Detection of CH⁺, SH⁺, and their ¹³C- and ³⁴S-isotopologues toward PKS 1830−211

¹ Department of Space, Earth and Environment, 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 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
⁴ Observatoire de Paris, LERMA, Collège de France, CNRS, PSL Univ., UPMC, Sorbonne Univ., 75014 Paris, France
⁵ School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, 6140 Wellington, New Zealand
⁶ Institut de Radioastronomie Millimétrique, 300 rue de la piscine, 38406 St Martin d’ Hères, France
⁷ Max-Planck-Institut für Radioastonomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Astron. Dept., King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁹ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
¹⁰ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, 7630355 Santiago, Chile
¹¹ Institut d’Astrophysique Spatiale, Bât. 121, Université Paris-Sud, 91405 Orsay Cedex, France
¹² Department of Physics, Catholic University of America, 620 Michigan Ave NE, Washington DC 20064, USA
¹³ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany

Received: 19 June 2017
Accepted: 21 July 2017

Abstract

The z = 0.89 molecular absorber toward PKS 1830−211 provides us with the opportunity to probe the chemical and physical properties of the interstellar medium in the disk of a galaxy at a look-back time of half the present age of the Universe. Recent ALMA observations of hydrides have unveiled the multi-phase composition of this source’s interstellar medium along two absorbing sightlines. Here, we report ALMA observations of CH⁺ and SH⁺, and of their ¹³C- and ³⁴S-isotopologues, as potential tracers of energetic processes in the interstellar medium. CH⁺ and ¹³CH⁺ are detected toward both images of PKS 1830−211, CH⁺ showing the deepest and broadest absorption among all species observed so far. The [CH⁺]/[¹³CH⁺] abundance ratio is ~100 in the south-west line of sight. This value is larger than any previous [¹²CX]/[¹³CX] ratios determined from other species toward this source and suggests either that the latter might be affected by fractionation or that CH⁺ might be tracing a different gas component. Toward the north-east image, we find an even larger value of [CH⁺]/[¹³CH⁺], 146 ± 43, although with a large uncertainty. This sightline intercepts the absorber at a larger galactocentric radius than the southwestern one, where material might be less processed in stellar nucleosynthesis. In contrast to CH⁺ and its ¹³C isotopologue, SH⁺ and ³⁴SH⁺ are only detected on the south-west sightline. These are the first detections of extragalactic SH⁺ and interstellar ³⁴SH⁺. The spectroscopic parameters of SH⁺ are reevaluated and improved rest frequencies of ³⁴SH⁺ are obtained. The [CH⁺]/[SH⁺] column density ratios show a large difference between the two lines of sight: ~25 and >600 toward the SW and NE image, respectively. We are not able to shed light on the formation process of CH⁺ and SH⁺ with these data, but the differences between the two sightlines toward PKS 1830−211 suggest that their absorptions arise from gas with a molecular fraction of ≳10%, with SH⁺ tracing significantly higher molecular fractions than CH⁺.