The chemistry of interstellar HnO+ beyond the Galaxy

TL;DR

This paper reviews the detection and chemistry of HnO+ ions in interstellar space, highlighting their role in water formation and as tracers of cosmic ray ionization, with a focus on extragalactic observations and chemical processes.

Contribution

It provides a comparative analysis of HnO+ chemistry within the Galaxy and beyond, emphasizing recent extragalactic detections and the need for further chemical rate data.

Findings

Low H2O/H3O+ ratio in M82 indicates rapid water photodissociation.

High OH+ and H2O+ abundances in Mrk 231 suggest radiative excitation or formation pumping.

Detection of HnO+ ions in external galaxies expands understanding of interstellar chemistry.

Abstract

The astrochemistry of the HnO+ (n=1..3) ions is important as the main gas-phase formation route for water, and as tracer of the interstellar ionization rate by cosmic rays and other processes. While interstellar H3O+ has been known since the early 1990's, interstellar OH+ and H2O+ have only recently been detected using the Herschel space observatory and also from the ground. This paper reviews detections of HnO+ toward external galaxies and compares with ground-based work. The similarities and differences of the HnO+ chemistry within the Galaxy and beyond are discussed. Special attention is given to the low H2O/H3O+ ratio in M82 of only 3.3, suggesting rapid H2O photodissociation, and the high apparent OH+ and H2O+ abundances in Mrk 231, suggesting radiative excitation and/or formation pumping. Photodissociation rates for H3O+ and collisional cross-sections for OH+ and H2O+ with H, He and electrons are needed to test these interpretations.