The phenomenon of counterrotation in galaxies

The phenomenon of counterrotation has been observed in about 60 galaxies ra nging from ellipticals to SO's and to spirals. It occurs when two component s of a galaxy rotate in opposite direction with respect to each other. Star s have been found to counterrotate with respect to other stars or gas and t he counterrotation of gas versus gas has been also detected. The origin and the nature of the phenomenon is discussed according to the k ind of counterrotation and the full list of galaxies hosting counterrotatin g components is presented.