Spatially resolved chemistry in nearby galaxies. I. The center of IC 342

We have imaged emission from the millimeter lines of eight molecules-C2H, (CS)-S-34, N2H+, CH3OH, HNCO, HNC, HC3N, and SO-in the central half-kiloparsec of the nearby spiral galaxy IC 342. The 5 (similar to50 pc) resolution images were made with the Owens Valley Millimeter Array. Using these and previously published CO and HCN images, we obtain a picture of the chemistry within the nuclear region on the size scales of individual giant molecular clouds. Bright emission is detected from all but SO. There are marked differences in morphology for the different molecules. A principal-component analysis is performed to quantify similarities and differences among the images. This analysis reveals that while all molecules are to zeroth order correlated, that is, that they are all found in dense molecular clouds, there are three distinct groups of molecules distinguished by the location of their emission within the nuclear region. N2H+, (CO)-O-18, HNC, and HCN are widespread and bright, good overall tracers of dense molecular gas. C2H and (CS)-S-34, tracers of photodissociation region chemistry, originate exclusively from the central 50-100 pc region, where radiation fields are high. The third group of molecules, CH3OH and HNCO, correlates well with the expected locations of bar-induced orbital shocks. The good correlation of HNCO with the established shock tracer molecule CH3OH is evidence that this molecule, whose chemistry has been uncertain, is indeed produced by the processing of grain mantles. HC3N is observed to correlate tightly with 3 mm continuum emission, demonstrating that the young starbursts are the sites of the warmest and densest molecular gas. We compare our HNC images with the HCN images of Downes and coworkers to produce the first high-resolution, extragalactic HCN/HNC map: the HNC/HCN ratio is near unity across the nucleus, and the correlation of both of these gas tracers with star formation is excellent. The ratio exhibits no obvious correlation with gas temperature or star formation strength.