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

Abstract

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 ∼1 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 NHI expressed in units of 1020 cm−2 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 H I mass fraction of the galaxies. The physical process responsible for these scale-invariant fluctuations is, however, at present not known.