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

ABSTRACT Hydroxyl ($\rm OH$) is known to form efficiently in cold gas (T ∼ 100 K) along with the molecule $\rm H_2$ and can be used as an efficient tracer of diffuse molecular interstellar medium (ISM). Using a simple formalism describing H\, I/H_2$ transition reduced network major chemical reactions, we present semi-analytical prescription estimate abundances O-bearing molecules ISM. We show that predictions based on our are good agreement estimates obtained using meudon pdr code which utilizes full reaction network. investigate dependence relative OH/H\, I$ OH/H_2$ variations physical conditions i.e. metallicity, number density (n), cosmic ray ionization rate (ζ), strength UV field (χ) medium. find observed Galactic ISM reproduced by models n 50 cm−3, χ 1 (Mathis field), ζ 3 × 10−17 s−1, variation about dex allowed around these values. constrained column distribution function at z 3, OH$ discuss future prospects upcoming large radio absorption line surveys.