Excited OH^+, H_2O^+, and H_3O^+ in NGC 4418 and Arp 220

Abstract

We report on Herschel/PACS observations of absorption lines of OH^+, H_2O^+ and H_3O^+ in NGC 4418 and Arp 220. Excited lines of OH^+ and H_2O^+ with E_(lower) 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^+/H_2O^+ 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) × 10^(16) 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^+/H^2O^+ ~ 5−10. H_3O^+ is detected in both sources with N(H_3O^+) ~ (0.5−2) × 10^(16) 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^+ → H_2O^+ → H_3O^+, 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 (≳10^4 cm^(-3)) interclump medium, in which case the ionization rate per H nucleus (including secondary ionizations) is ζ > 10^(-13) s^(-1), a lower limit that is several × 10^2 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.

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