A new, large-scale map of interstellar reddening derived from HI emission

We present a new map of interstellar reddening, covering the 39\% of the sky with low {\rm HI} column densities (N_{rm HI} < 4times10^{20},rm cm^{-2} or E(B-V)approx 45rm, mmag) 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_{rm HI}/E(B-V) = 8.8times10^{21}, rm, cm^{2}, mag^{-1} for gas with |v_{rm LSR}| < 90,rm km, 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 simeq 5,rm mmag. Further, the differences between our map and the SFD map are in excellent agreement with the low resolution (4circ{.}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 (http://dx.doi.org/10.7910/DVN/AFJNWJ).