Parker Instability in a Self-gravitating Magnetized Gas Disk. III. Nonlinear Development of the Parker Instability

Using a total variation diminishing MHD code, we have simulated the nonlinear development of the Parker instability in an isothermal magnetized gas disk that is under the influence of self-gravity. Our objective is to investigate how the Jeans and Parker instabilities compete with the disruptive tendency of the convection in the nonlinear stage of evolution and to know whether the Parker-Jeans instability can be a mechanism for the formation of the larger scale structures in the Galaxy. When the perturbation wavelengths are larger than a Jeans critical wavelength, a cooperative action between the Parker and Jeans instabilities can suppress completely the disruptive behavior of the convective instability and lead the interstellar medium gas material into large-scale structures of high density, whose masses and sizes correspond to H I superclouds rather than to giant molecular clouds. The gas disk develops the vertical filamentary structures near the dense core instead of the chaotic sheet structures that are often seen from simulations of the classical Parker instability. The low-density filaments connect the dense part to the diffuse region far from the disk central plane. The filamentary structure is similar to galactic diffuse vertical structure. When the wavelength of the given perturbations is so short that the Jeans instability may not get triggered, the self-gravitating, magnetized gas disk seems to reach an equilibrium state different from the initial one.