Atomic carbon emission from individual molecular clouds in M33

We present observations of the 492 GHz [C I] emission for four individual giant molecular clouds in the Local Group galaxy M33 obtained with the James Clerk Maxwell Telescope. The average [C I] to CO J = 1-0 integrated intensity ratio of 0.10 ± 0.03 is similar to what is observed in Galactic molecular clouds but is smaller than what is seen in starburst galaxies. Similarly, the column density ratio N(C)/N(CO) is similar to that observed in the Orion Bar but is smaller than values obtained for starburst galaxies. The [C I] line is found to be a more important coolant than the lowest three rotational transitions of CO for all the clouds in the sample. The [C I] luminosity does not appear to be enhanced significantly in two low-metallicity clouds, which may be a result of the unusual ionization environment of the clouds. © 1997. The American Astronomical Society. All rights reserved.