Dense gas in luminous infrared galaxies

TL;DR

This study investigates the molecular composition of luminous infrared galaxies, revealing how high-density tracers relate to infrared luminosity and differentiating their nuclear environments based on molecular line ratios.

Contribution

It provides a comprehensive multi-molecule, multi-transition analysis of (ultra-)luminous infrared galaxies, highlighting the connection between molecular line ratios and nuclear activity environments.

Findings

High-density tracer emissions correlate strongly with infrared luminosity.

Diagnostic diagrams differentiate environments based on radiation fields.

OH megamaser galaxies show line ratios typical of photon-dominated regions.

Abstract

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. The HCN(1-0), HNC(1-0), HCO+(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 have been augmented with data available in the literature. The integrated emissions of high-density tracer molecules show a strong relation to the far-infrared luminosity. Ratios of integrated line luminosities have been 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 initial conditions and the radiative excitation environment. Initial differentiation has been 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.