Peculiar galaxy

[1] Between five and ten percent of known galaxies are categorized as peculiar.

[3] When two galaxies come close to each other, their mutual gravitational forces can cause them to acquire highly irregular shapes.

[4] As such, peculiar galaxies tend to host more active galactic nuclei than normal galaxies, indicating that they contain supermassive black holes.

[2] 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.

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."