Velocity-anisotropy-driven bending instability in the rapidly rotating stellar discs

The gas-dynamic model of the disc of highly flattened, rapidly and non-uniformly rotating galaxies is studied to determine its instability against bending perturbations (e.g. those produced by a spontaneous gravity disturbance). The bending perturbations do not cause density enhancements. This type of the vertical, normal to the symmetry plane collective motions makes the disc bend in the same way as the plane of an oscillating membrane does. It is found that a vertical-to-plane anisotropy of peculiar (random) velocities of stars drives the instability: the disc would be unstable to the bending mode when the ratio of plane to vertical velocity dispersions exceeds about 2. The extent to which our results on the disc's stability can have a bearing on observable gas-rich spiral galaxies with a high star formation rate is discussed as well. In particular, I argue that this bending instability may be an efficient heating mechanism for thin galactic discs that increases both the disc thickness and the vertical velocity dispersion of stars with time after they born.