Counterrotation in Galaxies
  The phenomenon of counterrotation is observed when two galaxy components have their angular momenta projected antiparallel onto the sky. It follows that if the two components rotate around the same axis, the counterrotation is intrinsic. On the contrary the counterrotation is only apparent if the rotation axes are misaligned and the line-of-sight lies in between the two vectors or their antivectors. In the case of intrinsic counterrotation the two components can be superimposed or radially separated. As far as the two components are concerned, stars are observed to counterrotate with respect to other stars or gas. The counterrotation of gas versus gas has been also detected. Up to now, the number of galaxies exhibiting these phenomena are $\sim60$, the morphological type of which ranges from ellipticals to S0's and to spirals.   When a second event occurs in a galaxy, such as the acquisition of material from outside, it is likely that the resulting angular momentum of the acquired material is decoupled from the angular momentum of the preexisting galaxy. Counterrotation is therefore a general signature of material acquired from outside the main confines of the galaxy. Good examples of such cases are ellipticals with a dust lane or gaseous disk along the minor axis and polar ring galaxies, where the angular momenta are perpendicular. It should be noted that recently attempts have been made to explain special cases of stars versus stars counterrotation in disk galaxies as due to a self induced phenomenon in non-axisymmetric potentials. We discuss the phenomenon according to the morphological type and to the kind of counterrotation. 
F. Bertola and E.M. Corsini