Argonium, ArH+, in Extragalactic Sources

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 38Ar+ 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 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.

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