N(H I)/E(B-V)
Abstract
We explore the relationship between dust-emission derived reddening \ and atomic hydrogen column density N(H I) derived from 21 cm emission surveys. We consider measurements at galactic latitude |b| 20\ and \ 0.1 mag where the interstellar gas is predominantly neutral and atomic and opacity corrections to 21cm H I profiles are small. Over the Galaxy at large at lower resolution in H I, and on smaller scales at higher resolution, we find that the reddening is always much smaller than would be expected from the usually-quoted relation N(H) = 5.8× 1021 \ based on stellar reddening and uv absorption toward early-type stars. On wide scales, we find N(H I) = 8.3 × 1021 . We cite various precedents for such a large N(H I)/\ ratio whenever wide-field 21cm emission surveys are considered, including when reddening based on galaxy counts or colors is substituted for the dust-emission derived reddening measure.
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