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Register a new userMapping the submillimeter spiral wave in NGC 6946
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P.B. Alton
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We have analysed SCUBA 850mum images of the (near) face-on spiral galaxy NGC 6946, and found a tight correlation between dust thermal emission and molecular gas. The map of visual optical depth relates well to the distribution of neutral gas (HI+H2) and implies a global gas-to-dust ratio of 90. There is no significant radial variation of this ratio: this can be understood, since the gas content is dominated by far by the molecular gas. The latter is estimated through the CO emission tracer, which is itself dependent on metallicity, similarly to dust emission. By comparing the radial profile of our visual optical depth map with that of the SCUBA image, we infer an emissivity (dust absorption coefficient) at 850mum that is 3 times lower than the value measured by COBE in the Milky Way, and 9 times lower than in NGC 891. A decomposition of the spiral structure half way out along the disk of NGC 6946 suggests an interarm optical depth of between 1 and 2. These surprisingly high values represent 40-80% of the visual opacity that we measure for the arm region (abridged).
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Star formation induced by a spiral shock wave, which in turn is generated by a spiral density wave, produces an azimuthal age gradient across the spiral arm, which has opposite signs on either side of the corotational resonance. An analysis of the spatial separation between young star clusters and nearby HII regions made it possible to determine the position of the corotation radius in the studied galaxies. Fourier analysis of the gas velocity field in the same galaxies independently confirmed the corotation radius estimates obtained by the morphological method presented here.
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.
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