HI power spectrum of the spiral galaxy NGC628

TL;DR

This study measures the HI power spectrum of galaxy NGC628, revealing a transition from 3D to 2D turbulence at large scales and estimating the galaxy's HI scale height.

Contribution

First measurement of the HI power spectrum in NGC628 showing a scale-dependent turbulence transition and estimating the galaxy's HI scale height.

Findings

Power spectrum follows a power law with slope -1.6.

Transition from 3D to 2D turbulence at ~800 pc.

Upper limit on galaxy's HI scale height is 800 pc.

Abstract

We have measured the HI power spectrum of the nearly face-on spiral galaxy NGC628 (M74) using a visibility based estimator. The power spectrum is well fitted by a power law $P(U)=AU^{\alpha}$, with $\alpha =- 1.6\pm0.2$ over the length scale $800 {\rm pc} {\rm to} 8 {\rm kpc}$. The slope is found to be independent of the width of the velocity channel. This value of the slope is a little more than one in excess of what has been seen at considerably smaller length scales in the Milky-Way, Small Magellanic Cloud (LMC), Large Magellanic Cloud (SMC) and the dwarf galaxy DDO210. We interpret this difference as indicating a transition from three dimensional turbulence at small scales to two dimensional turbulence in the plane of the galaxy's disk at length scales larger than galaxy's HI scale height. The slope measured here is similar to that found at large scales in the LMC. Our analysis also places an upper limit to the galaxy's scale height at $800\ {\rm pc}$ .