Stability of bending gravity perturbations in rotating galactic discs of finite thickness: fluid dynamics description

ABSTRACT A method is provided for solving the self-consistent system of Euler fluid dynamics and Poisson equations that describe the collective oscillations of a self-gravitating, rotationally flattened galactic disc of finite but small thickness, using a linear perturbation framework. The dispersion properties of bending gravity perturbations, which compress/disperse the system’s medium in the direction normal to the mid-plane and propagate horizontally, are studied for the first time. It is shown that these antisymmetric to the plane perturbations in a rotating disc, behaving like a compressible flow, are always stable macroscopically. The hypothesis is advanced that the small-sized vertical corrugation observed in flat galaxies, including the Milky Way Galaxy, even in the absence of close neighbours, results from the kinetic wave-star resonances by inverse Landau damping. A Boltzmann kinetic theory and particle-based computer modelling will be required to examine these resonances in detail, considering the behaviour of the galactic disc over time at the microscopic level.