REGIONAL VARIATIONS IN THE DENSE GAS HEATING AND COOLING IN M51 FROM HERSCHEL**Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. FAR-INFRARED SPECTROSCOPY

We present Herschel PACS and SPIRE spectroscopy of the most important far-infrared cooling lines in M51, [C ii](158 μm), [N ii](122 and 205 μm), [O i](63 and 145 μm), and [O iii](88 μm). We compare the observed flux of these lines with the predicted flux from a photon-dominated region model to determine characteristics of the cold gas such as density, temperature, and the far-ultraviolet (FUV) radiation field, G0, resolving details on physical …