Gravitational Runaway and Turbulence Driving in Star‐Gas Galactic Disks

Galactic disks consist of both stars and gas. The gas is more dynamicallyresponsive than the stars, and strongly nonlinear structures and velocities candevelop in the ISM even while stellar surface density perturbations remainfractionally small. We use 2D numerical simulations to explore formation ofbound clouds and turbulence generation in the gas of two-component galacticdisks. We represent the stars with collisionless particles and follow theirorbits using a PM method, and treat the gas as an isothermal, unmagnetizedfluid. The two components interact through a combined gravity. Using stellarparameters typical of mid-disk conditions, we find that models with gaseousToomre parameter Q_g < Q_c ~ 1.4 experience gravitational runaway andeventually form bound condensations. This Q_c value is nearly the same aspreviously found for razor-thin, gas-only models, indicating that thedestabilizing effect of live stars is offsets the reduced self-gravity of thickdisks. This result is also consistent with empirical studies showing that starformation is suppressed when Q_g > 1-2. The bound gaseous clouds that form havemass 6x10^7 Msun each; these represent superclouds that would subsequentlyfragment into GMCs. Self-gravity and sheared rotation also interact to driveturbulence in the gas when Q_g > Q_c. This turbulence is anisotropic, with morepower in sheared than compressive motions. The gaseous velocity dispersion is ~0.6 times the thermal speed when Q_g ~ Q_c. This suggests that gravity isimportant in driving ISM turbulence in many spiral galaxies, since the lowefficiency of star formation naturally leads to a state of marginalinstability.Comment: 41 pages, 13 figures, Accepted for publication in Ap