1995A&A...295..571H

Ten detections and five tentative detections of hydrogen isocyanide (HNC) J=1-0 emission are reported from a survey including sixteen galaxies. Full maps are presented for the nuclear regions of NGC253 and IC342, partial maps for Maffei2, M82, and M83. Toward IC342, 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. Relative 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 occurrence of shocks in the nuclear regions of the galaxies observed. As in prominent Galactic star forming regions, 3mm HNC line emission tends to be weaker than the corresponding emission from HCN and HCO⁺. Toward Arp220, however, the 3mm HNC/HCN line intensity ratio is >1. HNC/HCO⁺, HNC/CO, and HNC to 20cm radio continuum luminosity ratios are also particularly large. A possible interpretation is the presence of cool quiescent gas outside the central region which contains the starburst. In the other ultraluminous galaxy observed, NGC6240, X(HNC)<10^–10. This is a factor of >10 smaller than in Arp220, demonstrating that the molecular composition in ultraluminous galaxies is far from being uniform. HNC/CN and HNC/CS abundance ratios are of order 0.1. The CN abundances are smaller than expected for an interstellar medium which is often considered to be `chemically young'.