There are three major 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. • 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, and is often shaped via collision or merger. Irregular galaxies with luminosities below 10^8 L_\odot fall into the dwarf irregular galaxy category. 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==