  The azimuthal power spectra of optical emission from star formation and dust
in spiral galaxies resembles the azimuthal power spectra of HI emission from
the LMC. These and other power spectra of whole galaxies all resemble that of
velocity in incompressible Kolmogorov turbulence. The reasons for this are
unknown but it could be simply that star and cloud formation are the result of
a mixture of processes and each gives a power spectrum similar to Kolmogorov
turbulence, within the observable errors. The important point is that star and
cloud formation are not random but are correlated over large distances by
forces that span several orders of magnitude in scale. These forces are
probably the usual combination of self-gravity, turbulence, and compression
from stellar winds and supernovae, but they have to work in concert to create
the structures we see in galaxies. In addition, the identification of
flocculant spirals with swing amplified instabilities opens the possibility
that a high fraction of turbulence in the ISM is the result of self-gravity.

Astrophysics