5.3. Counter-rotating components
The phenomenon of counter-rotating components is a tracer of galaxy interactions, mass accretion or mergers. It has been first discovered in ellipticals with kinematically decoupled cores (likely to be merger remnants, e.g. Barnes & Hernquist 1992). It has been observed also in many spirals; either two components of stars are counter-rotating, or the gas with respect to the stars, or even two components of gas, in different regions of the galaxies (Galletta 1987, Bertola et al 1992). In the special case of NGC 4550 (Rubin et al. 1992), two almost identical counter-rotating stellar disks are superposed along the line of sight.
These systems pose a number of questions, first from their formation scenario, but also about their stability, their life-time, etc.. Do special waves and instabilities develop in counter-rotating disks, and does this favor central gas accretion?