Tracing the Formation of Molecular Clouds in a Low-Metallicity Galaxy: A
  HI Narrow Self-Absorption Survey of the Large Magellanic Cloud
  Cold atomic hydrogen clouds are the precursors of molecular clouds. Due to
self-absorption, the opacity of cold atomic hydrogen may be high, and this gas
may constitute an important mass component of the interstellar medium (ISM).
Atomic hydrogen gas can be cooled to temperatures much lower than found in the
cold neutral medium (CNM) through collisions with molecular hydrogen. In this
paper, we search for HI Narrow Self-Absorption (HINSA) features in the Large
Magellanic Cloud (LMC) as an indicator of such cold HI clouds, and use the
results to quantify atomic masses and atomic-to-molecular gas ratio. Our search
for HINSA features was conducted towards molecular clouds in the LMC using the
ATCA+Parkes HI survey and the MAGMA CO survey. HINSA features are prevalent in
the surveyed sightlines. This is the first detection of HINSA in an external
galaxy. The HINSA-HI/$\rm{H}_{2}$ ratio in the LMC varies from 0.5\e{-3} to
3.4\e{-3} (68\% interval), with a mean value of $(1.31 \pm 0.03)$\e{-3}, after
correcting for the effect of foreground HI gas. This is similar to the Milky
Way value and indicates that similar fractions of cold gas exist in the LMC and
the Milky Way, despite their differing metallicities, dust content and
radiation fields. The low ratio also confirms that, as with the Milky Way, the
formation timescale of molecular clouds is short. The ratio shows no radial
gradient, unlike the case for stellar metallicity. No correlation is found
between our results and those from previous HI absorption studies of the LMC.
