SOFIA/FIFI-LS Full-disk [CII] Mapping and CO-dark Molecular Gas across
  the Nearby Spiral Galaxy NGC 6946
  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 $\lesssim2\,M_{sun}\,pc^{-2}\,(K\,km\,s^{-1})^{-1}$) 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.
