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

摘要

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.