2017A&A...606A.109M

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⁺.