===== 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\\
**[[https://doi.org/10.1051/0004-6361/201527254|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.\\
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We know from [[molecules:ism:arh-plus|Galactic observations]] that argonium is an excellent tracer of the almost purely atomic and very diffuse interstellar medium.\\
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A. M. Jacob, K. M. Menten, F. Wyrowski, B. Winkel, D. A. Neufeld and B. S. Koribalski
reported on\\
**[[https://doi.org/10.1051/0004-6361/202142544|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.\\
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Contributor(s): H. S. P. Müller; 10, 2015; 04, 2022
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