E(B – V), N(H I), and N(H{sub 2})

Journal Article · Sat Mar 01 04:00:00 EST 2014 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/783/1/17· OSTI ID:22351380

Liszt, Harvey ^[1]

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1. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475 (United States)

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 E(B – V) 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 yr that follow a single power-law relationship ∫τ(H I)dv=14.07 km s{sup −1} E(B – V){sup 1.074} at 0.02 ≲ E(B – V) ≲ 3 mag. Corrections to 21 cm line-derived H I column densities are too small to have had any effect on the ratio N(H I)/E(B – V) = 8.3 × 10{sup 21} cm{sup –2} mag{sup –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(H I)-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 H{sub 2} formation and we show that models of H{sub 2} formation in a low-density diffuse molecular gas can readily explain the inflected N (H I) – E(B – V) relationship. Below |b| = 20° N(H I)/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(H I)/E(B – V) measured over all E(B – V) declines to N(H I)/E(B – V) = 5-6 × 10{sup 21} cm{sup –2} mag{sup –1} at |b| ≲ 30°, perhaps providing an explanation of the difference between our results and the gas/reddening ratios measured previously using stellar spectra.

• OSTI ID: 22351380
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 783; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
ATOMS
CORRECTIONS
DENSITY
DUSTS
FAR INFRARED RADIATION
GALAXIES
INTERFEROMETRY
SPECTRA
STARS