Kinematics of Spiral-arm Streaming in M51

We use CO and H 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 consistentwith the predictions of density wave theory and support the existence of shocks. The streamingmotions 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 2000 (800 pc) from the center. Velocity profile fits from CO and H are typically similar, suggesting that most of the H emission originates from regions of recent star formation. We also explore vortensity andmass 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 quasiYsteady state in any rotating frame, and/or that out-of-plane motions may be significant. Subject headinggs: galaxies: individual (M51) — galaxies: kinematics and dynamics — galaxies: spiral