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With the help of high-resolution long-slit and integral-field spectroscopyobservations, the number of confirmed cases of galaxies with counterrotation isincreasing rapidly. The evolution of such counterrotating galaxies remains farfrom being well understood. In this paper we study the dynamics ofcounterrotating collisionless stellar disks by means of $N$-body simulations.We show that, in the presence of counterrotation, an otherwise gravitationallystable disk can naturally generate bending waves accompanied by strong diskheating across the disk plane, that is in the vertical direction. Suchconclusion is found to hold even for dynamically warm systems with typicalvalues of the initial vertical-to-radial velocity dispersion ratio $\sigma_{\rmz}/\sigma_{\rm R} \approx 0.5$, for which the role of pressure anisotropyshould be unimportant. We note that, during evolution, the $\sigma_{\rmz}/\sigma_{\rm R}$ ratio tends to rise up to values close to unity in the caseof locally Jeans-stable disks, whereas in disks that are initiallyJeans-unstable it may reach even higher values, especially in the innermostregions. This unusual behavior of the $\sigma_{\rm z}/\sigma_{\rm R}$ ratio ingalaxies with counterrotation appears not to have been noticed earlier. Ourinvestigations of systems made of two counterrotating components with differentmass-ratios suggest that even apparently normal disk galaxies (i.e., with aminor counterrotating component so as to escape detection in currentobservations) might be subject to significant disk heating especially in thevertical direction.

Disk heating and bending instability in galaxies with counterrotation