Power spectra in V band and Hα of nine irregular galaxies

Abstract

Fourier transform power spectra of major axis cuts in V and Hα images were made for a sample of nine irregular galaxies. These power spectra reveal structure over a wide range of scales. For six of the galaxies the power spectrum slopes at intermediate scales (10-400 pc) in the V-band images range from -1.3 to -1.5. The similarity of slopes suggests that the same processes are structuring these systems. These slopes are slightly shallower than those observed in other galaxies in H I, molecular emission, dust extinction, and optical light. Three of the galaxies have flat power spectra like sky noise; these three galaxies are relatively indistinct in the direct images. The power spectrum slope for Hα steepens with increasing star formation rate, ranging from a shallow value comparable to the noise at low rates to a steep value with a slope of ∼-1.5 at high rates. This change reflects the increasing areal filling factor of Hα emission with increasing star formation rate and an apparently universal slope inside the Hα regions that is comparable to that for Kolmogorov turbulence. The power spectrum of H I in one galaxy has a steeper power law, with a slope of ∼-2.9. The fact that the power laws of star formation are about the same for dwarf galaxies and giant spiral galaxies suggests the microscopic processes are the same, independent of spiral density waves and galaxy size. © 2005. The American Astronomical Society. All rights reserved.