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.
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 galaxy 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.
NGC1427A, an example of an irregular galaxy.