Water in the interstellar media of galaxies

TL;DR

This paper reviews recent observations of water in galactic interstellar media, highlighting how water chemistry varies with radiation environment and ionization rates across different galactic regions.

Contribution

It provides new insights into water chemistry driven by photodissociation and desorption in PDRs and reveals differences in ionization rates in galactic nuclei versus disks.

Findings

Water chemistry in PDRs is driven by photodissociation and desorption.

Ionization rates are lower in galactic disks compared to nuclei.

Water ionization rates decrease by at least 10 times from nuclei to disks.

Abstract

This paper reviews recent observations of water in Galactic interstellar clouds and nearby galactic nuclei. Two results are highlighted: (1) Multi-line H$_2$O mapping of the Orion Bar shows that the water chemistry in PDRs is driven by photodissociation and -desorption, unlike in star-forming regions. (2) High-resolution spectra of H$_2$O and its ions toward 5 starburst / AGN systems reveal low ionization rates, unlike as found from higher-excitation lines. We conclude that the chemistry of water strongly depends on radiation environment, and that the ionization rates of interstellar clouds decrease by at least 10 between galactic nuclei and disks.