DENSE GAS IN NEARBY GALAXIES .9. A SURVEY FOR HNC

Ten detections and five tentative detections of hydrogen isocyanide (H NC) J=1-0 emission are reported from a survey including sixteen galaxi es. Full maps are presented for the nuclear regions of NGC 253 and IC 342, partial maps for Maffei 2, M 82, and M 83. Toward IC 342, the HNC and HCO+ distributions differ from those observed in 12CO, 13CO, HCN, CS, and NH3. This is likely a consequence of the density structure. R elative HNC abundances are with 10(-10)-10(-9) much smaller than those measured in nearby dark clouds and appear to be slightly smaller than those in regions of massive star formation of the Galactic disk. This is consistent with the presence of dense warm gas or a frequent occur rence of shocks in the nuclear regions of the galaxies observed. As in prominent Galactic star forming regions, 3 mm HNC line emission tends to be weaker than the corresponding emission from HCN and HCO+. Towar d Arp 220, however, the 3 mm HNC/HCN line intensity ratio is > 1. HNC/ HCO+, HNC/CO, and HNC to 20 cm radio continuum luminosity ratios are a lso particularly large. A possible interpretation is the presence of c ool quiescent gas outside the central region which contains the starbu rst. In the other ultraluminous galaxy observed, NGC 6240, X(HNC)< 10( -10). This is a factor of > 10 smaller than in Arp 220, demonstrating that the molecular composition in ultraluminous galaxies is far from b eing uniform. HNC/CN and HNC/CS abundance ratios are of order 0.1. The CN abundances are smaller than expected for an interstellar medium wh ich is often considered to be 'chemically young'.