The irradiated ISM of ULIRGs

TL;DR

This paper investigates the molecular gas chemistry in ULIRGs' nuclei, distinguishing X-ray dominated regions from UV-dominated regions using specific molecular line ratios and high-J CO lines.

Contribution

It introduces diagnostic line ratios and high-J CO observations to differentiate between XDRs and PDRs in ULIRGs' molecular gas.

Findings

HCO+/HCO+ and HNC/HCN ratios distinguish PDRs from XDRs.

High-J CO lines effectively identify XDRs versus PDRs.

XDRs produce abundant warm H2O and OH, unlike PDRs.

Abstract

The nuclei of ULIRGs harbor massive young stars, an accreting central black hole, or both. Results are presented for molecular gas that is exposed to X-rays (1-100 keV, XDRs) and far-ultraviolet radiation (6-13.6 eV, PDRs). Attention is paid to species like HCO+, HCN, HNC, OH, H2O and CO. Line ratios of HCN/HCO+ and HNC/HCN discriminate between PDRs and XDRs. Very high J (>10) CO lines, observable with HIFI/Herschel, discriminate very well between XDRs and PDRs. In XDRs, it is easy to produce large abundances of warm (T>100 K) H2O and OH. In PDRs, only OH is produced similarly well.