Local stability criterion for the gaseous subsystem of a flat galaxy

A local stability criterion is derived: to stabilize an inhomogeneous, differentially rotating gaseous subsystem of a flat galaxy against all nonaxisymmetric disturbances in its plane, the gas clouds should have a velocity dispersion c/sub g/ = [1+f(dΩ/dr, dsigma0/dr)]c0. Here c0 = πGsigma0/kappa is the velocity dispersion sufficient to stabilize axisymmetric perturbations; Ω, sigma0 denote the angular rotational velocity and surface density of the gaseous subsystem; and kappa is the epicyclic frequency. The function f is evaluated for a model disk representing the gaseous component of the Galaxy (estimates for the solar neighborhood indicate that froughly-equal1.6). Gradient waves can exist in such models, analogous to terrestrial Rossby waves and internal gravity waves; their observational manifestations in flat galaxies are discussed