Variations of Interstellar Gas-to-dust Ratios at High Galactic Latitudes

Abstract

Interstellar dust at high Galactic latitudes can influence astronomical foreground subtraction, produce diffuse scattered light, and soften the UV spectra of quasars. In a sample of 94 sight lines toward quasars at high latitude and low extinction, we evaluate the interstellar “gas-to-dust ratio” N H / E ( B − V ), using hydrogen column densities (H i and H 2 ) and far-IR (FIR) estimates of dust reddening. In the Galactic plane, this ratio is 6.0 ± 0.2 (in units of 10 21 cm −2 mag −1 ). On average, recent Planck estimates of E ( B − V ) in low-reddening sight lines are 12% higher than those from Schlafly & Finkbeiner, and N H I exhibits significant variations when measured at different radio telescopes. In a sample of 51 quasars with measurements of both H i and H 2 and 0.01 ≤ E ( B − V ) ≲ 0.1, we find mean ratios 10.3 ± 0.4 (gas at all velocities) and 9.2 ± 0.3 (low-velocity only) using Planck E ( B − V ) data. High-latitude H 2 fractions are generally small (2%–3% on average), although nine of 39 sight lines at ∣ b ∣ ≥ 40° have f H2 of 1%–17%. Because FIR-inferred E ( B − V ) is sensitive to modeled dust temperature T d and emissivity index β , gas-to-dust ratios have large, asymmetric errors at low E ( B − V ). The ratios are elevated in sight lines with high-velocity clouds, which contribute N H but little reddening. In Complex C, the ratio decreases by 40% when high-velocity gas is excluded. Decreases in dust content are expected in low-metallicity gas above the Galactic plane, resulting from grain destruction in shocks, settling to the disk, and thermal sputtering in hot halo gas.