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

Abstract

Despite the vastly different angular scales on which they are measured, the integrated λ 21 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.