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. 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.
Peculiar galaxies have been mapped by Halton Arp. 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".
If a galaxy is coming toward us, the specific colors of atoms become slightly bluer. If the galaxy is going away they become redder. Astronomers call this blueshifting and redshifting. All of the distant galaxies Arp observed were redshifted, meaning they are travelling away from us. Astronomers interpret this to mean that the fabric of the universe is expanding, pulling all the galaxies away from all the other galaxies. The farther a galaxy is away from us, the more intervening space there is to expand. This extra distance "pushes" the galaxy away faster, making the specific colors appear redder the farther away a galaxy is from us. The distance to all but the nearest galaxies is virtually impossible to measure by most techniques, and the remaining techniques require a supernova to erupt in a galaxy to facilitate the measurement of the distance. Astronomer Edwin Hubble solved this problem by applying the redshift-distance relationship in reserve, assuming the distance to a galaxy is related to its redshift by Hubble's Law, so all an astronomer needs to do is measure the galaxy's redshift, and the distance to that galaxy can be computed. 
- "peculiar galaxy". Daviddarling.info. 2007-02-01. Retrieved 2012-08-14.
- Millis, John P. "Peculiar Galaxies". http://space.about.com. About.com. Retrieved 17 December 2014.
- "Arp's Catalog Of Peculiar Galaxies". Arpgalaxy.com. 2007-11-12. Retrieved 2012-08-14.
- "Formation of Structure in the Universe". Ned.ipac.caltech.edu. Retrieved 2012-08-14.
- Benson, Mike. "Arp Peculiar Galaxy Introduction". astroleague.
[[Category:Galaxy morphological types]