Detection of deuterated molecules, but not of lithium hydride, in the z = 0.89 absorber 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 LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 75014 Paris, France
⁴ Institut de Radioastronomie Millimétrique, 300, Rue de la Piscine, 38406 St Martin d’Hères, France
⁵ Max-Planck-Institut für Radioastonomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Astron. Dept., King Abdulaziz University, PO Box 80203, Jeddah 21589, Saudi Arabia
⁷ Observatoire de Paris, LERMA, College de France, CNRS, PSL Univ., Sorbonne Univ., 75014 Paris, France
⁸ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁹ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
¹⁰ Observatori Astronòmic, Universitat de València, Parc Científic, C. Catedràtico José Beltrán 2, 46980 Paterna, València, Spain
¹¹ Departament d’Astronomia i Astrofísica, Universitat de València, C. Dr. Moliner 50, 46100 Burjassot, València, Spain

Received: 31 January 2020
Accepted: 18 March 2020

Abstract

Deuterium and lithium are light elements of high cosmological and astrophysical importance. In this work we report the first detection of deuterated molecules and a search for lithium hydride, ⁷LiH, at redshift z = 0.89 in the spiral galaxy intercepting the line of sight to the quasar PKS 1830−211. We used ALMA to observe several submillimeter lines of ND, NH₂D, and HDO, and their related isotopomers NH₂, NH₃, and H₂¹⁸O, in absorption against the southwest image of the quasar, allowing us to derive XD/XH abundance ratios. The absorption spectra mainly consist of two distinct narrow velocity components for which we find remarkable differences. One velocity component shows XD/XH abundances that is about 10 times larger than the primordial elemental D/H ratio, and no variability of the absorption profile during the time span of our observations. In contrast, the other component shows a stronger deuterium fractionation. Compared to the first component, this second component has XD/XH abundances that are 100 times larger than the primordial D/H ratio, a deepening of the absorption by a factor of two within a few months, and a rich chemical composition, with relative enhancements of N₂H⁺, CH₃OH, SO₂ and complex organic molecules. We therefore speculate that this component is associated with the analog of a Galactic dark cloud, while the first component is likely more diffuse. Our search for the ⁷LiH (1–0) line was unsuccessful and we derive an upper limit ⁷LiH/H₂ = 4 × 10⁻¹³ (3σ) in the z = 0.89 absorber toward PKS 1830−211. Besides, with ALMA archival data, we could not confirm the previous tentative detections of this line in the z = 0.68 absorber toward B 0218+357; we derive an upper limit ⁷LiH/H₂ = 5 × 10⁻¹¹ (3σ), although this is less constraining than our limit toward PKS 1830−211. We conclude that, as in the Milky Way, only a tiny fraction of lithium nuclei are possibly bound in LiH in these absorbers at intermediate redshift.