INTERSTELLAR H I AND H{sub 2} IN THE MAGELLANIC CLOUDS: AN EXPANDED SAMPLE BASED ON ULTRAVIOLET ABSORPTION-LINE DATA

Journal Article · Wed Feb 01 04:00:00 EST 2012 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/745/2/173· OSTI ID:22011694

Welty, Daniel E ^[1]; Rui, Xue; Wong, Tony ^[2]

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1. Current address: Astronomy and Astrophysics Center, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637 (United States)
2. Astronomy Department, University of Illinois at Urbana/Champaign, 1002 W. Green Street, Urbana, IL 61801 (United States)

We have determined column densities of H I and/or H{sub 2} for sight lines in the Magellanic Clouds from archival Hubble Space Telescope and Far-Ultraviolet Spectroscopic Explorer spectra of H I Ly{alpha} and H{sub 2} Lyman-band absorption. Together with some similar data from the literature, we now have absorption-based N(H I) and/or N(H{sub 2}) for 285 Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) sight lines (114 with a detection or limit for both species)-enabling more extensive, direct, and accurate determinations of molecular fractions, gas-to-dust ratios, and elemental depletions in these two nearby, low-metallicity galaxies. For sight lines where the N(H I) estimated from 21 cm emission is significantly higher than the value derived from Ly{alpha} absorption (presumably due to emission from gas beyond the target stars), integration of the 21 cm profile only over the velocity range seen in Na I or H{sub 2} absorption generally yields much better agreement. Conversely, N(21 cm) can be lower than N(Ly{alpha}) by factors of 2-3 in some LMC sight lines-suggestive of small-scale structure within the 21 cm beam(s) and/or some saturation in the emission. The mean gas-to-dust ratios obtained from N(H{sub tot})/E(B - V) are larger than in our Galaxy, by factors of 2.8-2.9 in the LMC and 4.1-5.2 in the SMC-i.e., factors similar to the differences in metallicity. The N(H{sub 2})/E(B - V) ratios are more similar in the three galaxies, but with considerable scatter within each galaxy. These data may be used to test models of the atomic-to-molecular transition at low metallicities and predictions of N(H{sub 2}) based on comparisons of 21 cm emission and the IR emission from dust.

• OSTI ID: 22011694
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 2 Vol. 745; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABSORPTION
ABUNDANCE
DUSTS
EMISSION
FAR ULTRAVIOLET RADIATION
HYDROGEN
MAGELLANIC CLOUDS
STARS
TELESCOPES