SOFIA/FIFI-LS Full-disk [C ii] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946
F. Bigiel, I. de Looze, A. Krabbe, D. Cormier, A. T. Barnes, C. Fischer, A. D. Bolatto, A. Bryant, S. Colditz, N. Geis, R. Herrera-Camus, C. Iserlohe, R. Klein, A. K. Leroy, H. Linz, L. W. Looney, S. C. Madden, A. Poglitsch, J. Stutzki, W. D. Vacca
Abstract
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.