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.

2018 Census of Interstellar, Circumstellar, Extragalactic, Protoplanetary Disk, and Exoplanetary Molecules

“…The (relatively) low number of gas-phase species so far detected in disks can likely be attributed to a combination of factors. Churchwell et al (1977) [4] Fuente et al (2006) [5] García-Burillo et al (2002) [6] Gardner & Whiteoak (1974) [7] Geballe et al (2006) [8] González-Alfonso et al (2004) [9] González-Alfonso et al (2013) [10] Heikkilä et al (1999) [11] Henkel & Bally (1985) [12] Henkel et al (1987) [13] Henkel et al (1988) [14] Johansson (1991) [15] Magain & Gillet (1987) [16] Martin & Ho (1979) [17] Martín et al (2003) [18] Martín et al (2006) [19] Matsuura et al (2002) [20] Mauersberger et al (1990) [21] [22] Mauersberger et al (1995) [23] [24] Monje et al (2011) [25] Muller et al (2011) [26] Muller et al (2013) [27] Muller et al (2014a) [28] Muller et al (2014b) [29] Müller et al (2015) [30] Muller et al (2016) [31] Muller et al (2017) [32] Nguyen-Q- Rieu et al (1991) [33] Petuchowski & Bennett (1992) [34] Qiu et al (2018) [35] Rangwala et al (2011) [36] Rickard et al (1975) [37] Rickard et al (1977) [38] Sage & Ziurys (1995) [39] Seaquist & Bell (1986) [40] Sewiło et al (2018) [41] Stark & Wolff (1979) [42] Thompson et al (...…”

The ortho-to-para ratio of H2Cl+: Quasi-classical trajectory calculations and new simulations in light of new observations

“…Molecules tracing gas with a low fraction of molecular hydrogen ( f H 2 ) preferentially, such as ArH + , OH + , H 2 O + , and H 2 Cl + , are found to have enhanced abundances along the NE line of sight relative to the SW one (Muller et al 2014a;Müller et al 2015;Muller et al 2016a). Among other evidence, this observation suggests that the absorbing gas in the NE line of sight is more diffuse.…”

Theoretical study of ArH+ dissociative recombination and electron-impact vibrational excitation

“…Moreover, features of 38 ArH + were subsequently found in Sg B2(M) as well and, consequently, Schilke et al suggested that argonium is ubiquitous in the ISM. More recently, Müller et al (2015) made extragalactic detections of the 36 Ar and 38 Ar isotopologues of argonium through absorption studies of a foreground galaxy E-mail: vincenzo.laporta@univ-lehavre.fr (VL) at z = 0.89 along two different lines of sight toward PKS 1830-211 within the band 7 of the ALMA interferometer, including the corresponding redshifted transitions.…”