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

González-Alfonso, E. and Fischer, J. and Bruderer, S. and Müller, H. S. P. and Graciá-Carpio, J. and Sturm, E. and Lutz, D. and Poglitsch, A. and Feuchtgruber, H. and Veilleux, S. and Contursi, A. and Sternberg, A. and Hailey-Dunsheath, S. and Verma, A. and Christopher, N. and Davies, R. and Genzel, R. and Tacconi, L. (2013) Excited OH^+, H_2O^+, and H_3O^+ in NGC 4418 and Arp 220. Astronomy and Astrophysics, 550 . Art. No. A25. ISSN 0004-6361.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription
Fischer, J.0000-0001-6697-7808
Müller, H. S. P.0000-0002-0183-8927
Lutz, D.0000-0003-0291-9582
Veilleux, S.0000-0002-3158-6820
Hailey-Dunsheath, S.0000-0002-8504-7988
Genzel, R.0000-0002-2767-9653
Tacconi, L.0000-0002-1485-9401
Additional Information:© 2013 ESO. Received: 29 September 2012. Accepted: 19 November 2012. Published online 18 January 2013. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. We thank the referee, David A. Neufeld, for useful indications that improved the manuscript. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KU Leuven, CSL, IMEC (Belgium); CEA, LAM (France); MPIA (Germany); INAFIFSI/OAA/OAP/OAT, LENS, SISSA (Italy); IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI/INAF (Italy), and CICYT/MCYT (Spain). E.G.-A. is a Research Associate at the Harvard-Smithsonian Center for Astrophysics, and thanks the support by the Spanish Ministerio de Ciencia e Innovación under project AYA2010-21697-C05-01. Basic research in IR astronomy at NRL is funded by the US ONR; J.F. also acknowledges support from the NHSC. H.S.P.M. acknowledges support by the Bundesministerium für Bildung und Forschung (BMBF) through project FKZ 50OF0901 (ICC HIFI Herschel). S.V. thanks NASA for partial support of this research via Research Support Agreement RSA 1427277, support from a Senior NPP Award from NASA, and his host institution, the Goddard Space Flight Center. This research has made use of NASA’s Astrophysics Data System (ADS) and of GILDAS software (
Funding AgencyGrant Number
Spanish Ministerio de Ciencia e InnovaciόnAYA2010-21697-C05-01
Office of Naval Research (ONR)UNSPECIFIED
Bundesministerium für Bildung und Forschung (BMBF) (Germany)FKZ 50OF0901
NASA Research Support AgreementRSA 1427277
Subject Keywords:line: formation; line: identification; ISM: abundances; galaxies: ISM; infrared: galaxies; molecular processes
Record Number:CaltechAUTHORS:20130409-152633063
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Official Citation:Excited OH+, H2O+, and H3O+ in NGC 4418 and Arp 220 A25 E. González-Alfonso, J. Fischer, S. Bruderer, H. S. P. Müller, J. Graciá-Carpio, E. Sturm, D. Lutz, A. Poglitsch, H. Feuchtgruber, S. Veilleux, et al. (8 more) Published online: 18 January 2013 DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:37838
Deposited By: Ruth Sustaita
Deposited On:09 Apr 2013 22:56
Last Modified:09 Mar 2020 13:19

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