A SOFIA Survey of [CII] in the galaxy M51 II. [CII] and CO kinematics across spiral arms

TL;DR

This study provides the first velocity-resolved [CII] imaging of M51, revealing detailed ISM dynamics across spiral arms and supporting the presence of shocks predicted by galaxy simulations.

Contribution

It offers new velocity-resolved [CII] data of M51 and compares observations with simulations to understand spiral arm dynamics and shocks.

Findings

Velocity gradients observed in HI, CO, and [CII] at spiral arms

Offset between upstream molecular gas and downstream star formation

Evidence supporting shocks in spiral arms from velocity data

Abstract

We present the first complete, velocity-resolved [CII] 158um image of the M51 grand-design spiral galaxy, observed with the upGREAT instrument on SOFIA. [CII] is an important tracer of various phases of the interstellar medium (ISM), including ionized gas, neutral atomic, and diffuse molecular regions. We combine the [CII] data with HI, CO, 24um dust continuum, FUV, and near-infrared 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 strong velocity gradients in HI, 12CO, and [CII] 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 12CO) and downstream star formation (traced by [CII]). 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 spiral arms that predict both tangential and radial velocities at the location of spiral arms. We find that our observations, based on the observed velocity gradients and associated offset between CO and [CII], are consistent with the presence of shocks in spiral arms in the inner parts of M51 and in the arm connecting the companion galaxy, M51b, in the outer parts of M51.