Dense gas in luminous infrared galaxies^*

¹ ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands e-mail: baan@astron.nl
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ Observatoire de l'Université de Bordeaux, 33270 Floirac, France
⁵ Onsala Space Observatory, 43900 Onsala, Sweden & Space Telescope Science Institute, Baltimore, Maryland 21218, USA

Received: 31 January 2007
Accepted: 16 October 2007

Abstract

Aims.Molecules that trace the high-density regions of the interstellar medium have been observed in (ultra-)luminous (far-)infrared galaxies, in order to initiate multiple-molecule multiple-transition studies to evaluate the physical and chemical environment of the nuclear medium and its response to the ongoing nuclear activity.

Methods.The HCN(1-0), HNC(1-0), (1-0), CN(1-0) and CN(2-1), CO(2-1), and CS(3-2) transitions were observed in sources covering three decades of infrared luminosity including sources with known OH megamaser activity. The data for the molecules that trace the high-density regions were augmented with data available in the literature.

Results.The integrated emissions of high-density tracer molecules show a strong relation to the far-infrared luminosity. Ratios of integrated line luminosities were used for a first-order diagnosis of the integrated molecular environment of the evolving nuclear starbursts. Diagnostic diagrams display significant differentiation among the sources that relate to the initial conditions and the radiative excitation environment. Initial differentiation was introduced between the FUV radiation field in photon-dominated-regions and the X-ray field in X-ray-dominated-regions. The galaxies displaying OH megamaser activity have line ratios typical of photon-dominated regions.