CO-dark Molecular Gas and Star Formation across the Nearby Spiral Galaxy NGC 6946

We present SOFIA/FIFI-LS observations of the [CII] 158 μm cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO and HI data to compare [CII] emission to dust properties, SFR, H₂ and HI 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 [CII] luminosity to arms, 19% and 8% to center and interarm region, respectively. [CII]/TIR, [CII]/CO and [CII]/PAH radial profiles are largely constant, but rise at large radii (> 8kpc) and drop in the center (“[CII]-deficit”). This increase at large radii and the observed decline with the 70/100 μm dust color are likely driven by radiation field hardness. We find a near proportional [CII]-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [CII] 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 [CII] as an H₂ and SFR tracer in such regimes. Based on the observed [CII] 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 and low [CII]/CO ratios imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.