Using Multiple Lines for Galactic PDR Analysis
Photo-dissociation regions (PDRs) are where molecular clouds are destroyed by UV radiation from forming massive stars. The molecular gas is photo-dissociated and then ionized by UV radiation. The UV radiation below 13.6 eV heats the gas via the photo-electric effect on dust grains. Cooling happens through the dust continuum and far infrared emission lines. The destruction of the star-forming cloud by its forming massive stars is an important feedback process. Understanding the process and its emission is important in understanding star formation on galactic scales and the interpretation of extra-galactic emission lines tracing star formation. For a detailed analysis of galactic PDRs and neighboring ionized and shocked gas regions, ideally multiple species and transitions enter the analysis to trace a broad range of physical conditions (gas density, temperature, radiation field) varying over the regions. I will present exemplary data and initial results for galactic PDRs (M17, M42, DR21) motivating the need for fast mapping of spectral lines over a large wavelength range at moderate spectral resolution. High spectral resolution is desirable, too, but trade-offs need to be made between mapping speed, wavelength coverage, and spectral resolution.