Spatially resolved chemistry in nearby galaxies. I. The center of IC 342 (original) (raw)

We have imaged emission from the millimeter lines of eight molecules-C 2 H, C 34 S, N 2 H + , CH 3 OH, HNCO, HNC, HC 3 N, and SO-in the central half kiloparsec of the nearby spiral galaxy IC 342. The 5 ′′ (∼ 50 pc) resolution images were made with the Owens Valley Millimeter Array (OVRO). Using these and previously published CO and HCN images we obtain a picture of the chemistry within the nuclear region on the sizescales 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, 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. N 2 H + , C 18 O, HNC and HCN are widespread and bright, good overall tracers of dense molecular gas. C 2 H and C 34 S, tracers of Photo-Dissociation Region chemistry, originate exclusively from the central 50-100 pc region, where radiation fields are high. The third group of molecules, CH 3 OH and HNCO, correlates well with the expected locations of bar-induced orbital shocks. The good correlation of HNCO with the established shock tracer molecule CH 3 OH is evidence that this molecule, whose chemistry has been uncertain, is indeed produced by processing of grains. HC 3 N is observed to correlate tightly with 3mm 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 et al. (1992) 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 the star formation is excellent. The ratio exhibits no obvious correlation with gas temperature or star formation strength.