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

Interstellar dust at high Galactic latitudes can influence astronomical foreground subtraction, produce diffuse scattered light, and soften the ultraviolet 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" $N_{\rm H}/E(B-V)$, using hydrogen column densities (H I and H$_2$) and far-infrared estimates of dust reddening. In the Galactic plane, this ratio is $6.0\pm0.2$ (in units of $10^{21}~{\rm cm}^{-2}~{\rm mag}^{-1}$). On average, recent Planck estimates of $E(B-V)$ in low-reddening sight lines are 12% higher than those from Schlafly & Finkbeiner (2011), and $N_{\rm HI}$ 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 $0.01 \leq E(B-V) \lesssim 0.1$, we find mean ratios $10.3\pm0.4$ (gas at all velocities) and $9.2\pm0.3$ (low velocity only) using Planck $E(B-V)$ data. High-latitude H$_2$ fractions are generally small (2-3% on average), although 9 of 39 sight lines at $|b| \geq 40^{\circ}$ have $f_{\rm H2}$ of 1-17%. Because FIR-inferred $E(B-V)$ is sensitive to modeled dust temperature $T_d$ and emissivity index $\beta$, 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 $N_{\rm H}$ 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.