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 N _H. 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 τ ₃₅₃, reddening E(B - V), and the total proton column density N _H in the range (1-30) × 10²⁰ cm^-2, along sightlines with no molecular gas detections in emission. We derive an N _H/E(B - V) ratio of (9.4 ± 1.6) × 10²¹ cm^-2 mag^-1 for purely atomic sightlines at , which is 60% higher than the canonical value of Bohlin et al. We report a ∼40% increase in opacity σ ₃₅₃ = τ ₃₅₃/N _H, when moving from the low column density (N _H < 5 × 10²⁰ cm^-2) to the moderate column density (N _H > 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 σ ₃₅₃ of the order of a further ∼20%. We estimate molecular hydrogen column densities from our derived linear relations, and hence derive the OH/H₂ abundance ratio of X _OH ∼ 1 × 10^-7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with in the range ∼ (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.