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The chemistry of interstellar HnO+ beyond the Galaxy

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.