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Hydrogen column densities inferred from X-ray absorption are typically 5 - 30times larger than the neutral atomic hydrogen column densities derived from21cm HI absorption toward radio-loud active galactic nuclei. Some part of thedifference is ascribed to uncertainty in the spin temperature \Tsp\ = 100 Kthat is often used to convert 21cm HI HI absorption to N(HI). Here we proposeanother way to infer the gas column from HI absorption. In our Galaxy there isa nearly linear correlation between the inteferometrically-measured integrated21cm HI absorption \WHI\ and reddening, \WHI\ $\propto$ \EBV$^{1.10}$ for \WHI\$\ga 0.7$ \kms\ or \EBV\ $\ga 0.04$ mag. Scaling \EBV\ then provides the totalgas column density N(H) from the same dust column that is responsible foroptical obscuration and X-ray absorption, without calculating N(HI). Values ofN(H) so derived typically exceed N(HI) by a factor 4 because the ubiquitousGalactic 21cm HI HI absorption samples only a portion of the interstellar gas.If the well-studied case of Hydra-A is a guide, even very large disparities inX-ray and 21cm HI gas column densities can be explained by resolving the coreradiocontinuum and inferring N(H) from 21cm HI absorption. Milky Way conditionsare often invoked in discussion of obscured AGN, so the empirical relationshipseen in the Milky Way should be a relevant benchmark.

Reconciling X-ray and 21cm HI absorption gas column densities toward obscured AGN