E(B-V), N(H I) and N(H2)
Abstract
We consider the structure of the N(H I) - E(B-V) relationship when H I is measured in the 21 cm radio line and \ is defined by far-IR dust-derived measures. We derive reddening-dependent corrections to N(H I) based on interferometric absorption measurements over the past 30 years that follow a single power-law relationship ∫ τ(H I) dv = 14.07 ~\ 1.074 at 0.02 \ 3 mag. Corrections to 21cm line-derived H I column densities are too small to have had any effect on the ratio N(H I)/\ = 8.3 × 1021 mag-1 we derived at 0.015 \ 0.075 mag and \ 20; they are also too small to explain the break in the slope of the N(H I) - \ relation at \ 0.1 mag that we demonstrated around the Galaxy at 20. The latter must therefore be attributed to the onset of -formation and we show that models of \ formation in a low density diffuse molecular gas can readily explain the inflected N(H I)- \ relationship. Below \ = 20\ N(H I)/\ measured at 0.015 \ 0.075 mag increases steadily down to \ = 8\ where sightlines with small \ no longer occur. By contrast, the ratio N(H I)/\ measured over all \ declines to N(H I)/\ = 5-6 × 1021 mag-1 at \ 30, perhaps providing an explanation of the difference between our results and the gas/reddening ratios measured previously using stellar spectra.
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