HI Clouds in LITTLE THINGS Dwarf Irregular Galaxies
  We identify 814 discrete HI clouds in 40 dwarf irregular galaxies from the
LITTLE THINGS survey using an automated cloud-finding algorithm. The cloud
masses range from ~10^3 to 10^7 Msolar, have a surface density averaged over
all of the clouds of ~9.65 Msolar/pc^2, and constitute 2-53% of the total HI
mass of the host galaxy. For individual clouds, the mass including He varies
with cloud radius as log M_{gas} = (2.11\pm0.04)x log R_{cl}+(0.78\pm0.08) and
the internal velocity dispersion varies as log V_{disp} = 0.5xlog
R_{cl}-0.57\pm0.21. The HI clouds tend to be in the outer regions of the
galaxies, with 72% of the galaxies having more than 70% of their clouds outside
one disk scale length, and 32% of the galaxies having more than 50% of their
clouds outside the radius encircling the HII emission. 36% of the clouds are
essentially non-self-gravitating from HI alone, with a virial parameter that
exceeds alpha_{vir}~10, and 5% have alpha_{vir}\le2. We estimate the missing
molecular mass, based on the total star formation rate and a typical molecular
consumption time of 2 Gyr, as observed in CO-rich galaxies. The resulting
molecular fraction has a value averaged over the galaxies of 0.23 and
correlates with both the surface density of star formation and the fraction of
HI clouds in the outer regions. We conclude that a significant fraction of the
inner parts of these dwarf galaxy disks is in the form of dark molecular gas,
and that this fraction could be high enough to make the inner disks mildly
gravitationally unstable as a precursor to star formation.
