2023MNRAS.523.4593R

We investigate the correlation between OH and H₂ column densities in diffuse Galactic clouds, in order to identify potential molecular tracers of interstellar H₂. For this, we analyse near-UV spectra extracted from the ESO/VLT archives towards seventeen sightlines (five of them new) with known N (H₂), along with nine sightlines with no H₂ information. N (OH) shows only marginal correlation with N (H₂) (10²⁰ to 2 × 10²¹ cm^–2), at the 95 per cent confidence level. We use orthogonal distance regression analysis to obtain N (OH) /N (H₂) = (1.32 ± 0.15) × 10^–7, which is ∼33 per cent higher than the previous estimates based on near-UV data. We also obtain N (CH) /N (H₂) = (3.83 ± 0.23) × 10^–8 and a significant correlation between N (OH) and N (CH), with N (OH) = (2.61 ± 0.19) × N (CH), both of which are consistent with previous results. Comparison with predictions of numerical models indicate that OH absorption arises from diffuse gas (n_H ∼ 50 cm^–3) illuminated by radiation fields ∼0.5-5 G₀, while CH is associated with higher density of 500 cm^–3. We posit that the apparent dichotomy in the properties of the diffuse clouds giving rise to OH and CH absorption could be due to either: (a) the presence of multiple spectroscopically unresolved clouds along the line-of-sight, or, (b) density gradients along the line-of-sight within a single cloud.