Interstellar H I and H_2 in the Magellanic Clouds: An Expanded Sample Based on UV Absorption-Line Data

TL;DR

This study expands the dataset of interstellar hydrogen in the Magellanic Clouds using UV absorption lines, enabling improved analysis of gas properties, molecular fractions, and dust relations in these low-metallicity galaxies.

Contribution

It provides the largest absorption-based measurements of H I and H_2 in the Magellanic Clouds, refining gas-to-dust ratios and molecular fractions with direct, high-accuracy data.

Findings

Gas-to-dust ratios are higher than in the Milky Way, consistent with lower metallicity.

Discrepancies between 21 cm emission and Lyman-alpha absorption reveal small-scale structure and saturation effects.

Data can test models of atomic-to-molecular transition at low metallicities.

Abstract

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.