Magnetorotationally-Driven Galactic Turbulence and the Formation of Giant Molecular Clouds

Giant molecular clouds (GMCs), where most stars form, may originate from self-gravitating instabilities in the interstellar medium. Using local threedimensional magnetohydrodynamic simulations, we investigate ways in which galactic turbulence associated with the magnetorotational instability (MRI) may influence the formation and properties of these massive, self-gravitating clouds. Our disk models are vertically stratified with both gaseous and stellar gravity, and subject to uniform shear corresponding to a flat rotation curve. Initial magnetic fields are assumed to be weak and purely vertical. For simplicity, we adopt an isothermal equation of state with sound speed cs = 7 km s . We find that MRI-driven turbulence develops rapidly, with the saturated-state Shakura & Sunyaev parameter α ∼ (0.15− 0.3) dominated by Maxwell stresses. Many of the dimensionless characteristics of the turbulence (e.g. the ratio of the Maxwell to Reynolds stresses) are similar to results from previous MRI studies of accretion disks, hence insensitive to the degree of vertical disk compression, shear rate, and the presence of self-gravity – although self-gravity enhances fluctuation amplitudes slightly. The density-weighted velocity dispersions in nonor weakly self-gravitating disks are σx ∼ σy ∼ (0.4−0.6)cs and σz ∼ (0.2−0.3)cs, suggesting