OH⁺ and H₂O⁺ absorption toward PKS 1830–211^⋆

¹ Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
e-mail: mullers@chalmers.se
² I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
³ Institut d’Astrophysique Spatiale, Bât. 121, Université Paris-Sud, 91405 Orsay Cedex, France
⁴ Observatoire de Paris, LERMA, CNRS, 61 avenue de l’Observatoire, 75014 Paris, France
⁵ School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington 6140, New Zealand
⁶ LERMA/LRA, École Normale Supérieure, Observatoire de Paris, CNRS UMR 8112, PSL Research University, Sorbonne Universités, UPMC Université Paris, 24 rue Lhomond, 75005 Paris, France
⁷ Institut de Radioastronomie Millimétrique, 300 rue de la piscine, 38406 St-Martin-d’Hères, France
⁸ Max-Planck-Institut für Radioastonomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁹ Astron. Dept., King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia
¹⁰ European Southern Observatory, Alonso de Córdova 3107, Vitacura Casilla 763 0355, Santiago, Chile
¹¹ Department of Physics, Catholic University of America, 620 Michigan Ave NE, Washington DC 20064, USA
¹² European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany

Received: 7 June 2016
Accepted: 26 August 2016

Abstract

We report the detection of OH⁺ and H₂O⁺ in the z = 0.89 absorber toward the lensed quasar PKS 1830–211. The abundance ratio of OH⁺ and H₂O⁺ is used to quantify the molecular hydrogen fraction (f_H2) and the cosmic-ray ionization rate of atomic hydrogen (ζ_H) along two lines of sight, located at ~ 2 kpc and ~ 4 kpc to either side of the absorber’s center. The molecular fraction decreases outward, from ~ 0.04 to ~ 0.02, comparable to values measured in the Milky Way at similar galactocentric radii. For ζ_H , we find values of ~ 2 × 10^-14 s^-1 and ~ 3 × 10^-15 s^-1, respectively, which are slightly higher than in the Milky Way at comparable galactocentric radii, possibly due to a higher average star formation activity in the z = 0.89 absorber. The ALMA observations of OH⁺, H₂O⁺, and other hydrides toward PKS 1830–211reveal the multi-phase composition of the absorbing gas. Taking the column density ratios along the southwest and northeast lines of sight as a proxy of molecular fraction, we classify the species ArH⁺, OH⁺, H₂Cl⁺, H₂O⁺, CH, and HF as tracing gases increasingly more molecular. Incidentally, our data allow us to improve the accuracy of H₂O⁺ rest frequencies and thus refine the spectroscopic parameters.