Molecular Gas and Star Formation in the Nucleus of IC 342: C18O and Millimeter Continuum Imaging

Abstract

We present high-resolution maps (D2A) maps of the J 1¨0 and J 2¨1 transitions of C18 Oi n the central D150 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 to date for IC 342. Because of their N H2 low opacities, the transitions of C18O give a more reliable estimate of the true molecular gas columndensity 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 re—ect 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, overestimates the amount of molecular gas in the X CO , center of IC 342 by a factor of D2¨3 at the molecular cloud peaks and by more than this in the diUuse 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.