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TwitterA&A 464, 193-200 (2007)
DOI: 10.1051/0004-6361:20066473
Overluminous HNC line emission in Arp 220, NGC 4418 and Mrk 231. Global IR pumping or XDRs?
S. Aalto1, M. Spaans2, M. C. Wiedner3, and S. Hüttemeister41 Onsala Rymdobservatorium, Chalmers Tekniska Högskola, 439 92 Onsala, Sweden
e-mail: susanne@oso.chalmers.se
2 Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
3 I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
4 Astronomisches Institut der Universität Bochum, Universitätsstraße 150, 44780 Bochum, Germany
(Received 29 September 2006 / Accepted 29 November 2006)
Abstract
Context.In recent studies of 3 mm J = 1-0 HNC emission from galaxies
it is found that the emission is often bright which is unexpected in warm, star forming clouds.
We propose that the main cause for the luminous HNC line emission is the extreme radiative and kinematical environment in starburst and active nuclei.
Aims.To determine the underlying excitational and chemical causes behind the luminous HNC emission
in active galaxies and to establish how HNC emission may serve to identify important properties of the nuclear source.
Methods.We present mm and submm JCMT, IRAM 30 m and CSO observations of the J = 3-2 line of HNC
and its isomer HCN in three luminous galaxies and J = 4-3 HNC observations of one galaxy.
The observations are discussed in terms of physical conditions and excitation as well as
in the context of X-ray influenced chemistry.
Results.The ultraluminous mergers Arp 220 and Mrk 231 and the luminous
IR galaxy NGC 4418 show the HNC J 3-2 emission being brighter than the HCN 3-2 emission by
factors of 1.5 to 2. We furthermore report the detection of HNC J = 4-3 in Mrk 231.
Overluminous HNC emission is unexpected in warm molecular gas in ultraluminous galaxies since
I(HNC) 
I(HCN) is usually taken as a signature of cold (10-20 K) dark clouds.
Since the molecular gas of the studied galaxies is warm (
K), we present
two alternative explanations to the overluminous HNC:
a) HNC excitation is affected by pumping of the rotational levels through the mid-infrared
continuum and b) XDRs (X-ray Dominated Regions) influence the abundances of HNC.
HNC may become pumped at 21.5 
m brightness temperatures of 
50 K,
suggesting that HNC-pumping could be common in warm, ultraluminous galaxies with compact IR-nuclei.
This means that the HNC emission is no longer dominated by collisions and its luminosity may not be
used to deduce information on gas density.
On the other hand, all three galaxies are either suspected of having buried AGN - or the presence of AGN is clear (Mrk 231) - indicating that X-rays may affect the ISM chemistry.
Conclusions.We conclude that both the pumping and XDR alternatives imply molecular cloud ensembles distinctly
different from those of typical
starforming regions in the Galaxy, or the ISM of less extreme starburst galaxies. The HNC molecule shows the potential of becoming an additional important tracer of extreme nuclear environments.
Key words: galaxies: evolution -- galaxies: general -- galaxies: starburst -- galaxies: active -- radio lines: galaxies -- ISM: molecules
© ESO 2007