H I power spectrum of the spiral galaxy NGC 628

We have measured the H I power spectrum of the nearly face-on spiral galaxy NGC 628 (M74) using a visibility-based estimator. The power spectrum is well fitted by a power law P(U) = AU , with α = −1.6 ± 0.2 over the length-scale 800 pc to 8 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, the Small Magellanic Cloud, the Large Magellanic Cloud (LMC) and the dwarf galaxy DDO 210. 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 disc at length-scales larger than the H I scaleheight of the galaxy. 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 scaleheight at 800 pc.