Giant molecular clouds ( GMCs ) , where most stars form , may originate from self-gravitating instabilities in the interstellar medium .
Using local three-dimensional 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 c _ { s } = 7 { km s ^ { -1 } } .
We find that MRI-driven turbulence develops rapidly , with the saturated-state Shakura & Sunyaev parameter \alpha \sim ( 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 non- or weakly self-gravitating disks are \sigma _ { x } \sim \sigma _ { y } \sim ( 0.4 - 0.6 ) c _ { s } and \sigma _ { z } \sim ( 0.2 - 0.3 ) c _ { s } , suggesting that MRI can contribute significantly to the observed level of galactic turbulence .
The saturated-state magnetic field strength \bar { B } \sim 2 \mu G is similar to typical galactic values .
When self-gravity is strong enough , MRI-driven high-amplitude density perturbations are swing-amplified to form Jeans-mass ( \sim 10 ^ { 7 } { M _ { \odot } } ) bound clouds .
Compared to previous unmagnetized or strongly-magnetized disk models , the threshold for nonlinear instability in the present models occurs for surface densities at least 50 % lower , corresponding to the Toomre parameter Q _ { th } \sim 1.6 .
We present evidence that self-gravitating clouds like GMCs formed under conditions similar to our models can lose much of their original spin angular momenta by magnetic braking , preferentially via fields threading near-perpendicularly to their spin axes .
Finally , we discuss the present results within the larger theoretical and observational context , outlining directions for future study .