Dust–Gas Scaling Relations and OH Abundance in the Galactic ISM

Abstract

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 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">∣</mml:mo> <mml:mi>b</mml:mi> <mml:mo stretchy="false">∣</mml:mo> <mml:mo>&gt;</mml:mo> <mml:mn>5</mml:mn> <mml:mo>°</mml:mo> </mml:math> , 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 &lt; 5 × 10 20 cm −2 ) to the moderate column density ( N H &gt; 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> in the range <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> ∼ (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.