Astrophys. J., 862, 49-49 (2018/July-3)
Dust-gas scaling relations and OH abundance in the galactic ISM.
NGUYEN H., DAWSON J.R., MIVILLE-DESCHENES M.-A., TANG N., LI D., HEILES C., MURRAY C.E., STANIMIROVIC S., GIBSON S.J., McCLURE-GRIFFITHS N.M., TROLAND T., BRONFMAN L. and FINGER R.
Abstract (from CDS):
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, 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 τ353, reddening E(B - V), and the total proton column density NH in the range (1-30) x 1020 cm–2, along sightlines with no molecular gas detections in emission. We derive an NH/E(B - V) ratio of (9.4 ± 1.6) x 1021 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 σ353 = τ353/NH, when moving from the low column density (NH < 5 x 1020 cm–2) to the moderate column density (NH > 5 x 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 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 x 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) x 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.
© 2018. The American Astronomical Society. All rights reserved.
dust, extinction - ISM: clouds - ISM: molecules
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