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 $N_{\mathrm{H}}$. By comparing $\text{Planck}$ 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 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 $N_{\mathrm{H}}$ in the range (1$-$30)$\times$10${}^{20}$cm${}^{-2}$, along sightlines with no molecular gas detections in emission. We derive an $N_{\mathrm{H}}$/$E(B-V)$ ratio of (9.4$\pm$1.6)$\times$10${}^{21}$cm${}^{-2}$mag${}^{-1}$ for purely atomic sightlines at $|b|$$>$5${}^{\circ }$, which is 60$\%$ higher than the canonical value of Bohlin et al. (1978). We report a $\sim$40$\%$ increase in opacity ${\sigma }_{353}$=${\tau }_{353}$/$N_{\mathrm{H}}$, when moving from the low column density ($N_{\mathrm{H}}$$<$5$\times$10${}^{20}$cm${}^{-2}$) to moderate column density ($N_{\mathrm{H}}$$>$5$\times$10${}^{20}$cm${}^{-2}$) regime, and suggest that this rise is due to the evolution of dust grains in the atomic ISM. Failure to account for HI opacity can cause an additional apparent rise in ${\sigma }_{353}$, of the order of a further $\sim$20$\%$. We estimate molecular hydrogen column densities $N_{{\mathrm{H}}_2}$ from our derived linear relations, and hence derive the OH/H${}_2$ abundance ratio of $X_{\mathrm{OH}}$$\sim$1$\times$10${}^{-7}$ for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with $N_{{\mathrm{H}}_2}$ in the range $N_{{\mathrm{H}}_2}$$\sim$(0.1$-$10)$\times$10${}^{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.

The revised manuscript is accepted for publication in The Astrophysical Journal