Bending instability in a rotating stellar disk

Most simulations on bending instability adopt the particle-mesh scheme and lack resolution in the direction normal to the disk plane, which can lead t o suppression of the bending instabilities. I present a high resolution par ticle simulation with particle number N = 3,000,000, and discover that the bending instability can be driven by moderately anisotropic velocity disper sion of stars, even without a bar in the center of galaxy or a counter-rota ting stellar disk. The simulation results show that a rigidly rotating disk can be unstable to bending perturbations, with the Toomre's Q value equal to 1.3 and the velocity anisotropy sigma(z)/sigma(h) equal to 1/root 2. Bes ides the velocity anisotropy, the Toomre's Q value is also a important para meter for bending instability in a rotating system.