Dust–Gas Scaling Relations and OH Abundance in the Galactic ISM (original) (raw)

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) × 10 20 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)×10 21 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 σ 353 =τ 353 /N H , when moving from the low column density (N H <5 × 10 20 cm −2) to the moderate column density (N H >5 × 10 20 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 N H 2 from our derived linear relations, and hence derive the OH/H 2 abundance ratio of X OH ∼1×10 −7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with N H 2 in the range N H 2 ∼(0.1−10)×10 21 cm −2. This suggests that OH may be used as a reliable proxy for H 2 in this range, which includes sightlines with both CO-dark and CO-bright gas.