2018ApJ...862...49N

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 τ₃₅₃, reddening E(B - V), and the total proton column density N_H in the range (1-30) x 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) x 10²¹ cm^–2 mag^–1 for purely atomic sightlines at | b| > 5^○, which is 60% higher than the canonical value of Bohlin et al. We report a ∼40% increase in opacity σ₃₅₃ = τ₃₅₃/N_H, when moving from the low column density (N_H < 5 x 10²⁰ cm^–2) to the moderate column density (N_H > 5 x 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 N_H2_ from our derived linear relations, and hence derive the OH/H₂ abundance ratio of X_OH ∼ 1 x 10^–7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with N_H2_ in the range N_H2_ ∼ (0.1-10) x 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.