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 1830211
Astron. Astrophys. 582, (2015) Art. No. L4.
The ground state rotational transitions of 36ArH+ and 38Ar+ were observed in absorption in the z = 0.89 foreground galaxy toward the quasar PKS 1830211. 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 36Ar/38Cl 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.