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Session 69 - Starburst Galaxies.
Display session, Thursday, June 11
We present aperture synthesis maps of the ^13CO(2-1), C^18O(1-0), C^18O(2-1) transitions of in the nucleus of the nearby starburst galaxy, IC 342. Because of its lower optical depth, ^13CO(2-1) better traces the density and temperature distribution of the bulk of the molecular gas in the regions associated with the starburst nucleus than do the more abundant ^12CO transitions, which trace the warmer, ``PDR'' regions, rather than the cooler cloud cores. These three transitions show that the emission is confined to the peaks found in the ^12CO transitions. The ^13CO(2-1)/^13CO(1-0) ratio varies smoothly across the region and has values ranging from 0.3 - 1.7. These very low line ratios are interpreted as due to a combination of excitation temperatures lower than the ^12CO and moderate (\sim a few) ^13CO opacities. There is one region with ratios significantly > 1 in which the ^13CO appears to be optically thin. Because the ^13CO appears to be optically thick over much the central molecular region, to accurately estimate the total column density we have observed C^18O. This region is confirmed to be optically thin by the ^18CO. The masses obtained from C^18O are consistent with the those found using the standard ^12CO conversion factor.
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