Abstract
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.
Highlights
The observation of emission from singly ionized carbon ([C II]) plays a central role in the study of the star-forming interstellar medium (ISM) in galaxies
The distribution of [C II] emission follows closely that of the warm dust and traces tightly the spiral arms in
We probe three key ratios across the disk of NGC 6946: the [C II]/total infrared (TIR) and [C II]/polycyclic aromatic hydrocarbons (PAHs) intensity ratios, often used as proxies for the photoelectric heating efficiency if the cooling from [O I] can be neglected (Tielens & Hollenbach 1985), and the [C II]/CO intensity ratio probing the star formation activity normalized to the bulk molecular gas reservoir
Summary
The observation of emission from singly ionized carbon ([C II]) plays a central role in the study of the star-forming interstellar medium (ISM) in galaxies. This is largely due to the role of [C II]) as one of the brightest cooling lines across the galaxy population and its accessibility across virtually all redshifts. In addition to the widespread occurrence of [C II] emission, the fine-structure splitting of its ground state leads to a relatively low excitation energy (E/k ∼ 92 K corresponding to λ ∼ 158 μm), which is excited via (predominantly neutral) collisions in the neutral (CNM, PDR) gas This provides an efficient cooling mechanism, in particular in dense. Throughout the paper, we assume a position angle of 243° and an inclination of 33° (Leroy et al 2013)
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