SOFIA/FIFI-LS Full-disk [C II] Mapping and CO-dark Molecular Gas across the Nearby Spiral Galaxy NGC 6946
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 αCO to increase with radius, in line with the observed metallicity gradient. The low αCO (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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- November 2020
- DOI:
- 10.3847/1538-4357/abb677
- arXiv:
- arXiv:2011.02498
- Bibcode:
- 2020ApJ...903...30B
- Keywords:
-
- Spiral galaxies;
- Interstellar medium;
- Interstellar dust;
- Interstellar line emission;
- Galaxy structure;
- 1560;
- 847;
- 836;
- 844;
- 622;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 12 pages, 6 Figures, published in ApJ
