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

Artikel i vetenskaplig tidskrift, 2020

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, (LiH)-Li-7, 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 (H2O)-O-18 H 2 18 O , 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 (LiH)-Li-7 (1-0) line was unsuccessful and we derive an upper limit (LiH)-Li-7/H-2 = 4x10(-13) (3 sigma) 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 (LiH)-Li-7/H-2 = 5x10(-11) (3 sigma), 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.