The large-scale J=3 -> 2 and J=2 -> 1 CO emission from M17 and its implications for extragalactic CO observations

We observed a 10 x 20 pc region of the molecular cloud M17 in the (CO)-C-12 and (CO)-C-13 J = 3 --> 2 and J = 2 -->1 transitions at the James Clerk Ma xwell Telescope to determine their global behavior and to assess the reliab ility of using ratios of CO line intensities integrated over an entire clou d to determine the physical conditions within the cloud. Both the (CO)-C-12 /(CO)-C-13 J = 2 --> 1 and J = 3 --> 2 line ratios correlate with the (CO)- C-13 integrated intensity, with smaller line ratios observed at locations w ith large integrated intensities. This correlation is likely due to variati ons in the column density from one position to another within M17. The (CO) -C-12 and (CO)-C-13 (J = 3 --> 2)/(J = 2 --> 1) line ratios show no signifi cant variation from place to place within M17, even on the peak of the phot on-dominated region. A large velocity gradient analysis of globally average d line ratios gives a kinetic temperature of 30 K, a density of 0.25 x 10(5 ) cm(-3), and a (CO)-C-12 column density of 1 x 10(18) cm(-2). This result is in reasonable agreement with the results obtained for individual lines o f sight through the cloud, which suggests that the typical physical conditi ons in a molecular cloud can be determined using CO line ratios integrated over the entire cloud. We estimate the global CO line ratios for M17 to be (CO)-C-12/(CO)-C-13 J = 2 --> 1: 4.5 +/- 0.7; (CO)-C-12/(CO)-C-13 J = 3 --> 2: 3.7 +/- 0.9; (CO)-C-12 (J = 3 --> 2)/(J = 2 --> 1): 0.76 +/- 0.19; (CO) -C-13 (J = 3 --> 2)/ (J = 2 --> 1): 1.3 +/- 0.3. These line ratios generall y agree quite well with previous measurements of individual Galactic molecu lar clouds. There appears to be a clear trend of increasing (CO)-C-12/(CO)- C-13 J = 2 --> 1 and J = 3 --> 2 line ratios as one moves from Galactic mol ecular cloud cores to entire Galactic molecular clouds to normal galaxies, similar to the trend seen previously for the (CO)-C-12/(CO)-C-13 J = 1 --> 0 line. These new observations of M17 show that the difference between the (CO)-C-12/(CO)-C-13 line ratios for Galactic molecular clouds and the disks of spiral galaxies occurs for all three of the lowest rotational transitio ns. The most likely explanation of the high line ratios for normal galaxies is a significant contribution to the CO emission by low column density mat erial, such as diffuse molecular clouds or the outer envelopes of giant mol ecular clouds. Radial gradients in the relative contribution of low and hig h column density material in galaxies could be a significant source of unce rtainty in derivations of the physical properties of molecular gas in exter nal galaxies.