Probing interstellar turbulence in spiral galaxies using HI power spectrum analysis

TL;DR

This study analyzes the HI intensity fluctuation power spectrum in 18 spiral galaxies, revealing a range of power-law indices and suggesting different turbulence regimes in the interstellar medium at various scales.

Contribution

It provides the first extensive analysis of HI power spectra across multiple galaxies, linking observed indices to 2D and 3D turbulence in the ISM.

Findings

Power spectra fit by power law with index $oldsymbol{ ext{α}}$ varying across galaxies.

No significant correlation between $oldsymbol{ ext{α}}$ and galaxy properties.

Different $oldsymbol{ ext{α}}$ ranges interpreted as 2D and 3D turbulence regimes.

Abstract

We estimate the \HI intensity fluctuation power spectrum for a sample of 18 spiral galaxies chosen from THINGS. Our analysis spans a large range of length-scales from $\sim 300 {\rm pc}$ to $\sim 16 {\rm kpc}$ across the entire galaxy sample. We find that the power spectrum of each galaxy can be well fitted by a power law $P_{\rm HI}(U) = A U^{\alpha}$, with an index $\alpha$ that varies from galaxy to galaxy. For some of the galaxies the scale-invariant power-law power spectrum extends to length-scales that are comparable to the size of the galaxy's disk. The distribution of $\alpha$ is strongly peaked with 50% of the values in the range $\alpha=-1.9$ to 1.5, and a mean and standard deviation of -1.3 and 0.5 respectively. We find no significant correlation between $\alpha$ and the star formation rate, dynamical mass, \HI mass or velocity dispersion of the galaxies. Several earlier studies that have measured the power spectrum within our Galaxy on length-scales that are considerably smaller than $500 {\rm pc}$ have found a power-law power spectrum with $\alpha$ in the range $\approx -2.8$ to -2.5. We propose a picture where we interpret the values in the range $\approx -2.8$ to -2.5 as arising from three dimensional (3D) turbulence in the Interstellar Medium (ISM) on length-scales smaller than the galaxy's scale-height, and we interpret the values in the range $\approx -1.9$ to -1.5 measured in this paper as arising from two-dimensional ISM turbulence in the plane of the galaxy's disk. It however still remains a difficulty to explain the small galaxy to galaxy variations in the values of $\alpha$ measured here.