A New, Large-scale Map of Interstellar Reddening Derived from H I Emission

We present a new map of interstellar reddening, covering the 39% of the sky with low HI column densities (N_(HI) < 4 × 10^(20) cm^(−2) or E(B−V) ≈ 45mmag) at 16.′1 resolution, based on all-sky observations of Galactic HI emission by the HI4PI Survey. In this low column density regime, we derive a characteristic value of N_(HI)/E(B−V) = 8.8 × 10^(21) cm^2 mag^(−1) for gas with |v_(LSR)| < 90km s^(−1) and find no significant reddening associated with gas at higher velocities. We compare our HI-based reddening map with the Schlegel, Finkbeiner, and Davis (1998, SFD) reddening map and find them consistent to within a scatter of ≃5mmag. Further, the differences between our map and the SFD map are in excellent agreement with the low resolution (4.∘5) corrections to the SFD map derived by Peek and Graves (2010) based on observed reddening toward passive galaxies. We therefore argue that our HI-based map provides the most accurate interstellar reddening estimates in the low column density regime to date. Our reddening map is made publicly available (this http URL)