Local stability criterion for a two component galactic disk

Abstract

The criterion for stability against local, axisymmetric perturbations in gravitationally coupled stars and gas in a galactic disk is obtained. The new feature is the inclusion of both stars and gas in the study, and a two-component analog of the standard one-component Q-criterion is obtained which is applicable to realistic galaxies consisting of stars and gas. The Qs-g value is shown to be always less than the Q values for the stars-alone and the gas-alone cases, thus a two-component system is more unstable than either component by itself. Hence, future studies of galaxies must use Qs-g as a measure of disk stability. The results obtained are general, and several applications for the stability, structure, and evolution of galaxies are discussed.