Systematic Variations of the CO J=2-1/1-0 Ratio Between the Arm and Interarm Regions of M51
Abstract
We report systematic variations in the emission line ratio of the CO J=2-1 and J=1-0 transitions (R2-1/1-0) in the grand-design spiral galaxy M51. The R2-1/1-0 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, R2-1/1-0 is typically <0.7 (and often 0.4-0.6). 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. R2-1/1-0 is also high, ~0.8-1.0, in the central region of M51. Analysis using the Large Velocity Gradient radiative transfer calculation suggests that cold and low density gas <10K, ~<300 cm-3) is required for the interarm giant molecular clouds (GMCs), but this gas must become warmer and/or denser in the more active star forming spiral arms. The ratio R2-1/1-0 is higher in areas of high 24micron 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.
- Publication:
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American Astronomical Society Meeting Abstracts #221
- Pub Date:
- January 2013
- Bibcode:
- 2013AAS...22123505K