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

Abstract

We present SOFIA/FIFI-LS observations of the [CII] 158$\mu$m cooling line across the nearby spiral galaxy NGC 6946. We combine these with UV, IR, CO, and H I data to compare [CII] emission to dust properties, star formation rate (SFR), H${}_2$, and HI at 560pc 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, and $19\%$ and $8\%$ to the center and interarm region, respectively. [CII]/TIR, [CII]/CO, and [CII]/PAH radial profiles are largely constant, but rise at large radii ($\gtrsim$8kpc) and drop in the center ("[CII] deficit"). This increase at large radii and the observed decline with the 70$\mu$m/100$\mu$m dust color are likely driven by radiation field hardness. We find a near proportional [CII]-SFR scaling relation for beam-sized regions, though the exact scaling depends on methodology. [CII] also becomes increasingly luminous relative to CO at low SFR (interarm or large radii), likely indicating more efficient photodissociation of CO and emphasizing the importance of [CII] as an H${}_2$ and SFR tracer in such regimes. Finally, based on the observed [CII] and CO radial profiles and different models, we find ${\alpha }_{CO}$ to increase with radius, in line with the observed metallicity gradient. The low ${\alpha }_{CO}$ (galaxy average $\lesssim 2M_{sun}p{c}^{-2}(Kkm{s}^{-1}{)}^{-1}$) and low [CII]/CO ratios ($\sim$400 on average) imply little CO-dark gas across NGC 6946, in contrast to estimates in the Milky Way.

12 pages, 6 Figures, published in ApJ

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