A Turbulent Origin for Flocculent Spiral Structure in Galaxies. II. Observations and Models of M33

Fourier transform power spectra of azimuthal scans of the optical structureof M33 are evaluated for B, V, and R passbands and fit to fractal models ofcontinuum emission with superposed star formation. Power spectra are alsodetermined for Halpha. The best models have intrinsic power spectra with 1Dslopes of around -0.7pm0.7, significantly shallower than the Kolmogorovspectrum (slope =-1.7) but steeper than pure noise (slope=0). A fit to thepower spectrum of the flocculent galaxy NGC 5055 gives a steeper slope ofaround -1.5pm0.2, which could be from turbulence. Both cases model the opticallight as a superposition of continuous and point-like stellar sources thatfollow an underlying fractal pattern. Foreground bright stars are clipped inthe images, but they are so prominent in M33 that even their residual affectsthe power spectrum, making it shallower than what is intrinsic to the galaxy. Amodel consisting of random foreground stars added to the best model of NGC 5055fits the observed power spectrum of M33 as well as the shallower intrinsicpower spectrum that was made without foreground stars. Thus the opticalstructure in M33 could result from turbulence too.Comment: accepted by ApJ, 13 pages, 10 figure