Optical Reddening, Integrated H i Optical Depth, and Total Hydrogen Column Density

Abstract

Abstract Despite the vastly different angular scales on which they are measured, the integrated λ21 cm H i optical depth measured interferometrically, , is a good proxy for the optical reddening derived from IR dust emission, with ∝ E(B–V)1.10 for 0.04 mag ≲ E(B–V) ≲ 4 mag. For E(B–V) ≲ 0.04 mag or < 0.7 , less-absorbent warm and ionized gases assert themselves and τ(H i) is a less reliable tracer of E(B–V). The –E(B–V) relationship can be inverted to give a broken power-law relationship between the total hydrogen column density N(H) and such that knowledge of alone predicts N(H) with an accuracy of a factor of 1.5 (±0.18 dex) across two orders of magnitude variation of . The –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.