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 H I column densities (N_HI< 4× 10²⁰ cm^-2 or E(B-V)≈ 45 mmag) at 16.1\prime resolution, based on all-sky observations of Galactic H I emission by the HI4PI Survey. In this low-column-density regime, we derive a characteristic value of N_HI/E(B-V)=8.8 × 10²¹ cm² mag^-1 for gas with | v_LSR| < 90 km s^-1 and find no significant reddening associated with gas at higher velocities. We compare our H I-based reddening map with the Schlegel et al. (SFD) reddening map and find them consistent to within a scatter of ≃ 5 mmag. 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 based on observed reddening toward passive galaxies. We therefore argue that our H I-based map provides the most accurate interstellar reddening estimates in the low-column-density regime to date. Our reddening map is made publicly available at doi.org/10.7910/DVN/AFJNWJ.