An irregular galaxy is a
galaxy
A galaxy is a Physical system, system of stars, stellar remnants, interstellar medium, interstellar gas, cosmic dust, dust, and dark matter bound together by gravity. The word is derived from the Ancient Greek, Greek ' (), literally 'milky', ...
that does not have a distinct regular shape, unlike a
spiral or an
elliptical galaxy. 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. 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
Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...
without the effects of density waves.
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. This is not necessarily true for dwarf irregulars. Irregular galaxies may also be formed in galaxy collisions.
Irregular galaxies are commonly small, about one tenth the mass of the
Milky Way
The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
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.
Types
There are three major types of irregular galaxies:
* An Irr-I galaxy (Irr I) is an irregular galaxy that features some
structure
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
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
Dwarf, dwarfs or dwarves may refer to:
Common uses
*Dwarf (folklore), a supernatural being from Germanic folklore
* Dwarf, a human or animal with dwarfism
Arts, entertainment, and media Fictional entities
* Dwarf (''Dungeons & Dragons''), a sh ...
irregular galaxy. 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.
Irregular galaxies are considered late-type along with
spiral galaxies as opposed to early type
elliptical galaxies.
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
A Magellanic spiral galaxy is a spiral galaxy with only one spiral arm. Magellanic spiral galaxies are classified as the type Sm (with sub-categories SAm, SBm, SABm); the prototype galaxy and namesake for Magellanic spirals is the Large Magell ...
). The
Small Magellanic Cloud
The Small Magellanic Cloud (SMC) is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about , and contains several hundred million stars. It has a total mass of approximately 7 bill ...
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
File:A frenzy of stars IC 4710.jpg, IC 4710 lies roughly 25 million light-years away in the southern constellation of Pavo.
File:Surveying the cosmos.jpg, ESO 486-21 is a spiral galaxy with a somewhat irregular and ill-defined structure.
File:Spotlight on IC 3583.jpg, Irregular galaxy IC 3583 has been found to have a bar of stars running through its center.
File:A lopsided lynx NGC 2337.jpg, NGC 2337 is an irregular galaxy that resides 25 million light-years away in the Lynx constellation.
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.
File:Meeting the neighbours.jpg, Dwarf irregular galaxy known as PGC 18431
File:A spattering of blue.jpg, IC 559 is classified as a type Sm galaxy.
File:A cosmic optical illusion.jpg, Irregular dwarf galaxy PGC 16389 covers its neighboring galaxy APMBGC 252+125-117.
See also
*
Blueberry galaxy – Small and very active galaxies.
*
Dwarf galaxy
*
Dwarf elliptical galaxy
*
Peculiar galaxy
*
Galaxy morphological classification
References
External links
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Galaxy morphological types