A Turbulent Origin for Flocculent Spiral Structure in Galaxies

The flocculent structure of star formation in 7 galaxies has a Fouriertransform power spectrum for azimuthal intensity scans with a power law slopethat increases systematically from -1 at large scales to -1.7 at small scales.This is the same pattern as in the power spectra for azimuthal scans of HIemission in the Large Magellanic Clouds and for flocculent dust clouds ingalactic nuclei. The steep part also corresponds to the slope of -3 fortwo-dimensional power spectra that have been observed in atomic and moleculargas surveys of the Milky Way and the Large and Small Magellanic Clouds. Thesame power law structure for star formation arises in both flocculent and granddesign galaxies, which implies that the star formation process is the same ineach. Fractal Brownian motion models that include discrete stars and anunderlying continuum of starlight match the observations if all of the emissionis organized into a global fractal pattern with an intrinsic 1D power spectrumhaving a slope between 1.3 and 1.8. We suggest that the power spectrum ofoptical light in galaxies is the result of turbulence, and that large-scaleturbulent motions are generated by sheared gravitational instabilities whichmake flocculent spiral arms first and then cascade to form clouds and clusterson smaller scales.Comment: accepted for ApJ, 31 pg, 9 figure