Linear theory of magnetized, viscous, self-gravitating gas disks

Motivated by the idea that gravitational instability in the gaseous disk of the galaxy may form giant molecular clouds, we consider the linear theory of viscous, magnetized, self-gravitating gas disks. The effective viscosity of the interstellar medium in the solar neighborhood is of order 0.8 km s-1 kpc. Viscous, self-gravitating disks are known to be unstable, and for the solar neighborhood the growth rate of the viscous instability is ≅10⁸ yr^-1. After defining a quantitative measure of the nonaxisymmetric responsiveness of the disk R, we show that R declines as viscosity is increased. Magnetized, inviscid self-gravitating disks in solid-body rotation are also known to be subject to an instability that is similar to the viscous instability. We show that this instability is not present in differentially rotating disks, and that magnetic fields also tend to reduce nonaxisymmetric responsiveness.