The Fine-Scale Structure of the neutral Interstellar Medium in nearby
  Galaxies
  We present an analysis of the properties of HI holes detected in 20 galaxies
that are part of "The HI Nearby Galaxy Survey" (THINGS). We detected more than
1000 holes in total in the sampled galaxies. Where they can be measured, their
sizes range from about 100 pc (our resolution limit) to about 2 kpc, their
expansion velocities range from 4 to 36 km/s, and their ages are estimated to
range between 3 and 150 Myr. The holes are found throughout the disks of the
galaxies, out to the edge of the HI; 23% of the holes fall outside R25. We find
that shear limits the age of holes in spirals (shear is less important in dwarf
galaxies) which explains why HI holes in dwarfs are rounder, on average than in
spirals. Shear, which is particularly strong in the inner part of spiral
galaxies, also explains why we find that holes outside R25 are larger and
older. We derive the scale height of the HI disk as a function of
galactocentric radius and find that the disk flares up in all galaxies. We
proceed to derive the surface and volume porosity (Q2D and Q3D) and find that
this correlates with the type of the host galaxy: later Hubble types tend to be
more porous. The size distribution of the holes in our sample follows a power
law with a slope of a ~ -2.9. Assuming that the holes are the result of massive
star formation, we derive values for the supernova rate (SNR) and star
formation rate (SFR) which scales with the SFR derived based on other tracers.
If we extrapolate the observed number of holes to include those that fall below
our resolution limit, down to holes created by a single supernova, we find that
our results are compatible with the hypothesis that HI holes result from star
formation.
