H. S. P. Müller, S. Muller, P. Schilke, E. A. Bergin, J. H. Black, M. Gérin, D. C. Lis, D. A. Neufeld, and S. Suri,
reported on the
Detection of Extragalactic Argonium, ArH⁺, toward PKS 1830–211
Astron. Astrophys. 582, (2015) Art. No. L4.
The ground state rotational transitions of ³⁶ArH⁺ and ³⁸ArH⁺ were observed in absorption in the z = 0.89 foreground galaxy toward the quasar PKS 1830–211. The Atacama Large Millimeter Array (ALMA) was used in its Early Science Cycle 2 phase. The transitions were redshifted from about 617 GHz to about 327 GHz. The interstellar abundance of ⁴⁰Ar is almost negligible, and ⁴⁰ArH⁺ was accordingly not detected.

The ³⁶Ar/³⁸Ar isotopic ratios were 3.46 ± 0.16 and 4.53 ± 0.33 toward the stronger SW and the weaker NE image, respectively. The values differ from 5.50 ± 0.01 in the Solar neighborhood, suggesting that high-mass supernovae played a greater role in creating elements heavier than helium (metals for astronomers) in the early Universe compared to today, as one would expect; the look-back time at z = 0.89 is about 7.5 billion years.

We know from Galactic observations that argonium is an excellent tracer of the almost purely atomic and very diffuse interstellar medium.

A. M. Jacob, K. M. Menten, F. Wyrowski, B. Winkel, D. A. Neufeld and B. S. Koribalski<br> reported on
ArH⁺ and H₂O⁺ Absorption towards Luminous Galaxies
Astron. Astrophys. 659, (2022) Art. No. 152.
The J = 1 − 0 transition of ³⁶ArH⁺ was detected with the APEX 12 m dish toward the nearby galaxies NGC 253, NGC 4945, and Arp 220. The molecular fraction of the gas traced by ArH⁺ was estimated to be ~10⁻³ and that it resides in gas volumes with low electon densities.

Contributor(s): H. S. P. Müller; 10, 2015; 04, 2022