Counterrotation in galaxies

The phenomenon of counterrotation occurs when the angular momenta of two different galaxian components are oriented in antiparallel way. Stars versus gas, stars versus stars and gas versus gas have been found to be counterrotating in a fair number of galaxies whose morphological type range from elliptical to S0 and spiral. This kinematical decoupling is supposed to have the same origin as the one observed in ellipticals with a dust lane or gaseous disk along the minor axis and in polar ring galaxies, where the angular momenta are perpendicular. Gaseous disks counterrotating with respect to stellar disks and counterrotating stellar disks are generally superimposed. Gas counterrotating versus gas is found in separate regions, although it is not excluded that in some cases gaseous disks can be apposed. In the case of counterrotating stellar disks, they can be of different luminosity and scale length, thus making very difficult the detection. This fact together with the high percentage of S0 with counterrotating gas indicates that the cause of the phenomenon, namely a second event in the history of the galaxy, could play a relevant role in the evolution of galaxies.