Dust–Gas Scaling Relations and OH Abundance in the Galactic ISM

Observations of interstellar dust are often used as a proxy for total gas column density NH. By comparing Planck thermal dust data (Release 1.2) and new dust reddening maps from Pan-STARRS 1 and 2MASS, with accurate (opacity-corrected) H I column densities and newly published OH data from the Arecibo Millennium survey and 21-SPONGE, we confirm linear correlations between dust optical depth τ353, reddening E(B − V), and the total proton column density NH in the range (1–30) × 1020 cm−2, along sightlines with no molecular gas detections in emission. We derive an NH/E(B − V) ratio of (9.4 ± 1.6) × 1021 cm−2 mag−1 for purely atomic sightlines at |b| \u3e 5°, which is 60% higher than the canonical value of Bohlin et al. We report a ~40% increase in opacity σ353 = τ 353/NH, when moving from the low column density (NH \u3c 5 × 1020 cm−2) to the moderate column density (NH \u3e 5 × 1020 cm−2) regime, and suggest that this rise is due to the evolution of dust grains in the atomic interstellar medium. Failure to account for H I opacity can cause an additional apparent rise in σ353 of the order of a further ~20%. We estimate molecular hydrogen column densities NH2 from our derived linear relations, and hence derive the OH/H2 abundance ratio of XOH ~ 1 × 10−7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with NH2 in the range NH2 ~ (0.1−10) × 1021 cm−2. This suggests that OH may be used as a reliable proxy for H2 in this range, which includes sightlines with both CO-dark and CO-bright gas