Detection of CH +, SH +, and their 13 C- and 34 S-isotopologues toward PKS 1830 - 211

Abstract

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+.