Physical Conditions in Molecular Clouds in the Arm and Interarm Regions
  of M51
  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 the
upstream interarm regions, passage into the spiral arms and back into the
downstream 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 giant
molecular clouds (GMCs) with low far-IR luminosities. However, the ratio rises
to >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 LVG
calculation provides insight into the changes in the gas physical conditions
between 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/or
denser in the more active star forming spiral arms. R is higher in areas of
high 24micron brightness (an approximate tracer of star formation rate surface
density) and high CO(1-0) integrated intensity (a well-calibrated tracer of
total 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 some
caution is needed when using CO(2-1) as a tracer of bulk molecular gas mass.
