The \HI column density power spectrum of six nearby spiral galaxies

TL;DR

This paper introduces a method to measure the amplitude of the ext{HI} column density power spectrum in nearby spiral galaxies, revealing a correlation with ext{HI} mass fraction and confirming power-law behavior across kiloparsec scales.

Contribution

The study provides the first estimates of the amplitude of ext{HI} column density power spectra in six galaxies, linking it to galaxy properties and refining understanding of ISM fluctuations.

Findings

Power spectra follow a -1.6 slope across 1-10 kpc scales.

Amplitude correlates with ext{HI} mass fraction.

Fluctuations range from 0.03 to 20 in mean square units.

Abstract

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.