Supercloud formation by nonaxisymmetric gravitational instabilities in sheared magnetic galaxy disks

The gravitational stability of sheared perturbations in magnetic gas disks is investigated. Realistic shapes and amplitudes for the initial perturbations are considered. The magnetic field destabilizes the gas for all densities. The collapse is primarily along the mean field direction, and for a pressure-equilibrium field strength, the growth rate is similar to that for the nonrotating, nonmagnetic Jeans instability. The ambient interstellar medium in normal disk galaxies should spontaneously form low-density, 10 million solar mass cloud complexes on time scales of 100 million yr. 84 references