CONSTRAINING THE INFLUENCE OF STAR-FORMATION ON THE LOWEST (CO)-C-12 LINE RATIOS IN M33

We present new (CO)-C-12 J = 2-1 maps for three regions in the nearby spiral galaxy M33, as well as the first (CO)-C-12 J = 3-2 observations of a Local Group galaxy. Two of the regions observed are centered on molecular clouds in the inner disk, while the third is centered on the giant H II region NGC 595, a region where the star formation rate and efficiency are higher than those of the inner disk by an order of mag nitude. These data have been combined with published J = 1-0 data to m easure the CO line ratios in M33. The ratio of the J = 2-1 to J = 1-0 integrated intensities is 0.67 +/- 0.19, in good agreement with measur ements of this line ratio in other galaxies, while the J = 3-2 to J = 2-1 integrated intensity ratio is 0.64 +/- 0.28. The J = 2-1 to J = 1- 0 line ratio is very uniform throughout the regions studied, and thus the presence of intense high-mass star formation and the lower metalli city in NGC 595 do not seem to have a measurable effect on the CO emis sion in the lowest two rotational transitions. A comparison of the J = 1-0 peak temperatures estimated for beam sizes ranging from 7'' to 54 '' reveals that the molecular cloud MC 32 has an average areal filling factor substantially less than unity (less than or equal to 0.3). In addition, CO emission observed with single-dish measurements but not p resent in interferometric observations (''the missing flux'') contribu tes substantially to the area filling factor of the CO emission within 0'.5-1' single-dish beams. The CO line ratios have been combined with a large velocity gradient model to constrain the density, column dens ity, and temperature of the molecular gas. Although our measurements a re consistent with recent results from multiple-transition CO studies of Galactic giant molecular clouds, with only two line ratios we canno t place very strong constraints on the physical conditions in the mole cular interstellar medium in M33. For an assumed kinetic temperature o f 20 K, the molecular hydrogen density must be less than 3000 cm(-3) a nd the CO column density must be 5 x 10(16)- x 10(19) cm(-2).