Molecular Gas and Star Formation in the Nucleus of IC 342:C$^{18}$O and Millimeter Continuum Imaging

Abstract

We present high resolution maps ($\sim 2^{''}$) maps of the J=1-0 and J=2-1 transitions of C$^{18}$O in the central $\sim 150$ pc of the gas-rich nucleus of IC 342, made with the Owens Valley Millimeter Array. From the C$^{18}$O maps we are able to obtain the most accurate map of N$_{H_{2}}$ to date for IC 342. Due to their low opacities, the transitions of C$^{18}$O give a more reliable estimate of the true molecular gas column density distribution than the more common $^{12}$CO and $^{13}$CO isotopomers. The morphology of the C$^{18}$O emission in the nucleus is a mini-spiral similar to that of the main isotopomer, $^{12}$CO, except it is more symmetric, lacking the enhancements to north. We suggest that the asymmetries present in $^{12}$CO images may reflect the viewing perspective of the starburst region biased by the high optical depths of $^{12}$CO, rather than true asymmetries in the amount of molecular gas present. The giant molecular clouds seen in C$^{18}$O appear to be nonspherical, probably due to tidal arm shearing. Column densities determined from C$^{18}$O 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 $\sim$2-3 at the molecular cloud peaks, and by $>$3 in the diffuse gas away from the starburst. Revised molecular masses based on this conversion factor imply that star formation efficiencies in the starburst region are very high. From the distribution of gas and star formation it appears that the sites of star formation are dynamically determined, rather than driven by density peaks. Near the central star-forming region, evidence is seen for chemical enrichment of C$^{18}$O due to massive stars.