Kinematics of Spiral Arm Streaming in M51

TL;DR

This study analyzes gas kinematics in M51's spiral arms using velocity fields, revealing complex streaming motions, non-coplanar disk features, and evidence against steady-state spiral arm models.

Contribution

It provides detailed measurements of radial and tangential velocities, tests vortensity conservation, and challenges the steady-state spiral density wave theory in M51.

Findings

Large radial and tangential streaming velocities consistent with density wave theory

Velocity field indicates the disk is not coplanar near the center

Mass fluxes vary significantly, inconsistent with steady global spiral modes

Abstract

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.