ArH+ and H2O+ absorption towards luminous galaxies

Context. Along several sight lines within the MilkyWay 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 similar to 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.