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

¹ Universidad de Alcalá de HenaresDepartamento de Física y Matemáticas, Campus Universitario, 28871 Alcalá de Henares, Madrid, Spain
e-mail: eduardo.gonzalez@uah.es
² Naval Research Laboratory, Remote Sensing Division, 4555 Overlook Ave SW, Washington, DC 20375, USA
³ Max-Planck-Institute for Extraterrestrial Physics (MPE), Giessenbachstraße 1, 85748 Garching, Germany
⁴ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
⁵ Department of Astronomy, University of Maryland, College Park, MD 20742, USA
⁶ Astroparticle Physics Laboratory, NASA Goddard Space Flight Center, Code 661, Greenbelt, MD 20771 USA
⁷ Sackler School of Physics & Astronomy, Tel Aviv University, 69978 Ramat Aviv, Israel
⁸ California Institute of Technology, Mail Code 301-17, 1200 E. California Blvd., Pasadena, CA 91125, USA
⁹ University of Oxford, Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK

Received: 29 September 2012
Accepted: 19 November 2012

Abstract

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 ~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 ζ > 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 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.