Molecular gas and star formation in the nucleus of IC 342:: C18O and millimeter continuum imaging

We present high-resolution maps (similar to2) maps of the J = 1-0 and J = 2-1 transitions of (CO)-O-18 in the central similar to 150 pc of the gas-rich nucleus, IC 342, made with the Owens Valley Millimeter Array. From the (CO)-O-18 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 (CO)-O-18 give a more reliable estimate of the true molecular gas column-density distribution than the more common (CO)-C-12 and (CO)-C-13 isotopomers. The morphology of the (CO)-O-18 emission in the nucleus is a minispiral similar to that of the main isotopomer, (CO)-C-12, except that it is more symmetric and lacks the enhancements to the north. We suggest that the asymmetries present in (CO)-C-12 images may reflect the viewing perspective of the starburst region biased by the high optical depths of (CO)-C-12 rather than true asymmetries in the amount of molecular gas present. The giant molecular clouds seen in (CO)-O-18 appear to be nonspherical, probably because of tidal arm shearing. Column densities determined from (CO)-O-18 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 similar to2-3 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 (CO)-O-18 caused by massive stars.