Daniel E. Welty, Rui Xue, Tony Wong
We have determined column densities of H I and/or H_2 for sight lines in the
Magellanic Clouds from archival HST and FUSE spectra of H I Lyman-alpha and H_2
Lyman-band absorption. Together with some similar data from the literature, we
now have absorption-based N(H I) and/or N(H_2) for 285 LMC and 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 Lyman-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_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_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_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_2) based on comparisons
of 21 cm emission and the IR emission from dust.
View original:
http://arxiv.org/abs/1111.3674
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