The structure and characteristic scales of the HI gas in galactic disks
  The spatial distribution of the HI gas in galaxies holds important clues on
the physical processes that shape the structure and dynamics of the
interstellar medium (ISM). In this work, we quantify the structure of the HI
gas in a sample of 33 nearby galaxies taken from the THINGS Survey using the
delta-variance spectrum. The THINGS galaxies display a large diversity in their
spectra, however, there are a number of recurrent features. In many galaxies,
we observe a bump in the spectrum on scales of a few to several hundred pc. We
find the characteristic scales associated with the bump to be correlated with
galactic SFR for values of the SFR > 0.5 M$_{sol}$ yr$^{-1}$ and also with the
median size of the HI shells detected in those galaxies. On larger scales, we
observe the existence of two self-similar regimes. The first one, on
intermediate scales is shallow and the power law that describes this regime has
an exponent in the range [0.1-1] with a mean value of 0.55 which is compatible
with the density field being generated by supersonic turbulence in the cold
phase of the HI gas. The second power law is steeper, with a range of exponents
between [0.5-1.5] and a mean value of 1.5. These values are associated with
subsonic turbulence which is characteristic of the warm phase of the HI gas.
The spatial scale at which the transition between the two regimes occurs is
found to be $\approx 0.5 R_{25}$ which is similar to the size of the molecular
disk in the THINGS galaxies. Overall, our results suggest that on scales < $0.5
R_{25}$, the structure of the ISM is affected by the effects of supernova
explosions. On larger scales (> 0.5 $R_{25}$), stellar feedback has no
significant impact, and the structure of the ISM is determined by large scale
processes that govern the dynamics of the gas in the warm neutral medium such
as the flaring of the HI disk and the effects of ram pressure stripping.
