2001ApJ...551..687M

We present high-resolution maps (∼2") maps of the J=1-0 and J=2-1 transitions of C¹⁸O in the central ∼150 pc of the gas-rich nucleus, IC 342, made with the Owens Valley Millimeter Array. From the C¹⁸O maps, we are able to obtain the most accurate map of N_H2 to date for IC 342. Because of their low opacities, the transitions of C¹⁸O give a more reliable estimate of the true molecular gas column-density distribution than the more common ¹²CO and ¹³CO isotopomers. The morphology of the C¹⁸O emission in the nucleus is a minispiral similar to that of the main isotopomer, ¹²CO, except that it is more symmetric and lacks the enhancements to the north. We suggest that the asymmetries present in ¹²CO images may reflect the viewing perspective of the starburst region biased by the high optical depths of ¹²CO rather than true asymmetries in the amount of molecular gas present. The giant molecular clouds seen in C¹⁸O appear to be nonspherical, probably because of tidal arm shearing. Column densities determined from C¹⁸O observations, 1.3 mm dust continuum, and the virial theorem indicate that the standard Galactic conversion factor, X_CO, overestimates the amount of molecular gas in the center of IC 342 by a factor of ∼2-3 at the molecular cloud peaks and by more than this in the diffuse gas away from the starburst. Revised molecular masses based on this conversion factor imply that star formation efficiencies in the starburst region are very high. From the distribution of gas and star formation, it appears that the sites of star formation are dynamically determined rather than driven by density peaks. Near the central star-forming region, evidence is seen for chemical enrichment of C¹⁸O caused by massive stars.