SOFIA/FIFI-LS Full-disk [C ii] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946

Abstract

We present SOFIA/FIFI-LS observations of the [C ii ] 158 μ m cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H i data to compare [C ii ] emission to dust properties, star formation rate (SFR), H 2 , and H i at 560 pc scales via stacking by environment (spiral arms, interarm, and center), radial profiles, and individual, beam-sized measurements. We attribute 73% of the [C ii ] luminosity to arms, and 19% and 8% to the center and interarm region, respectively. [C ii ]/TIR, [C ii ]/CO, and [C ii ]/PAH radial profiles are largely constant, but rise at large radii (≳ 8 kpc) and drop in the center (“[C ii ] deficit”). This increase at large radii and the observed decline with the 70  μ m/100 μ m dust color are likely driven by radiation field hardness. We find a near proportional [C ii ]–SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [C ii ] also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of [C ii ] as an H 2 and SFR tracer in such regimes. Finally, based on the observed [C ii ] and CO radial profiles and different models, we find α CO to increase with radius, in line with the observed metallicity gradient. The low α CO (galaxy average ≲ 2 M ⊙ pc −2 (K km s −1 ) −1 ) and low [C ii ]/CO ratios (∼400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.