2013MNRAS.436L..49D

We propose a method to determine the power spectrum of Hi 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 ∼ 1 to ∼ 10kpc, the amplitude however was undetermined. In this work, we have determined the amplitude of the Hi column density power spectrum. We find that the Hi column density NHi expressed in units of 10²⁰/cm2 has mean square fluctuations in the range ∼ 0.03 to ∼ 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.