Linear kinetic theory of instabilities of a gravitating stellar disk (Q1581032)

The authors develop a linear kinetic theory to describe collective oscillations propagating in a rapidly rotating disk of stars, typical of a highly flattened galaxy. The analysis is carried out for a self-graviting, infinitesimally thin and spatially inhomogeneous system, taking into account the thermal motions of stars and gravitational encounters between stars and giant molecular clouds in interstellar medium. The star-cloud encounters are described by Landau collision integral, and the dynamics of gravity perturbations with rare interparticle encounters is also taken into account. Such a model can be treated by using the mathematical formalism from plasma perturbation theory, employing the normal-mode analysis. In particular, the authors solve the Boltzmann equation by integration along paths, neglegting the influence of star-cloud encounters on the star distribution in zeroth-order approximation. The kinetic effect is introduced via the drift notion of stars, which can be computed from the equations of stellar motion in unperturbed central force field of the galaxy. The authors obtain two dispersion laws for two main branches of disk oscillations, i.e. the classical Jeans branch, and an additional gradient branch. The resonant Landau-type instabilities of hydrodynamically stable Jeans branch and gradient gravity perturbations are shown to be long-term generating mechanisms for propagating density waves, thereby leading to spiral-like and/or ring-like patterns in flat galaxies.