Detection of extragalactic argonium, ArH$^+$, toward PKS~1830$-$211

TL;DR

This study reports the first detection of argonium (ArH+) in an extragalactic source, demonstrating its potential as a tracer of diffuse atomic interstellar medium and providing insights into cosmic ray ionization and nucleosynthesis at high redshift.

Contribution

First extragalactic detection of ArH+ using ALMA, establishing its role as a tracer of diffuse atomic ISM in distant galaxies and analyzing isotopic ratios for nucleosynthesis insights.

Findings

ArH+ detected in absorption toward PKS 1830-211

Argonium absorption is stronger in more diffuse regions

Isotopic ratio 36Ar/38Ar is lower than solar value

Abstract

Argonium has recently been detected as a ubiquitous molecule in our Galaxy. Model calculations indicate that its abundance peaks at molecular fractions in the range of 1E-4 to 1E-3 and that the observed column densities require high values of the cosmic ray ionization rate. Therefore, this molecular cation may serve as an excellent tracer of the very diffuse interstellar medium (ISM), as well as an indicator of the cosmic ray ionization rate. We attempted to detect ArH+ in extragalactic sources to evaluate its diagnostic power as a tracer of the almost purely atomic ISM in distant galaxies. We obtained ALMA observations of a foreground galaxy at z = 0.89 in the direction of the lensed blazar PKS 1830-211. Two isotopologs of argonium, 36ArH+ and 38ArH+, were detected in absorption along two different lines of sight toward PKS 1830-211, known as the SW and NE images of the background blazar. The argonium absorption is clearly enhanced on the more diffuse line of sight (NE) compared to other molecular species. The isotopic ratio 36Ar/38Ar is 3.46 +- 0.16 toward the SW image, i.e., significantly lower than the solar value of 5.5. Our results demonstrate the suitability of argonium as a tracer of the almost purely atomic, diffuse ISM in high-redshift sources. The evolution of the isotopic ratio with redshift may help to constrain nucleosynthetic scenarios in the early Universe.