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, so many are experiencing starbursts, or episodes of enhanced star formation. Astronomers have identified two types of peculiar galaxies: interacting galaxies and active galactic nuclei (AGN). Combined, they constitute between 5% and 10% of the known galaxy population.
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.
Peculiar galaxies show a great diversity of form. They can be highly irregular in shape due to the immense gravitational forces which act on them during encounters with other galaxies. The vast majority of peculiar galaxies can be attributed to these encounters, to the extent that the terms ‘peculiar galaxy’ and ‘interacting galaxy’ are now virtually synonymous. They also, on average, host more active galactic nuclei 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".
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