Gravitational runaway and turbulence driving in star-gas galactic disks

Galactic disks consist of both stars and gas. The gas is more dynamically responsive than the stars, and strongly nonlinear structures and velocities can develop in the interstellar medium even while stellar surface density perturbations remain fractionally small. Yet, the stellar component still significantly influences the gas. We use two-dimensional numerical simulations to explore formation of bound condensations and turbulence generation in the gas of two-component galactic disks. We represent the stars with collisionless particles, follow their orbits using a particle-mesh method, and treat the gas as an isothermal, unmagnetized fluid. The two components interact through a combined gravitational potential that accounts for the distinct vertical thickness of each disk. Using stellar parameters typical of mid-disk conditions, we find that models with gaseous Toomre parameter Q(g) < Q(g,crit) - 1.4 experience gravitational runaway and eventually form bound condensations. This Q(g,crit) value is nearly the same as previously found for razor- thin, gas-only models, indicating that the destabilizing effect of live'' stars offsets the reduced self-gravity of thick disks. This result is also consistent with empirical studies showing that star formation is suppressed when Qg greater than or similar to 1-2. The bound gaseous structures that form have mass 6 x 10(7) M circle dot each, representing superclouds that would subsequently fragment into GMCs. Self-gravity and sheared rotation also interact to drive turbulence in the gas when Q(g)greater than or similar to Q(g,crit). This turbulence is anisotropic, with more power in sheared than compressive motions. The gaseous velocity dispersion is - 0.6 times the thermal speed when Q(g) approximate to Q(g,crit). This suggests that gravity is important in driving interstellar turbulence in many spiral galaxies, since the low efficiency of star formation naturally leads to a state of marginal instability.