2013A&A...550A..25G

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 E_lower of at least 285 and ∼200K, 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)x10¹⁶cm^–2 and ∼2x10^–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)x10¹⁶cm^–2, and in Arp 220 the pure inversion, metastable lines indicate a high rotational temperature of ∼500K, 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 (>10⁴cm^–3) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ζ>10^–13/s, a lower limit that is several x10² times the highest current rate estimates for Galactic regions. In Arp 220, our lower limit for ζ 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.