High Spectral Resolution Observations of the Interstellar Medium in the Nuclei of Nearby Galaxies using Herschel and SOFIA

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The heterodyne instruments on Herschel and SOFIA will provide for the first time an extensive inventory of the major cooling lines of the ISM in galactic nuclei sampling a wide range of physical conditions hardly found in the Milky Way. Previous observations with the KAO and ISO have left many questions unanswered which the FIR observatories with their high angular and high spectral resolutions can now address. We plan to focus on the following three key questions: 1) What are the physical conditions of the emitting star-forming gas in galactic nuclei, and what does the cooling emission tell us about the major pathways heating the gas? This will also tell us the importance of star formation over AGNs, of FUV photons over shocks, or of FUV photons over X-rays. 2) A second key questions is the origin of [CII] emission, the most important cooling line of the ISM. The relative importance of the different phases of the ISM is yet unclear will vary with physical environment. 3) Including the Magellanic Clouds in the sample will allow to study at small spatial scales how variations in the metallicity influence the other physical and chemical properties of the ISM.