Volume 550, February 2013
|Number of page(s)||23|
|Published online||18 January 2013|
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
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
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 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.
Key words: line: formation / line: identification / ISM: abundances / galaxies: ISM / infrared: galaxies / molecular processes
© ESO, 2013
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