Overluminous HNC line emission in Arp 220, NGC 4418 and Mrk 231. Global IR pumping or XDRs?

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) greater than or similar to 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 (T(k) greater than or similar to 40 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 mu m brightness temperatures of T(B) greater than or similar to 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.