Models of Vertically Stratified Two-Phase ISM Disks with MRI-Driven Turbulence

Abstract

We have performed time-dependent numerical simulations of the interstellar medium (ISM) which account for galactic shear and magnetic fields, vertical gravity, and a radiative cooling function for atomic gas. This allows us to study the magnetorotational instability (MRI) in cloudy, vertically-stratified disks. As in previous unstratified models, we find that thermal instability interacts with MRI-driven turbulence and galactic shear to produce a network of cold, dense, filamentary clouds embedded in a warm diffuse ambient medium. This structure strongly resembles the morphology of HI gas observed in the 21 cm line. There is significant thermally-unstable gas, but the density and temperature distributions retain the twin peaks of the classical two-phase ISM. We analyze the vertical distributions of density and various pressure terms, and address what supports the ISM vertically. Turbulent velocities of the cold gas increase as the cold mass fraction decreases, but are generally low ~1-3 km/s near the midplane; they increase to > 5 km/s at high z. Finally, we argue that in the outer parts of galactic disks, MRI is likely able to prevent the development of self-gravitating instabilities and hence suppress star formation, even if cold gas is present.