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A peculiar galaxy is a galaxy which is unusual in its size, shape, or composition. Peculiar galaxies come about as a result of interactions between galaxies, and they may contain atypical amounts of dust or gas, may have higher or lower surface brightness than a typical galaxy, or may have features such as nuclear jets. They can be highly irregular in shape due to the immense gravitational forces which act on them during encounters with other galaxies. They are the result of recent mergers between two or more normal (i.e. spiral galaxies, elliptical galaxies, etc.) galaxies. Specifically, Peculiar galaxies may be from galaxy mergers and collisions that happened in the recent past. Peculiar galaxies are of similar size to regular sized Spiral and Elliptical galaxies, and seem to have features and properties similar to these galaxy types. In addition, peculiar galaxies can be used by astronomers in order to determine the structure of regular galaxies, and can also, in rare cases, be used to "probe large scale mass distributions". Astronomers have identified two types of peculiar galaxies, interacting galaxies and active galactic nuclei (AGN). Also, during collisions it is expected that there would be a spike in activity such as star formation or ignition of the nucleus of one or both galaxies; a common property of Peculiar galaxies as well. Peculiar galaxies are designated by "p" or "pec" in some catalogs. They are snapshots of galaxy mergers in the first millions of years after the collision. That's when the resulting galaxy is in an active state and still maintains some common features of the host galaxies. They constitute between 5% and 10% of the known galaxy population, although most ‘normal’ galaxies will show peculiar features if examined carefully. Peculiar galaxies show a great diversity of form. The vast majority can be attributed to strong gravitational tides generated in the close passage of two galaxies, to the extent that the terms ‘peculiar galaxy’ and ‘interacting galaxy’ are now virtually synonymous. Many peculiar galaxies are experiencing episodes of enhanced star formation, called starbursts. They also show a greater tendency to host ACTIVE GALACTIC NUCLEI (AGN) compared with the normal galaxy population
Peculiar galaxies have been mapped by Halton Arp in his 1966 Atlas of Peculiar Galaxies. Arp says that "The peculiarities of the galaxies pictured in this Atlas represent perturbations, deformations, and interactions which should enable us to analyze the nature of the real galaxies which we observe and which are too remote to experiment on directly".