2016ApJ...821..112K

We present observations of the 12CO(J = 6-5) line and 686 GHz continuum emission in NGC 253 with the Submillimeter Array at an angular resolution of ∼4''. The 12CO(J = 6-5) emission is clearly detected along the disk and follows the distribution of the lower 12CO line transitions with little variation of the line ratios. A large velocity gradient analysis suggests a two-temperature model of the molecular gas in the disk, likely dominated by a combination of low-velocity shocks and the disk-wide photodissociation regions. Only marginal 12CO(J = 6-5) emission is detected in the vicinity of the expanding shells at the eastern and western edges of the disk. While the eastern shell contains gas even warmer (Tkin > 300 K) than the hot gas component (Tkin = 300 K) of the disk, the western shell is surrounded by gas much cooler (Tkin = 60 K) than the eastern shell but somewhat hotter than the cold gas component of the disk (for similar H2 and CO column densities), indicative of different (or differently efficient) heating mechansisms. The continuum emission at 686 GHz in the disk agrees well in shape and size with that at lower (sub)millimeter frequencies, exhibiting a spectral index consistent with thermal dust emission. We find dust temperatures of ∼10-30 K and largely optically thin emission. However, our fits suggest a second (more optically thick) dust component at higher temperatures ({T}_{°}\gt 60 K), similar to the molecular gas. We estimate a global dust mass of ∼106 {M}_☉ for the disk, translating into a gas-to-dust mass ratio of a few hundred, consistent with other nearby active galaxies.