E(B-V), N(HI), and N(H2)

Abstract

We consider the structure of the N(HI)-E(B-V) relationship when HI is measured in the 21 cm radio line and E(B-V) is defined by far-IR dust-derived measures. We derive reddening-dependent corrections to N(HI) based on interferometric absorption measurements over the past 30 yr that follow a single power-law relationship E(B-V)1.074 at 0.02 ≲ E(B-V) ≲ 3 mag. Corrections to 21 cm line-derived HI column densities are too small to have had any effect on the ratio N(H I)/E(B-V) = 8.3 × 1021 cm-2 mag -1 we derived at 0.015 ≲ E(B-V) ≲ 0.075 mag and |b| ≥ 20°; they are also too small to explain the break in the slope of the N(HI)-E(B-V) relation at E(B-V) ≳ 0.1 mag that we demonstrated around the Galaxy at |b|≥20°. The latter must therefore be attributed to the onset of H2 formation and we show that models of H2 formation in a low-density diffuse molecular gas can readily explain the inflected N (H I)-E(B-V) relationship. Below |b| = 20° N(HI)/E(B-V) measured at 0.015 ≲ E(B-V) ≲ 0.075 mag increases steadily down to |b| = 8° where sightlines with small E(B-V) no longer occur. By contrast, the ratio N(HI)/E(B-V) measured over all E(B-V) declines to N(H I)/E(B-V) = 5-6 × 1021 cm-2 mag -1 at |b| ≲ 30°, perhaps providing an explanation of the difference between our results and the gas/reddening ratios measured previously using stellar spectra. © 2014. The American Astronomical Society. All rights reserved.