A polar-ring galaxy is a type of galaxy with an outer ring of gas and stars that rotates over the poles of the galaxy.[1] These polar rings are thought to form when two galaxies gravitationally interact with each other. One possibility is that a material is tidally stripped from a passing galaxy to produce the polar ring.[2] The other possibility is that a smaller galaxy collides orthogonally with the plane of rotation of the larger galaxy, with the smaller galaxy effectively forming the polar-ring structure.[3]
The best-known polar-ring galaxies are S0s (lenticular galaxies), but from the physical point of view they are part of a wider category of galaxies, including several ellipticals.
The first four S0 galaxies that were identified as polar-ring galaxies were NGC 2685,[4] NGC 4650A,[5][6] A 0136 -0801,[3] and ESO 415 -G26.[6] While these galaxies have been extensively studied, many other polar-ring galaxies have since been identified.[7] Polar-ring S0 galaxies may be found around 0.5% of all nearby lenticular galaxies, and it is possible that 5% of lenticular galaxies may have had polar rings at some point during their lifetimes.[7]
The first polar-ring elliptical galaxies were identified in 1978. They were NGC 5128, NGC 5363, NGC 1947 and Cygnus A,[8] while the polar-ring S0 galaxies NGC 2685 and NGC 4650A were at that time indicated as resulting from similar formation processes.[8] Only some years later, when the first observations of the stellar and gas motion of polar-ring elliptical and S0 galaxies were possible with a better spectroscopic technology, the external origin of the gaseous rings was clarified.[3][6][9][10] In addition to the best-known example, NGC 5128 (Cen A), a very regular polar ring elliptical, is NGC 5266[10]
