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 × 20 pc region of the molecular cloud M17 in the 12CO and 13CO J=3→2 and J=2→1 transitions at the James Clerk Maxwell Telescope to determine their global behavior and to assess the reliability of using ratios of CO line intensities integrated over an entire cloud to determine the physical conditions within the cloud. Both the 12CO/13CO J=2→1 and J=3→2 line ratios correlate with the 13CO integrated intensity, with smaller line …