Models of vertically stratified two-phase ISM disks with MRI-driven turbulence

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. There is significant thermally unstable gas, but the density and temperature distributions retain the twin peaks of the classical two-phase ISM. Independent of the total gas surface density and vertical gravity levels adopted, the midplane ratios of thermal to magnetic pressure are are β = 0.3-0.6, when the mean vertical magnetic field is 0.26 μG. We analyze the vertical distributions of density and various pressure terms and address what supports the ISM vertically. All models become differentially stratified by temperature; only when the cold mass fraction is small does turbulent mixing maintain a large cold-medium scale height. Turbulent velocities of the cold gas also increase as the cold mass fraction decreases, but are generally low (∼ 1-3 km s ^-1) near the midplane; they increase to >5 km s^-1 at high |z|. Turbulent amplitudes are higher in the warm gas. The central thermal pressure is similar for all models even though the total weight varies by a factor of 7 for a range of imposed vertical gravity; in higher gravity models the increased weight is supported by increased magnetic pressure gradients. Approximate vertical equilibrium holds for all models. 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.