  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 ISM even while stellar surface density perturbations remain
fractionally small. We use 2D numerical simulations to explore formation of
bound clouds and turbulence generation in the gas of two-component galactic
disks. We represent the stars with collisionless particles and follow their
orbits using a PM method, and treat the gas as an isothermal, unmagnetized
fluid. The two components interact through a combined gravity. Using stellar
parameters typical of mid-disk conditions, we find that models with gaseous
Toomre parameter Q_g < Q_c ~ 1.4 experience gravitational runaway and
eventually form bound condensations. This Q_c value is nearly the same as
previously found for razor-thin, gas-only models, indicating that the
destabilizing effect of live stars is offsets the reduced self-gravity of thick
disks. This result is also consistent with empirical studies showing that star
formation is suppressed when Q_g > 1-2. The bound gaseous clouds that form have
mass 6x10^7 Msun each; these represent superclouds that would subsequently
fragment into GMCs. Self-gravity and sheared rotation also interact to drive
turbulence in the gas when Q_g > Q_c. 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 ~ Q_c. This suggests that gravity is
important in driving ISM turbulence in many spiral galaxies, since the low
efficiency of star formation naturally leads to a state of marginal
instability.
