Physical conditions in molecular clouds in the arm and interarm regions of M51

We report systematic variations in the emission line ratio of the CO J = 2-1 and J = 1-0 transitions (R_2-1/1-0) in the grand-design spiral galaxy M51. The R_2-1/1-0 ratio shows clear evidence for the evolution of molecular gas from the upstream interarm regions into the spiral arms and back into the downstream interarm regions. In the interarm regions, R _2-1/1-0 is typically <0.7 (and often 0.4-0.6); this is similar to the ratios observed in Galactic giant molecular clouds (GMCs) with low far-infrared luminosities. However, the ratio rises to >0.7 (often 0.8-1.0) in the spiral arms, particularly at the leading (downstream) edge of the molecular arms. These trends are similar to those seen in Galactic GMCs with OB star formation (presumably in the Galactic spiral arms). R_2-1/1-0is also high, ∼0.8-1.0, in the central region of M51. Analysis of the molecular excitation using a Large Velocity Gradient radiative transfer calculation provides insight into the changes in the physical conditions of molecular gas between the arm and interarm regions: cold and low-density gas (≲ 10 K, ≲ 300 cm^-3) is required for the interarm GMCs, but this gas must become warmer and/or denser in the more active star-forming spiral arms. The ratio R_2-1/1-0 is higher in areas of high 24 μm dust surface brightness (which is an approximate tracer of star formation rate surface density) and high CO(1-0) integrated intensity (i.e., 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, especially when galactic structures are resolved.