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A peculiar galaxy is a galaxy of unusual size, shape, or composition. These galaxies represent between five and ten percent of the known galaxy population. Astronomers have identified two types of peculiar galaxies: interacting galaxies and active galactic nuclei (AGN).
Scientists currently believe that many peculiar galaxies are formed by the collision of two or more galaxies. As a consequence of these galaxies merging, many peculiar galaxies experience starbursts or episodes of rapid star formation. Both the periods of elevated star formation and the luminosity resulting from active galactic nuclei cause peculiar galaxies to be slightly bluer in color than other galaxies.
Peculiar galaxies are diverse in form. During encounters with other galaxies, immense gravitational forces can cause peculiar galaxies to acquire highly irregular shapes. The terms 'peculiar galaxy' and 'interacting galaxy' have now become synonymous because the majority of peculiar galaxies attribute their forms to the interacting gravitational forces. Similarly, peculiar galaxies tend to host more active galactic nuclei than of the normal galaxy population, indicating that they contain a supermassive black hole.
Halton Arp mapped peculiar galaxies in his 1966 Atlas of Peculiar Galaxies. Arp states 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".
Thus, studying peculiar galaxies can offer insights on regular galaxies, providing useful information for understanding the formation and evolution of any type of galaxy.
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