Molecular tracers of PDR-dominated galaxies

TL;DR

This paper investigates how molecular tracers can identify photon-dominated regions in galaxies, revealing how different physical conditions influence molecular chemistry and enabling differentiation between starburst and AGN activity.

Contribution

It models PDR chemistry across various galaxy types, demonstrating the sensitivity of molecular tracers to physical conditions and proposing specific tracers to distinguish excitation sources.

Findings

Molecular tracers vary significantly with physical conditions.

Different tracers are recommended for starburst versus AGN-dominated galaxies.

Models can help interpret observations from Herschel and ALMA.

Abstract

Photon-dominated regions (PDRs) are powerful molecular line emitters in external galaxies. They are expected in galaxies with high rates of massive star formation due to either starburst (SB) events or starburst coupled with active galactic nuclei (AGN) events. We have explored the PDR chemistry for a range of physical conditions representing a variety of galaxy types. Our main result is a demonstration of the sensitivity of the chemistry to changes in the physical conditions. We adopt crude estimates of relevant physical parameters for several galaxy types and use our models to predict suitable molecular tracers of those conditions. The set of recommended molecular tracers differs from that which we recommended for use in galaxies with embedded massive stars. Thus, molecular observations can in principle be used to distinguish between excitation by starburst and by SB+AGN in distant galaxies. Our recommendations are intended to be useful in preparing Herschel and ALMA proposals to identify sources of excitation in galaxies.