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Dense gas in nearby galaxies IX. A survey for HNC

S. Hüttemeisterl, C. Henkel, R. Mauersberger, N. Brouillet, T. Wiklind, T.J. Millar
Max-Planck-Institut fr Radioastronomie, Auf dem Hiigel 69, D-53121 Bonn, Germany
Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
Observatoire de Bordeaux, BP 89, F-33270 Floirac, France
Onsala Space Observatory, S-43992 Onsala, Sweden
DEMIRM, Observatoire de Paris, Section de Meudon, F-92195 Meudon, France
Mathematics Department, UMIST, PO Box 88, Manchester M60 1QD, UK
Abstract: Ten detections and five tentative detections of hydrogen isocyanide ) J=1-0 emission are reported from a survey including sixteen galaxies. Full maps are presented for the nuclear regions of NGC 253 and IC 342, partial maps for Maffei 2, M82, and M83. Toward IC342, the HNC and HCO distributions differ from those observed in CO, CO, HCN, CS, and NH. This is likely a consequence of the density structure. Relative HNC abundances are with 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, 3 mm HNC line emission tends to be weaker than the corresponding emission from HCN and HCO. Toward 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 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) . This is a factor of >10 smaller than in Arp 220, 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'.

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Next: COHCO and Up: Scientific Results Previous: Scientific Results



Robert Lucas
Thu Mar 9 12:14:01 MET 1995