OH in the diffuse interstellar medium: physical modelling and prospects with upcoming SKA precursor/pathfinder surveys

Hydroxyl (OH) is known to form efficiently in cold gas (T ∼ 100 K) along with the molecule H₂ and can be used as an efficient tracer of the diffuse molecular gas in the interstellar medium (ISM). Using a simple formalism describing the H\, I/H₂ transition and a reduced network of major chemical reactions, we present a semi-analytical prescription to estimate the abundances of O-bearing molecules in the diffuse ISM. We show that predictions based on our prescription are in good agreement with the estimates obtained using the meudon pdr code which utilizes the full reaction network. We investigate the dependence of the relative abundances of OH/H\, I and OH/H₂ on the variations of physical conditions i.e. the metallicity, number density (n), cosmic ray ionization rate (ζ), and strength of UV field (χ) in the medium. We find that the OH/H\, I abundances observed in the Galactic ISM can be reproduced by models with n ∼ 50 cm−3, χ ∼ 1 (Mathis field), and ζ ∼ 3 × 10−17 s−1, with a variation of about 1 dex allowed around these values. Using the constrained H₂ column density distribution function at z ∼ 3, we estimate the OH column density distribution function and discuss future prospects with the upcoming large radio absorption line surveys.