2019ApJ...881...29L

Despite the vastly different angular scales on which they are measured, the integrated λ21 cm H I optical depth measured interferometrically, Υ_HI, is a good proxy for the optical reddening derived from IR dust emission, with Υ_HI ∝ E(B-V)^1.10 for 0.04 mag <= E(B-V) <= 4 mag. For E(B-V) <= 0.04 mag or Υ_HI < 0.7 kms^–1, less-absorbent warm and ionized gases assert themselves and τ(H I) is a less reliable tracer of E(B-V). The Υ_HI-E(B-V) relationship can be inverted to give a broken power-law relationship between the total hydrogen column density N(H) and Υ_HI such that knowledge of Υ_HI alone predicts N(H) with an accuracy of a factor of 1.5 (±0.18 dex) across two orders of magnitude variation of Υ_HI. The Υ_HI-N(H) relation is invariant under a linear rescaling of the reddening measure used in the analysis and does not depend on knowing properties of the H I such as the spin temperature.