N(H I)/E(B – V)

Abstract

We explore the relationship between dust-emission derived reddening E(B – V) and atomic hydrogen column density N(H I) derived from 21 cm emission surveys. We consider measurements at galactic latitudes |b| ≳ 20{sup o} and E(B – V) ≲ 0.1 mag where the interstellar gas is predominantly neutral and atomic, and opacity corrections to 21 cm H I profiles are small. Over the Galaxy at large, at lower resolutions in H I, and on smaller scales at higher resolutions, we find that the reddening is always much smaller than would be expected from the usually quoted relation N(H) =5.8 × 10{sup 21} cm{sup –2} E(B – V) based on stellar reddening and UV absorption toward early-type stars. On wide scales we find N(H I) = 8.3 × 10{sup 21} cm{sup –2} E(B – V). We cite various precedents for such a large N(H I)/E(B – V) ratio whenever wide-field 21 cm emission surveys are considered, including when reddening based on galaxy counts and colors is substituted for the dust-emission derived reddening measure.