H I Kinematics and the E(B-V)/N(H I) ratio

TL;DR

This study examines the gas-to-dust ratio at high Galactic latitudes using H I emission, revealing contributions from extragalactic material with different properties and confirming the typical Galactic ratio for high-latitude gas.

Contribution

It isolates and characterizes the kinematic components of H I gas, clarifying their impact on the gas-to-dust ratio measurements at high latitudes.

Findings

Extragalactic H I contributes to high-latitude measurements with lower reddening per H I.

The typical Galactic gas-to-dust ratio is confirmed as 8.3×10^{21} cm^{-2} mag^{-1}.

Kinematically isolated gas shows five times less reddening per H I than the average high-latitude gas.

Abstract

The $\lambda 21$cm H I emission that is used to trace the gas to dust ratio at high Galactic latitudes has contributions from material beyond the Milky Way disk, with uncertain and likely sub-Solar metallicity and dust content. These contributions can be isolated kinematically and their presence is clear for sightlines with small mean reddening $<$E(B-V)$>$ $\la$ 0.03 mag, which have mean ratios $<$N(H I)$>$/$<$E(B-V)$>$ that are 20-50\% above the high latitude Galactic average $<$N(H I)$>/<$E(B-V)$>=8.3\times10^{21}$cm$^{-2}$mag$^{-1}$. By mapping N(H I) and E(B-V) across H I High Velocity Cloud complexes and the Magellanic Clouds we show that the reddening of this kinematically-isolated gas is on average five times smaller per H I than the high latitude average. However, the aggregate contribution of this gas is small and $<$N(H I)$>/<$E(B-V)$>=8.3\times10^{21}$cm$^{-2}$mag$^{-1}$ is the appropriate value for Galactic gas seen at high latitude using the H I and reddening measures employed here and in our previous work.