A SOFIA Survey of [C II] IN THE GALAXY M51 II. [C II] AND CO KINEMATICS ACROSS SPIRAL ARMS

We present the first complete, velocity–resolved [C II ] 158μm image of the M51 grand-design spiral galaxy, observed with the upGREAT instrument on SOFIA. [C II ] is an important tracer of various phases of the inter-stellar medium, including ionized gas, neutral atomic, and diffuse molecular regions. We combine the [C II ] datawith H I , CO, 24μm dust continuum, FUV, and near–infrared (NIR) K–band observations to study the evolution of the ISM across M51’s spiral arms in both position–position, and position–velocity space. Our data show velocity discontinuities in H I , 12 CO, and [C II ] at the locations of stellar arms (traced by K–band data) with a clear offset in position–velocity space between upstream molecular gas (traced by 12 CO) and downstream star formation (traced by [C II ]). We compare the observed position–velocity maps across spiral arms with synthetic observations from numerical simulations of galaxies with both dynamical and quasi–stationary steady spiralarms that predict both tangential and radial velocities at the location of spiral arms. We find that our observations, based on the observed velocity discontinuities and associated offset between CO and [C II ], are consistent with the presence of shocks in spiral arms in the inner parts of M51 and in the arm connecting the companiongalaxy, M51b, in the outer parts of M51.