Submillimeter absorption from SH+, a new widespread interstellar radical, 13CH+ and HCl

TL;DR

This study used the APEX telescope to detect and analyze submillimeter absorption lines of SH+, HCl isotopologues, and 13CH+ in the interstellar medium, revealing the widespread presence of SH+ in the Galactic center and spiral arms.

Contribution

First detection of SH+ hyperfine lines at 683 GHz in the interstellar medium, expanding understanding of its distribution and properties in diffuse Galactic gas.

Findings

SH+ is a widespread component of the diffuse interstellar medium.

Detected hyperfine structure lines of SH+ near 683 GHz.

Absorption features from HCl isotopologues and 13CH+ were also observed.

Abstract

We have used the Atacama Pathfinder Experiment 12 m telescope (APEX) to carry out an absorption study of submillimeter wavelength rotational ground-state lines of H35Cl, H37Cl, 13CH+, and, for the first time, of the SH+ radical (sulfoniumylidene or sulfanylium). We detected the quartet of ground-state hyperfine structure lines of SH+ near 683 GHz with the CHAMP+ array receiver against the strong continuum source Sagittarius B2, which is located close to the center of our Galaxy. In addition to absorption from various kinematic components of Galactic center gas, we also see absorption at the radial velocities belonging to intervening spiral arms. This demonstrates that SH+ is a ubiquitous component of the diffuse interstellar medium. We do not find clear evidence for other SH+ lines we searched for, which is partially due to blending with lines from other molecules. In addition to SH+, we observed absorption from H35Cl, H37Cl, and 13CH+. The observed submillimeter absorption is compared in detail with absorption in 3 mm transitions of H13CO+ and c-C3H2 and the CO J = 1-0 and 3-2 transitions.