2000ApJ...538..134T

We have mapped a giant molecular cloud in the giant H II region NGC 604 in M33 in the 492 GHz ³P₁⟶³P₀ transition of neutral atomic carbon using the James Clerk Maxwell Telescope. We find the distribution of the [C I] emission to be asymmetric with respect to the CO J=1⟶0 emission, with the peak of the [C I] emission offset toward the direction of the center of the H II region. In addition, the line ratio I_[CI]_/ICO_ is highest (∼0.2) facing the H II region and lowest (≲0.1) away from it. These asymmetries indicate an edge-on morphology where the [C I] emission is strongest on the side of the cloud facing the center of the H II region and not detected at all on the opposite side This suggests that the sources of the incident flux creating C from the dissociation of CO are the massive stars of the H II region. The lowest line ratios are similar to what is observed in Galactic molecular clouds, while the highest are similar to starburst galaxies and other regions of intense star formation. The column density ratio, N(C)/N(H₂) is a few x10^–6, in general agreement with models of photodissociation regions.