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Published October 2015 | Published + Submitted
Journal Article Open

Detection of extragalactic argonium, ArH+, toward PKS 1830−211

Abstract

Context. 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 10^(-4) to 10^(-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. Aims. 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. Methods. We obtained ALMA observations of a foreground galaxy at z = 0.89 in the direction of the lensed blazar PKS 1830−211. Results. Two isotopologs of argonium, ^(36)ArH+ and ^(38)ArH+, 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 ^(36)Ar/^(38)Ar is 3.46 ± 0.16 toward the SW image, i.e., significantly lower than the solar value of 5.5. Conclusions. 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.

Additional Information

© 2015 ESO. Article published by EDP Sciences. 26 August 2015; Accepted 22 September 2015; Published online 07 October 2015. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00296.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. The present investigations have been supported by the Deutsche Forschungsgemeinschaft (DFG) in the framework of the collaborative research grant SFB 956, projects A4 and C3. Support for this work was also provided by NASA through an award issued by JPL/Caltech. Our research benefited from NASA's Astrophysics Data System (ADS).

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Submitted - 1509.06917v1.pdf

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Additional details

Created:
August 20, 2023
Modified:
October 25, 2023