Unraveling the Evolution of the Interstellar Medium and Star Formation in the M51 Grand-Design Spiral Galaxy with SOFIA

We present results on a joint impact project to map the entire extent of the M51 grand-design spiral galaxy in the [CII] 158um line with the upGREAT and FIFI-LS instruments on SOFIA. The [CII] 158um line is an important tool to diagnose the physical state of the ISM as it can reveal the distribution of the gas that is in the transition between atomic and molecular phases, including the CO-dark H2 gas (hydrogen is molecular, but carbon is not, resulting in this gas being traced neither by CO nor by HI). Additionally, the [CII] line is an important tracer of star formation in galaxies. In this talk we present results on the distribution of star formation activity in the disk of M51 and its companion galaxy M51b. We will also present results on the distribution of [CII] across M51 and on the relationship between the different constituents of the ISM traced by FUV, Halpha, CO, HI, and [CII] emission both spatially and spectrally in the arms and interarm regions of M51.