Spatially Resolved Chemistry in Nearby Galaxies. I. The Center of IC 342

We have imaged emission from the millimeter lines of eight molecules--C2H,C34S, N2H+, CH3OH, HNCO, HNC, HC3N, and SO--in the central half kpc of thenearby spiral galaxy IC 342. The 5" (~50 pc) resolution images were made withOVRO. Using these maps we obtain a picture of the chemistry within the nuclearregion on the sizescales of individual GMCs. Bright emission is detected fromall but SO. There are marked differences in morphology for the differentmolecules. A principal component analysis is performed to quantify similaritiesand differences among the images. This analysis reveals that while allmolecules are to zeroth order correlated, that is, they are all found in densemolecular clouds, there are three distinct groups of molecules distinguished bythe location of their emission within the nuclear region. N2H+, C18O, HNC andHCN are widespread and bright, good overall tracers of dense molecular gas. C2Hand C34S, tracers of PDR chemistry, originate exclusively from the central50-100 pc region, where radiation fields are high. The third group ofmolecules, CH3OH and HNCO, correlates well with the expected locations ofbar-induced orbital shocks. The good correlation of HNCO with the establishedshock tracer molecule CH3OH is evidence that this molecule, whose chemistry hasbeen uncertain, is indeed produced by processing of grains. HC3N is observed tocorrelate tightly with 3mm continuum emission, demonstrating that the youngstarbursts are the sites of the warmest and densest molecular gas. We compareour HNC images with the HCN images of Downes et al. (1992) to produce the firsthigh resolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unityacross the nucleus and the correlation of both of these gas tracers with thestar formation is excellent. (Abridged).Comment: 54 pages including 10 figures and 8 tables. Accepted for publication in Ap