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 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" $N_{\mathrm{H}}/E(B-V)$, using hydrogen column densities (H I and H${}_2$) and far-infrared estimates of dust reddening. In the Galactic plane, this ratio is $6.0\pm 0.2$ (in units of $10^{21}{\mathrm{cm}}^{-2}{\mathrm{mag}}^{-1}$). On average, recent Planck estimates of $E(B-V)$ in low-reddening sight lines are 12% higher than those from Schlafly & Finkbeiner (2011), and $N_{\mathrm{HI}}$ 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 $0.01\le E(B-V)\lesssim 0.1$, we find mean ratios $10.3\pm 0.4$ (gas at all velocities) and $9.2\pm 0.3$ (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 $|b|\ge 40^{\circ }$ have $f_{H2}$ 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 $N_{\mathrm{H}}$ 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.

Minor changes, accepted to Astrophysical Journal, 37 pages, 7 tables, 8 figures