Interstellar H I and H2in the Magellanic Clouds: An Expanded Sample Based on Ultraviolet Absorption-Line Data

We have determined column densities of H I and/or H₂ for sight lines in the Magellanic Clouds from archival HST and FUSE spectra of H I Lyman-alpha and H₂ Lyman-band absorption. Together with some similar data from the literature, we now have absorption-based N (H I) and/or N (H₂) 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 H₂ 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 (Hₜot) /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 (H₂) /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 (H₂) based on comparisons of 21 cm emission and the IR emission from dust.