Probing Turbulence in the Interstellar Medium using
  Radio-Interferometric Observations of Neutral Hydrogen
  We develop a visibility based power spectrum estimator to probe the power
spectrum directly from the interferometric observations and also estimate the
errors in it. A numerical simulation of synthetic observations is also
performed to access the efficacy and limitations of this estimator. We use this
estimator to evaluate the power spectrum of three individual spiral galaxies, a
dwarf galaxy sample and THINGS * spiral galaxies. In each case, the power
spectrum is found to follow a power law PHI (U) = AU {\alpha} over a specific
length scale range. The estimated value for the slope {\alpha} ranges from ~
~1.5 to ~ -2.6 for the sample of dwarf galaxies. We interpret this bi-modality
as arising due to 2D turbulence on length scales much larger than the
scale-height of the galaxy disk and 3D otherwise. The power law slope also show
a weak correlation with the surface density of star formation rate for these
galaxies. We found for the external spiral galaxies the power spectrum is a
power law up to a length scale of 10 to 16 kpc, indicating turbulence to be
operational at these large length scales. We measure the scale-height of the
external nearly face-on spiral galaxy NGC 1058 to be ~ 500 pc interpreting a
change in the slope observed in it's power spectrum. Power spectrum of the
harassed galaxy NGC 4254 is found to be affected by galaxy harassment. For 18
spiral galaxies derived from the THINGS sample we found no correlation between
the power law slope and the different dynamical parameters of these galaxies.
For most of the dwarf and spiral galaxies, the intensity fluctuation power
spectrum is found to be a direct probe of the density fluctuation and not
affected by the velocity fluctuations in ISM. We would attempt to understand
these new observations in terms of physical models of the ISM in future.
