  We study the dynamical evolution of spiral structure in the stellar disks of
isolated galaxies using high resolution Smoothed Particle Hydrodynamics (SPH)
simulations that treat the evolution of gas, stars, and dark matter
self-consistently. We focus this study on the question of self-excited spiral
structure in the stellar disk and investigate the dynamical coupling between
the cold, dissipative gaseous component and the stellar component. We find that
angular momentum transport from the gas to the stars inside of corotation leads
to a roughly time-steady spiral structure in the stellar disk. To make this
point clear, we contrast these results with otherwise identical simulations
that do not include a cold gaseous component that is able to cool radiatively
and dissipate energy, and find that spiral structure, when it is incipient,
dies out more rapidly in simulations that do not include gas. We also employ a
standard star formation prescription to convert gas into stars and find that
our results hold for typical gas consumption time scales that are in accord
with the Kennicutt-Schmidt relation. We therefore attribute the long-lived
roughly time steady spiral structure in the stellar disk to the dynamical
coupling between the gas and the stars and the resultant torques that the
self-gravitating gaseous disk is able to exert on the stars due to an azimuthal
phase shift between the collisionless and dissipative components.
