We use CO and H alpha velocity fields to study the gas kinematics in the
spiral arms and interarms of M51 (NGC 5194), and fit the 2D velocity field to
estimate the radial and tangential velocity components as a function of spiral
phase (arm distance). We find large radial and tangential streaming velocities,
which are qualitatively consistent with the predictions of density wave theory
and support the existence of shocks. The streaming motions are complex, varying
significantly across the galaxy as well as along and between arms. Aberrations
in the velocity field indicate that the disk is not coplanar, perhaps as far in
as 20\arcsec\ (800 pc) from the center. Velocity profile fits from CO and H
alpha are typically similar, suggesting that most of the H alpha emission
originates from regions of recent star formation. We also explore vortensity
and mass conservation conditions. Vortensity conservation, which does not
require a steady state, is empirically verified. The velocity and density
profiles show large and varying mass fluxes, which are inconsistent with a
steady flow for a single dominant global spiral mode. We thus conclude that the
spiral arms cannot be in a quasi-steady state in any rotating frame, and/or
that out of plane motions may be significant.Comment: 50 pages, including 20 figures; Accepted for publication in ApJ. PDF
version with high resolution figures available at
http://www.astro.umd.edu/~shetty/Research
