The Canis Major Dwarf Galaxy (CMa Dwarf) or Canis Major Overdensity (CMa Overdensity) is a disputed dwarf irregular galaxy in the Local Group, located in the same part of the sky as the constellation Canis Major.

The supposed small galaxy contains a relatively high percentage of red giants and is thought to contain an estimated one billion stars in all.

The Canis Major Dwarf Galaxy is classified as an irregular galaxy and is now thought to be the closest neighboring galaxy to the Earth's location in the Milky Way, being located about 25,000 light-years (7.7 kiloparsecs) away from the Solar System and 42,000 ly (13 kpc) from the Galactic Center. It has a roughly elliptical shape and is thought to contain as many stars as the Sagittarius Dwarf Elliptical Galaxy, the previous contender for closest galaxy to our location in the Milky Way.

IC 10

IC 10 is an irregular galaxy in the constellation Cassiopeia. It was discovered by Lewis Swift in 1887 and in 1935 Nicholas Mayall became the first to suggest that the object is extragalactic. Edwin Hubble suspected it might belong to the Local Group of galaxies, but its status remained uncertain for decades. The radial velocity of IC 10 was measured in 1962, and it was found to be approaching the Milky Way at approximately

350 km/s, strengthening the evidence for its membership in the Local Group. Its membership in the group was finally confirmed in 1996 by direct measurements of its

distance based on observations of Cepheids. Despite its closeness, the galaxy is rather difficult to study because it lies near the plane of the Milky Way and is therefore

heavily obscured by interstellar matter.The apparent distance between IC 10 and the Andromeda Galaxy is about the same as the apparent distance between the Andromeda Galaxy and the Triangulum Galaxy, which suggests that IC 10 may belong to the M31 subgroup.IC 10 is the only known starburst galaxy in the Local Group of galaxies. It has many more Wolf–Rayet stars per square kiloparsec (5.1 stars/kpc2) than the Large Magellanic Cloud (2.0 stars/kpc2) or the Small Magellanic Cloud (0.9 stars/kpc2). Although the galaxy has a luminosity similar to the SMC, it is considerably smaller. Its higher metallicity compared to the SMC suggests that star formation activity has continued for a longer time period. The evolutionary status of the Wolf–Rayet stars suggests that they all formed in a relatively short timespan. The ratio between the two types of Wolf–Rayet stars (WC stars and WN stars) in IC 10 is very different from the ratio in other galaxies in the Local Group, which may be somehow due to the starburst nature of the galaxy. Currently, the galaxy produces stars at the rate of 0.04–0.08 solar masses per year, which means that the gas supply in the galaxy can last for only a few billion years longer.Observations of IC 10 in the far-infrared show that the dust in this mild starburst galaxy is deficient in small grains. It is hypothesized that any small grains that formerly existed were destroyed by strong ultraviolet radiation in the areas around the hot luminous stars that were formed in the galaxy's recent burst of star formation.The galaxy has a huge envelope of hydrogen gas, with an apparent size measuring 68′ × 80′, which is far larger than the apparent size of the galaxy in visible light (5.5′ × 7.0′). IC 10 is also unusual in the respect that the visible part of the galaxy seems to rotate in a different direction than the outer envelope. It has a H II nucleus.

Local Group

The Local Group is the galaxy group that includes the Milky Way. The Local Group comprises more than 54 galaxies, most of them dwarf galaxies. Its gravitational center is located somewhere between the Milky Way and the Andromeda Galaxy. The Local Group has a diameter of 10 Mly (3.1 Mpc) (about 1023 meters) and has a binary (dumbbell)

distribution. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster.

The three largest members of the group (in descending order) are the Andromeda Galaxy, the Milky Way and the significantly smaller Triangulum Galaxy. The larger two of these spiral galaxies each have their own system of satellite galaxies.

The Andromeda Galaxy's satellite system consists of Messier 32 (M32), Messier 110 (M110), NGC 147, NGC 185, Andromeda I (And I), And II, And III, And V, And VI (also known as Pegasus Dwarf Spheroidal Galaxy, or Pegasus DSph), And VII (also known as Cassiopeia Dwarf Galaxy), And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.

The Milky Way's satellite galaxies system comprises Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy (disputed, considered by some not a galaxy), Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I (a dwarf galaxy), Leo II (a dwarf galaxy), and Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies.

The Triangulum Galaxy may or may not be a companion to the Andromeda Galaxy. Pisces Dwarf Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either.

The membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy, is uncertain due to extreme distances from the center of the Local Group.

The other members of the group are likely gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf Galaxy, Leo A, Tucana Dwarf Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte, Aquarius Dwarf Galaxy, and Sagittarius Dwarf Irregular Galaxy.

Messier 82

Messier 82 (also known as NGC 3034, Cigar Galaxy or M82) is a starburst galaxy approximately 12 million light-years away in the constellation Ursa Major. A member of the M81 Group, it is about five times more luminous than the whole Milky Way and has a center one hundred times more luminous than our galaxy's center. The starburst activity is thought to have been triggered by interaction with neighboring galaxy M81. As the closest starburst galaxy to Earth, M82 is the prototypical example of this galaxy type. SN 2014J, a type Ia supernova, was discovered in the galaxy on 21 January 2014. In 2014, in studying M82, scientists discovered the brightest pulsar yet known, designated M82 X-2.

NGC 14

NGC 14 is an irregular galaxy in the Pegasus constellation. It was included in Halton Arp's Atlas of Peculiar Galaxies, under the section "Galaxies with the appearance of fission," since the irregular appearance of this galaxy causes it to look like it is separating apart. It was discovered on September the 18th 1786 by William Herschel.

NGC 1531

NGC 1531 is a dwarf galaxy in the constellation Eridanus that is interacting with the larger spiral galaxy NGC 1532.

It was discovered by John Herschel on 19 October 1835. Although technically classified as a peculiar lenticular galaxy, the galaxy's structure is better described as amorphous.

NGC 4214

NGC 4214 is a dwarf barred irregular galaxy located around 10 million light-years away in the constellation Canes Venatici.

NGC 4485

NGC 4485 is an irregular galaxy located in the constellation of Canes Venatici. It is interacting with the spiral galaxy NGC 4490 and as a result both galaxies are distorted and are undergoing intense star formation.

NGC 4630

NGC 4630 is an irregular galaxy located about 54 million light-years away in the constellation of Virgo. NGC 4630 was discovered by astronomer William Herschel on February 2, 1786. NGC 4630 is part of the Virgo II Groups which form a southern extension of the Virgo Cluster.

NGC 5195

NGC 5195 (also known as Messier 51b or M51b) is a dwarf galaxy that is interacting with the Whirlpool Galaxy (also known as M51a or NGC 5194). Both galaxies are located approximately 25 million light-years away in the constellation Canes Venatici. Together, the two galaxies are one of the most famous interacting galaxy pairs.

NGC 5253

NGC 5253 is an irregular galaxy in the constellation Centaurus. It was discovered by John Frederick William Herschel on 15 March 1787.

NGC 5408

NGC 5408 is an irregular galaxy in the constellation Centaurus. It was discovered by John Herschel on June 5, 1834.

NGC 55

NGC 55, also occasionally referred to as The Whale Galaxy, is a Magellanic type barred spiral galaxy located about 7 million light-years away in the constellation Sculptor. Along with its neighbor NGC 300, it is one of the closest galaxies to the Local Group, probably lying between the Milky Way and the Sculptor Group.

NGC 5829

NGC 5829 is a spiral galaxy in the constellation Boötes that is interacting with the irregular galaxy IC 4526. Together, the two form the galaxy pair Arp 42.

NGC 6745

NGC 6745 (also known as UGC 11391) is an irregular galaxy about 206 million light-years (63.5 mega-parsecs) away in the constellation Lyra. It is actually a trio of galaxies in the process of colliding.

The three galaxies have been colliding for hundreds of millions of years. After passing through the larger galaxy (NGC 6745A), the smaller one (NGC 6745B) is now moving away. The larger galaxy was probably a spiral galaxy before the collision, but was damaged and now appears peculiar. It is unlikely that any stars in the two galaxies collided directly because of the vast distances between them. The gas, dust, and ambient magnetic fields of the galaxies, however, do interact directly in a collision. As a result of this interaction, the smaller galaxy has probably lost most of its interstellar medium to the larger one.

NGC 87

NGC 87 is a diffuse, highly disorganized barred irregular galaxy, part of Robert's Quartet, a group of four interacting galaxies.

Pegasus Dwarf Irregular Galaxy

The Pegasus Dwarf Irregular Galaxy (also known as Peg DIG or the Pegasus Dwarf) is a dwarf irregular galaxy in the direction of the constellation Pegasus. It was discovered by A. G. Wilson in the 1950s. The Pegasus Dwarf is a companion of the Andromeda Galaxy in the Local Group.

Sculptor Dwarf Irregular Galaxy

The Sculptor Dwarf Irregular Galaxy (SDIG) is an irregular galaxy in the constellation Sculptor. The galaxy was discovered in 1976.

Active nuclei
Energetic galaxies
Low activity
See also

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