, 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 
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'.