2024ApJ...961..204S

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 H₂) and far-IR (FIR) estimates of dust reddening. In the Galactic plane, this ratio is 6.0 ± 0.2 (in units of 10²¹ 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 H₂ 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 H₂ 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.