Detection of extragalactic argonium, ArH⁺, toward PKS 1830−211^⋆

¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany
e-mail: hspm@ph1.uni-koeln.de
² Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
³ Department of Astronomy, The University of Michigan, 500 Church Street, Ann Arbor, MI 48109-1042, USA
⁴ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, École normale supérieure, 75005 Paris, France
⁵ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, 75014 Paris, France
⁶ Cahill Center for Astronomy and Astrophysics 301-17, California Institute of Technology, Pasadena, CA 91125, USA
⁷ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

Received: 26 August 2015
Accepted: 22 September 2015

Abstract

Context. Argonium has recently been detected as a ubiquitous molecule in our Galaxy. Model calculations indicate that its abundance peaks at molecular fractions in the range of 10^-4 to 10^-3 and that the observed column densities require high values of the cosmic ray ionization rate. Therefore, this molecular cation may serve as an excellent tracer of the very diffuse interstellar medium (ISM), as well as an indicator of the cosmic ray ionization rate.

Aims. We attempted to detect ArH⁺ in extragalactic sources to evaluate its diagnostic power as a tracer of the almost purely atomic ISM in distant galaxies.

Methods. We obtained ALMA observations of a foreground galaxy at z = 0.89 in the direction of the lensed blazar PKS 1830−211.

Results. Two isotopologs of argonium, ³⁶ArH⁺ and ³⁸ArH⁺, were detected in absorption along two different lines of sight toward PKS 1830−211, known as the SW and NE images of the background blazar. The argonium absorption is clearly enhanced on the more diffuse line of sight (NE) compared to other molecular species. The isotopic ratio ³⁶Ar/³⁸Ar is 3.46 ± 0.16 toward the SW image, i.e., significantly lower than the solar value of 5.5.

Conclusions. Our results demonstrate the suitability of argonium as a tracer of the almost purely atomic, diffuse ISM in high-redshift sources. The evolution of the isotopic ratio with redshift may help to constrain nucleosynthetic scenarios in the early Universe.