N(H I)/E(B–V)

Abstract

We explore the relationship between dust-emission derived reddening \EBV\ and atomic hydrogen column density N(H I) derived from 21 cm emission surveys. We consider measurements at galactic latitude $|b| \ga 20$\degr\ and \EBV\ $\la$ 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\times 10^{21}\pcc$ \EBV\ based on stellar reddening and {\it uv} absorption toward early-type stars. On wide scales, we find N(H I) = $8.3 \times 10^{21}\pcc$ \EBV. We cite various precedents for such a large N(H I)/\EBV\ 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.