Parker Instability in a Self-gravitating Magnetized Gas Disk. I. Linear Stability Analysis

To be a formation mechanism of such large-scale structures as giant molecular clouds (GMCs) and H I superclouds, the classical Parker instability driven by external gravity has to overcome three major obstacles: The convective motions accompanying the instability would generate thin sheets rather than large condensations. The degree of density enhancement achieved by the instability turns out too low to make even diffuse interstellar clouds. The time and the length scales of the instability are longer and larger than the estimated formation time and the observed mean separation of the GMCs, respectively. This study examines whether a replacement of the driving agent from the external to the self-gravity might remove these obstacles by activating the gravitational instability in the Galactic ISM disk. Self-gravitating, magnetized, gas disk bound by a hot halo medium is subject to a Parker-type instability, the usual Jeans gravitational instability, and convection. Under the external gravity growth rate of the convection triggered by interchange mode perturbations increases without bound as the perturbation wavelength decreases; however, under the self-gravity, it reaches a finite value asymptotically. The presence of self-gravity can suppress the convective motions and enhance the density sufficiently high to form clouds. However, the mass and mean separation of the structures resulting from odd-parity undular mode perturbations under the self-gravity are consistent with the H I superclouds rather than the GMCs. To have structures of both scales, it seems necessary to consider a sum of the self and external gravities as the driving agent of the Parker instability.