Dust-gas scaling relations and oh abundance in the galactic ISM

Abstract

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 tau(353), reddening E(B - V), and the total proton column density N-H in the range (1-30) x 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) x 10(21) cm(-2) mag(-1) for purely atomic sightlines at |b| > 5 degrees, which is 60% higher than the canonical value of Bohlin et al. We report a similar to 40% increase in opacity sigma(353) = tau(353)/N-H, when moving from the low column density (N-H < 5 x 10(20) cm(-2)) to the moderate column density (N-H > 5 x 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 sigma(353) of the order of a further similar to 20%. We estimate molecular hydrogen column densities N-H2 from our derived linear relations, and hence derive the OH/H-2. abundance ratio of X-OH similar to 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 similar to (0.1-10) x 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.