Molecular properties of (U)LIRGs: CO, HCN, HNC and HCO+

TL;DR

This study investigates molecular line ratios in FIR-luminous and OH megamaser galaxies, revealing how these ratios trace star formation, gas density, and the influence of PDR and XDR chemistry, supporting classical OH-MM models.

Contribution

It demonstrates that specific molecular line ratios effectively diagnose gas density and radiation environments in (U)LIRGs and OH megamasers, advancing understanding of their molecular chemistry.

Findings

HCO+/HCN and HCO+/HNC ratios serve as density proxies.

HNC/HCN ratio distinguishes PDR and XDR sources.

OH-MM properties align with PDR chemistry at high densities.

Abstract

The observed molecular properties of a sample of FIR-luminous and OH megamaser (OH-MM) galaxies have been investigated. The ratio of high and low-density tracer lines is found to be determined by the progression of the star formation in the system. The HCO+/HCN and HCO+/HNC line ratios are good proxies for the density of the gas, and PDR and XDR sources can be distinguished using the HNC/HCN line ratio. The properties of the OH-MM sources in the sample can be explained by PDR chemistry in gas with densities higher than 10^5.5 cm^-3, confirming the classical OH-MM model of IR pumped amplification with (variable) low gains.