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Title: Constraining the H 2 column densities in the diffuse interstellar medium using dust extinction and H I data
Context. Carbon monoxide (CO) is a poor tracer of H2in the diffuse interstellar medium (ISM), where most of the carbon is not incorporated into CO molecules, unlike the situation at higher extinctions. Aims. We present a novel, indirect method for constraining H2column densities (NH2) without employing CO observations. We show that previously recognized nonlinearities in the relation between the extinction,AV(H2), derived from dust emission and the H Icolumn density (NH I) are due to the presence of molecular gas. Methods. We employed archival (NH2) data, obtained from the UV spectra of stars, and calculatedAV(H2) toward these sight lines using 3D extinction maps. The following relation fits the data: logNH2= 1.38742 (logAV(H2))3− 0.05359 (logAV(H2))2+ 0.25722 logAV(H2) + 20.67191. This relation is useful for constrainingNH2in the diffuse ISM as it requires onlyNH Iand dust extinction data, which are both easily accessible. In 95% of the cases, the estimates produced by the fitted equation have deviations of less than a factor of 3.5. We constructed aNH2map of our Galaxy and compared it to the CO integrated intensity (WCO) distribution. Results. We find that the average ratio (XCO) betweenNH2andWCOis approximately equal to 2 × 1020cm−2(K km s−1)−1, consistent with previous estimates. However, we find that theXCOfactor varies by orders of magnitude on arcminute scales between the outer and the central portions of molecular clouds. For regions withNH2≳ 1020cm−2, we estimate that the average H2fractional abundance,fH2= 2NH2/(2NH2+NH I), is 0.25. Multiple (distinct) largely atomic clouds are likely found along high-extinction sightlines (AV≥ 1 mag), hence limitingfH2in these directions. Conclusions. More than 50% of the lines of sight withNH2≥ 1020cm−2are untraceable by CO with aJ= 1−0 sensitivity limitWCO= 1 K km s−1.  more » « less
Award ID(s):
2009234 2108318
PAR ID:
10558828
Author(s) / Creator(s):
; ; ;
Publisher / Repository:
A&A
Date Published:
Journal Name:
Astronomy & Astrophysics
Volume:
682
ISSN:
0004-6361
Page Range / eLocation ID:
A161
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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