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SOFIA/FIFI-LS full-disk [C II] mapping and CO-dark molecular gas across the nearby spiral galaxy NGC 6946

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Geis,  N.
Infrared and Submillimeter Astronomy, MPI for Extraterrestrial Physics, Max Planck Society;

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Poglitsch,  A.
Infrared and Submillimeter Astronomy, MPI for Extraterrestrial Physics, Max Planck Society;

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Citation

Bigiel, F., de Looze, I., Krabbe, A., Cormier, D., Barnes, A. T., Fischer, C., et al. (2020). SOFIA/FIFI-LS full-disk [C II] mapping and CO-dark molecular gas across the nearby spiral galaxy NGC 6946. The Astrophysical Journal, 903(1): 30. doi:10.3847/1538-4357/abb677.


Cite as: http://hdl.handle.net/21.11116/0000-0007-EBEE-5
Abstract
We present SOFIA/FIFI-LS observations of the [C ii] 158 μm 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), H2, 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 μm/100 μm 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 H2 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.