The \HI column density power spectrum of six nearby spiral galaxies
  We propose a method to determine the power spectrum of \HI column density
fluctuations using radio-interferometric observations of 21-cm emission from
the ISM of galaxies. We have used this to estimate the power spectra of six
nearly face on nearby spiral galaxies. Earlier work has shown that these power
spectra are well fitted by power laws with slopes around -1.6 across
length-scales $\sim 1 \, {\rm kpc}$ to $\sim 10 \, {\rm kpc}$, the amplitude
however was undetermined. In the present work we have determined the amplitude
of the \HI column density power spectrum. We find that the \HI column density
$N_{\rm HI}$ expressed in units of $10^{20} \, {\rm cm}^{-2}$ has mean square
fluctuations in the range $\sim 0.03$ to $\sim 20$. The amplitude of the power
spectrum is found to be tightly correlated with the \HI mass fraction of the
galaxies. The physical process responsible for these scale-invariant
fluctuations is, however, at present not known.
