Detection of deuterated molecules, but not of lithium hydride, in the z = 0.89 absorber toward PKS 1830-211

doi:10.1051/0004-6361/202037628

Detection of deuterated molecules, but not of lithium hydride, in the z = 0.89 absorber toward PKS 1830-211

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.