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We report systematic variations in the CO(2-1)/CO(1-0) line ratio (R) in M51.The ratio shows clear evidence for the evolution of molecular gas from theupstream interarm regions, passage into the spiral arms and back into thedownstream interarm regions. In the interarm regions, R is typically low <0.7(and often 0.4-0.6); this is similar to the ratios observed in Galactic giantmolecular clouds (GMCs) with low far-IR luminosities. However, the ratio risesto >0.7 (often 0.8-1.0) in the spiral arms, particularly at their leading(downstream) edge. R is also high, 0.8-1.0, in the central region. An LVGcalculation provides insight into the changes in the gas physical conditionsbetween the arm and interarm regions: cold and low density gas (~10K, ~300cm-3)is required for the interarm GMCs, but this gas must become warmer and/ordenser in the more active star forming spiral arms. R is higher in areas ofhigh 24micron brightness (an approximate tracer of star formation rate surfacedensity) and high CO(1-0) integrated intensity (a well-calibrated tracer oftotal molecular gas surface density). The systematic enhancement of the CO(2-1)line relative to CO(1-0) in luminous star forming regions suggests that somecaution is needed when using CO(2-1) as a tracer of bulk molecular gas mass.

Physical Conditions in Molecular Clouds in the Arm and Interarm Regions of M51