We investigate the susceptibility of gaseous , magnetized galactic disks to formation of self-gravitating condensations using two-dimensional , local models .
We focus on two issues : ( 1 ) determining the threshold condition for gravitational runaway , taking into account nonlinear effects , and ( 2 ) distinguishing the magneto-Jeans instability ( MJI ) that arises under inner-galaxy rotation curves from the modified swing amplification ( MSA ) that arises under outer-galaxy rotation curves .
For axisymmetric density fluctuations , instability is known to require a Toomre parameter Q < 1 .
For nonaxisymmetric fluctuations , any nonzero shear q \equiv - d \ln \Omega / d \ln R winds up wavefronts such that in linear theory amplification saturates .
Any Q threshold for nonaxisymmetric gravitational runaway must originate from nonlinear effects .
We use numerical magnetohydrodynamic simulations to demonstrate the anticipated threshold phenomenon , to analyze the nonlinear processes involved , and to evaluate the critical value Q _ { c } for stabilization .
We find Q _ { c } \sim 1.2 - 1.4 for a wide variety of conditions , with the largest values corresponding to nonzero but subthermal mean magnetic fields .
Our findings for Q _ { c } are similar to those inferred from thresholds for active star formation in the outer regions of spiral galaxies .
MJI is distinct from MSA in that opposition to Coriolis forces by magnetic tension , rather than cooperation of epicyclic motion with kinematic shear , enables nonaxisymmetric density perturbations to grow .
We suggest that under low-shear inner-disk conditions , Q _ { c } will be larger than that in outer disks by a factor \sim ( v _ { A } / qc _ { s } ) ^ { 1 / 2 } , where v _ { A } and c _ { s } are the respective Alfvén and sound speeds .