Gaseous Spiral Structure and Mass Drift in Spiral Galaxies
  We use hydrodynamic simulations to investigate nonlinear gas responses to an
imposed stellar spiral potential in disk galaxies. The gaseous medium is
assumed to be infinitesimally thin, isothermal, and unmagnetized. We consider
various spiral-arm models with differing strength and pattern speed. We find
that the extent and shapes of gaseous arms as well as the related mass drift
rate depend rather sensitively on the arm pattern speed. In models where the
arm pattern is rotating slow, the gaseous arms extend across the corotation
resonance (CR) all the way to the outer boundary, with a pitch angle slightly
smaller than that of the stellar counterpart. In models with a fast rotating
pattern, on the other hand, spiral shocks are much more tightly wound than the
stellar arms, and cease to exist in the regions near and outside the CR where
$\mathcal{M}_\perp/{\rm sin} p_* \ge 25-40$, with $\mathcal{M}_\perp$ denoting
the perpendicular Mach number of a rotating gas relative to the arms with pitch
angle $p_*$. Inside the CR, the arms drive mass inflows at a rate of $\sim
0.05-3.0 {\rm M}_\odot {\rm yr}^{-1}$ to the central region, with larger values
corresponding to stronger and slower arms. The contribution of the shock
dissipation, external torque, and self-gravitational torque to the mass inflow
is roughly 50%, 40%, and 10%, respectively. We demonstrate that the
distributions of line-of-sight velocities and spiral-arm densities can be a
useful diagnostic tool to distinguish if the spiral pattern is rotating fast or
slow.
