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

J. Michael Shull, Georgia V. Panopoulou

Published 2023-10-18Version 1

Interstellar dust at high Galactic latitudes can influence astronomical foreground subtraction, produce diffuse scattered light, and result in harder (de-reddened) ultraviolet spectra of quasars. In a sample of 94 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 $10^{21}~{\rm cm}^{-2}~{\rm mag}^{-1}$). In a sub-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). Including H$_2$ increases $N_{\rm H}$ by $\sim10$%, even at high latitude. 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. We also show that $E(B-V)$ is sensitive to far-infrared modeled dust temperature $T_d$ and emissivity index $\beta$. Consequently, 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.