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 and 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_crit ~ 1.4
experience gravitational runaway and eventually form bound condensations.
This Q_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 Q_g > 1-2. The bound gaseous structures that form have
mass 6x10^7 Msun 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 > Q_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 ~ Q_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.