A turbulent origin for flocculent spiral structure in galaxies. II. Observations and models of M33

Abstract

Fourier transform power spectra of azimuthal scans of the optical structure of M33 are evaluated for B, V, and R passbands and fitted to fractal models of continuum emission with superposed star formation. Power spectra are also determined for Hα. The best models have intrinsic power spectra with one-dimensional slopes of around -0.7 ± 0.7, significantly shallower than the Kolmogorov spectrum (slope = -1.7) but steeper than pure noise (slope = 0). A fit to the power spectrum of the flocculent galaxy NGC 5055 gives a steeper slope of around -1.5 ± 0.2, which could be from turbulence. Both cases model the optical light as a superposition of continuous and pointlike stellar sources that follow an underlying fractal pattern. Foreground bright stars are clipped in the images, but they are so prominent in M33 that even their residual affects the power spectrum, making it shallower than what is intrinsic to the galaxy. A model consisting of random foreground stars added to the best model of NGC 5055 fits the observed power spectrum of M33 as well as the shallower intrinsic power spectrum that was made without foreground stars. Thus the optical structure in M33 could result from turbulence as well.