Interstellar H I and H_2 in the Magellanic Clouds: An Expanded Sample   Based on UV Absorption-Line Data
  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. 
Daniel E. Welty (1), Rui Xue (2), Tony Wong (2) ((1) University of   Chicago, (2) University of Illinois at Urbana/Champaign)
astro-ph.GA