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For refcode 1995A&A...295..571H:
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Copyright by European Southern Observatory (ESO). Reproduced by permission
1995A&A...295..571H Dense gas in nearby galaxies IX. A survey for HNC S. Huttemeister, C. Henkel, R. Mauersberger, N. Brouillet, T. Wiklind, and T.J. Millar Max-Planck-Institut fur Radioastronomie, Auf dem Hugel 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, 5-43992 Onsala, Sweden DEMIRM, Observatoire de Paris, Section de Meudon, F-92195 Meudon, France Mathematics Department, UMIST, PO Box 88, Manchester M60 lQD, UK Received 17 September 1994/Accepted 22 October 1994 Abstract. 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 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 ^12^CO, ^13^CO, HCN, CS, and NH_3_. 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, 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, 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 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'. Key words: ISM: molecules: - galaxies: ISM - galaxies: nuclei - galaxies: starburst - radio lines: galaxies
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