2009ApJ...694..115B

Fourier transform power spectra of azimuthal scans in 33 galaxies imaged with the Infrared Array Camera on the Spitzer Space Telescope show an approximate power-law structure over a wide range of wavenumbers with a gradual steepening from 3.6 or 4.5 µm to 8.0 µm, in the order of an increasing contribution from gas and dust. At radii with active star formation, the average of the slopes of the 8 µm power spectra at intermediate spatial frequencies is about the same for all galaxies, independent of spiral arm morphology. These power spectra are reminiscent of turbulence, although the 8 µm slopes, averaging -2.0, are slightly steeper than expected (-1.7). Reconstructed images using only these intermediate spatial frequencies at 4.5 µm show aging star complexes distorted by shear. These sheared features illustrate the transition from a hierarchical structure during star formation into azimuthal star streams like the Pleiades moving group in the Solar neighborhood. This is the first time that young star streams have been observed in spiral galaxies other than the Milky Way.