  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.
