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) × 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) × 1021 cm-2 mag-1 for purely atomic sightlines at , 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 × 1020 cm-2) to the moderate column density (N H > 5 × 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 σ 353 of the order of a further ∼20%. We estimate molecular hydrogen column densities from our derived linear relations, and hence derive the OH/H2 abundance ratio of X OH ∼ 1 × 10-7 for all molecular sightlines. Our results show no evidence of systematic trends in OH abundance with in the range ∼ (0.1-10) × 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.
|State||Published - Jul 20 2018|
Bibliographical noteFunding Information:
J.R.D. is the recipient of an Australian Research Council (ARC) DECRA Fellowship (project number DE170101086). D.L. thanks the supports from the National Key R&D Program of China (2017YFA0402600) and the CAS International Partnership Program (No.114A11KYSB20160008). N.M.-G. acknowledges the support of the ARC through Future Fellowship FT150100024. L.B. acknowledges the support from CONICYT grant PFB06. We are indebted to Professor Mark Wardle for providing us with valuable advice and support. We gratefully acknowledge discussions with Dr. Cormac Purcell and Anita Petzler. Finally, we thank the anonymous referee for comments and criticisms that allowed us to improve the paper.
© 2018. The American Astronomical Society. All rights reserved.
- ISM: clouds
- ISM: molecules
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science