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 (Green et al. 2018), with accurate (opacity-corrected) Hi column densities and newlypublished 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)×1020cm−2 , along sightlines with no molecular gas detections in emission. We derive an NH/E(B−V ) ratio of (9.4±1.6)×1021cm−2mag−1 for purely atomic sightlines at |b|>5 ◦ , which is 60% higher than the canonical value of Bohlin et al. (1978). We report a ∼40% increase in opacity σ353=τ353/NH, when moving from the low column density (NH<5×1020cm−2 ) to moderate column density (NH>5×1020cm−2 ) regime, and suggest that this rise is due to the evolution of dust grains in the atomic ISM. Failure to account for Hi 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)×1021cm−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