Unraveling the evolution of the interstellar medium and star formation in the M51 grand-desing spiral galaxy with SOFIA.

We present results on a joint impact project to map theentire extent of the M51 grand-design spiral galaxy in the [CII] 158um line with the upGREAT and FIFI-LS instruments on SOFIA. Spiral density waves play a fundamental role in the conversionof atomic gas to molecular gas, which is followed by gravitationalcontraction leading to star formation. Understanding the impact ofspiral density waves on the life cycle of the interstellar medium(ISM) and on star formation is a critical step for understandinggalaxy evolution, and requires having a complete picture of allconstituents of the ISM in galaxies. The [CII] 158_m line isan important tool to diagnose the physical state of the ISM as it canreveal the distribution of the gas that is in the transition betweenatomic and molecular phases, including the CO-dark H2 gas (hydrogenis molecular, but carbon is not, resulting in this gas being tracedneither by CO nor by HI. In this talk we present preliminary 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.