STAR FORMATION AND THE MOLECULAR CONTENT OF IC-342

An almost fully sampled map, at 45" resolution, of CO J = 1 --> 0 emission (115.2712 GHz) from the inner 3' of IC 342 is presented, along with sparse observations out to 10' from the center (in the plane of the sky). The CO emission is centrally concentrated. CO is enhanced near the spiral arms but is observed over the whole map. The H2 content can be described in terms of two components: the nuclear region and a molecular disk. The CO emission is strongest from the nuclear region, which extends 1.5 kpc from the center. The molecular disk lies between radii of 1.5 kpc and 13 kpc, across which the CO emission drops slowly. The CO does not drop exponentially across either the nuclear region or the disk. Beyond 13 kpc is an atomic disk in which H I dominates. In the nuclear region H2 comprises 27% of the dynamical mass and 99% of the gaseous interstellar medium. The ratio of central H2 to H I surface density is 140. In the molecular disk H2 is approximately 8% of the total mass and 75% of the gas mass. The fraction of gas in molecular form steadily decreases with increasing radius. H I and H2 have about equal surface mass densities at a radius of 13 kpc, where the total gas surface density is approximately 10 M. pc-2. This is remarkably similar to the transition surface density of the Milky Way. We use spectra with very good signal to noise to determine the inner rotation curve of IC 342. We find the region of solid body rotation to extend approximately 1.7 kpc from the center, much less than the previously reported 5 kpc. An earlier study suggested that the major difference in the distribution of molecular gas between the Milky Way and IC 342, the presence of a "ring" of H2 in the former, could be attributed to the large zone of solid body rotation in IC 342. We can now rule out this explanation. The efficiency of massive star formation in the nuclear region, determined from the ratio of far-infrared luminosity to molecular mass, is only slightly lower than in the Galactic region W49, and approaches values seen in strongly interacting and merging galaxies. Averaged over the whole disk of IC 342 the massive star formation efficiency is comparable to the average for all molecular clouds in the disk of the Milky Way. We suggest that the efficiency of star formation near the centers of most spiral galaxies is much greater than in the disks and may be related to the presence of large quantities of dense gas.