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