dwarf irregular galaxy
Sagittarius Dwarf Irregular Galaxy. Image: Hubble.
A dwarf irregular galaxy is a dwarf galaxy that lacks any apparent structure or uniformity of shape. Dwarf irregulars (dI) are becoming increasingly important in understanding the evolution of galaxies in general, because, with many examples nearby, they enable detailed study of important issues such as the occurrence of galactic winds, the chemical enrichment of the interstellar and intergalactic media, and the photometric evolution of galaxies. In addition, their low level of evolution, as implied by their low metallicity and high gas content, makes these systems the most similar to primeval galaxies and, therefore, the most useful to infer the primordial galaxy conditions. It has also been suggested that dwarf irregulars represent the local counterpart of faint blue galaxies found in excess in deep galaxy counts.