Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220.

We report on Herschel/PACS observations of absorption lines of OH+, H2O+ and H3O+ in NGC 4418 and Arp 220. Excited lines of OH+ and H2O+ with Elower of at least 285 and ~200 K, respectively, are detected in both sources, indicating radiative pumping and location in the high radiation density environment of the nuclear regions. Abundance ratios OH+/H2O+ of 1-2.5 are estimated in the nuclei of both sources. The inferred OH+ column and abundance relative to H nuclei are (0.5-1) × 1016 cm-2 and ~2 × 10-8, respectively. Additionally, in Arp 220, an extended low excitation component around the nuclear region is found to have OH+/H2O+ ~ 5-10. H3O+ is detected in both sources with N(H3O+) ~ (0.5-2) × 1016 cm-2, and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ~500 K, indicative of formation pumping and/or hot gas. Simple chemical models favor an ionization sequence dominated by H+ → O+ → OH+ → H2O+ → H3O+, and we also argue that the H+ production is most likely dominated by X-ray/cosmic ray ionization. The full set of observations and models leads us to propose that the molecular ions arise in a relatively low density (≳104 cm-3) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ς > 10-13 s-1, a lower limit that is several × 102 times the highest current rate estimates for Galactic regions. In Arp 220, our lower limit for ς is compatible with estimates for the cosmic ray energy density inferred previously from the supernova rate and synchrotron radio emission, and also with the expected ionization rate produced by X-rays. In NGC 4418, we argue that X-ray ionization due to an active galactic nucleus is responsible for the molecular ion production. [ABSTRACT FROM AUTHOR]