ArH+ and H2O+ absorption towards luminous galaxies

TL;DR

This study detects ArH+ and H2O+ in nearby luminous galaxies to evaluate their effectiveness as tracers of atomic interstellar medium and cosmic-ray ionization rates, expanding extragalactic observations beyond the Milky Way.

Contribution

First detection of ArH+ in two nearby galaxies and simultaneous observation of H2O+ lines, extending the use of these molecules as tracers of atomic gas in extragalactic environments.

Findings

ArH+ detected in NGC 253 and NGC 4945, not in Arp 220.

Line profiles of ArH+ and H2O+ compared with other atomic and molecular tracers.

Cosmic-ray ionization rates inferred from chemical modeling.

Abstract

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 red shift 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. 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. 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 NKaKc=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. By further 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.