Irregular galaxy
|
NGC1427A, an example of an irregular galaxy. |
An
irregular galaxy is a
galaxy that does not fall into the
Hubble classification for galaxies. These are galaxies that feature neither
spiral nor
elliptical morphology. They are often chaotic in appearance, with neither a nuclear bulge or any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by gravitational action.
There are two major Hubble types of irregular galaxies:
* 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.
de Vaucouleurs subtypes this into galaxies that have some spiral structure
Sm, and those that do not
Im.
* 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 third classification of irregular galaxies are the dwarf irregulars, labelled as
dI or
dIrrs. 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 irregular galaxies are small
spiral galaxies that are being distorted by the gravity of a larger neighbour.
The
Magellanic Cloud galaxies were once classified as irregular galaxies, but have since been found to contain barred spiral structures, and have been since re-classified as "SBm", a fourth type of
barred spiral galaxy.
*
IC 1613*
Irregular Galaxy IC 10*
Leo A*
Messier 82 (The Cigar Galaxy)*
NGC 1569*
NGC 3109*
Pegasus Dwarf Irregular Galaxy*
Phoenix Dwarf*
Sculptor Dwarf Irregular Galaxy*
Sextans A*
Wolf-Lundmark-Melotte*
Active galaxy*
Barred spiral galaxy*
Dwarf galaxy*
Dwarf elliptical galaxy*
Dwarf spheroidal galaxy*
Elliptical galaxy*
Galaxy classification*
Galaxy formation and evolution*
Groups and clusters of galaxies*
Lenticular galaxy*
List of galaxies*
List of nearest galaxies*
Ring galaxy*
Spiral galaxy*
Starburst galaxy*
Seyfert galaxy*
Timeline of galaxies, clusters of galaxies, and large scale structure
