The H I column density power spectrum of six nearby spiral galaxies

We propose a method to determine the power spectrum of H I column density fluctuations using radio-interferometric observations of 21-cm emission from the interstellar medium 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 similar to 1 to similar to 10 kpc, the amplitude however was undetermined. In this work, we have determined the amplitude of the H I column density power spectrum. We find that the H I column density N-H (I) expressed in units of 10(20) cm(-2) has mean square fluctuations in the range similar to 0.03 to similar to 20. The amplitude of the power spectrum is found to be tightly correlated with the H I mass fraction of the galaxies. The physical process responsible for these scale-invariant fluctuations is, however, at present not known.