Distribution and Dynamics of Molecular Gas in the Galaxy M51. III. Kinematics of Molecular Gas

We present here a study of the dynamics of the molecular gas in the spiral galaxy M51 using CO data obtained with the Nobeyama 45-m telescope. From the distribution of the radial velocity of the CO gas, the velocity components parallel and perpendicular to the spiral arms were determined for each spiral phase. Although the variations in the velocity components across the arms are in qualitatively good agreement with density-wave theory, their magnitudes are much larger than those predicted by models with small-amplitude perturbations (5–10% of axisymmetric field). By tracing the velocity vectors at each spiral phase, the orbit of the CO gas was observationally determined. The orbits deviate largely from pure circular motions, forming an oval pattern, as expected from density-wave theory. The arm-to-interarm ratio of the surface density of the gas can be explained in terms of this oval motion. In addition, both the smooth deceleration across the arms and the symmetric density distribution with respect to the arms suggest the lack of the galactic shock expected in a continuous fluid model.