Southern Galactic Plane Survey measurements of the spatial power spectrum of interstellar H I in the inner Galaxy

Using data from the Southern Galactic Plane Survey, we have measured the spatial power spectrum of the interstellar neutral atomic hydrogen in the fourth Galactic quadrant. This function shows the same power-law behavior that has been found for H I in the second quadrant of the Milky Way and in the Magellanic Clouds with the same slope. When we average over velocity intervals broader than the typical small-scale velocity dispersion, we find that the slope steepens, from similar or equal to -3 to similar or equal to -4 for the warm gas, as predicted by theories of interstellar turbulence if the column density fluctuations are dominated by variations in the gas density on small spatial scales. The cool gas shows a different increase of slope, which suggests that it is in the regime of turbulence dominated by fluctuations in the velocity field. Overall, these results confirm that the small-scale structure and motions in the neutral atomic medium are well described by a turbulent cascade of kinetic energy.