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.
