Probing interstellar turbulence in spiral galaxies using HI power spectrum analysis

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 $\sim 300\mathrm{pc}$ to $\sim 16\mathrm{kpc}$ across the entire galaxy sample. We find that the power spectrum of each galaxy can be well fitted by a power law $P_{\mathrm{HI}}(U)=A{U}^{\alpha }$, 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 $500\mathrm{pc}$ 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.

20 pages, 9 figures, 3 tables. accepted for publication in New Astronomy

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• Probing Turbulence in the Interstellar Medium using Radio-Interferometric Observations of Neutral Hydrogen
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• Effect of turbulent velocity on the \HI intensity fluctuation power spectrum from spiral galaxies
• HI column density statistics of the cold neutral medium from absorption studies
• Bending Waves in Velocity Space: a First Look at the THINGS sample