ArH⁺ and H₂O⁺ absorption towards luminous galaxies

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: ajacob@mpifr-bonn.mpg.de
² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD s 21218, USA
³ Australia Telescope National Facility, CSIRO Astronomy and Space Science, PO Box 76, Epping, NSW 1710, Australia

Received: 28 October 2021
Accepted: 27 December 2021

Abstract

Context. Along several sight lines within the Milky Way ArH⁺ has been ubiquitously detected with only one detection in extragalactic environments, namely along two sight lines in the redshift z = 0.89 absorber towards the lensed blazar PKS 1830-211. Being formed in predominantly atomic gas by reactions between Ar⁺, which were initially ionised by cosmic rays and molecular hydrogen, ArH⁺ has been shown to be an excellent tracer of atomic gas as well as the impinging cosmic-ray ionisation rates.

Aims. In this work, we attempt to extend the observations of ArH⁺ in extragalactic sources to examine its use as a tracer of the atomic interstellar medium (ISM) in these galaxies.

Methods. We report the detection of ArH⁺ towards two luminous nearby galaxies, NGC 253 and NGC 4945, and the non-detection towards Arp 220 observed using the SEPIA660 receiver on the APEX 12 m telescope. In addition, the two sidebands of this receiver allowed us to observe the N_KaKc = 1_1,0 − 1_0,1 transitions of another atomic gas tracer p-H₂O⁺ at 607.227 GHz with the ArH⁺ line, simultaneously. We modelled the optically thin spectra of both species and compared their observed line profiles with that of other well-known atomic gas tracers such as OH⁺ and o-H₂O⁺ and diffuse and dense molecular gas tracers HF and CO, respectively.

Results. Assuming that the observed absorption from the ArH⁺, OH⁺, and H₂O⁺ molecules are affected by the same flux of cosmic rays, we investigate the properties of the different cloud layers. Based on a steady-state analysis of the chemistry of these three species and using statistical equilibrium calculations, we estimate the molecular fraction traced by ArH⁺ to be ∼​10⁻³ and find that ArH⁺ resides in gas volumes with low electron densities. We further study the ortho-to-para ratio of H₂O⁺ and find that the derived ratios do not significantly deviate from the equilibrium value of three with spin temperatures greater than 15 and 24 K.