Accretion-Driven Turbulence and the Transition to Global Instability in
  Young Galaxy Disks
  A simple model of gas accretion in young galaxy disks suggests that fast
turbulent motions can be driven by accretion energy for a time
t_acc~2(epsilon^{0.5} GM^2/xi V^3)^{0.5} where epsilon is the fraction of the
accretion energy going into disk turbulence, M and V are the galaxy mass and
rotation speed, and xi is the accretion rate. After t_acc, accretion is
replaced by disk instabilities as a source of turbulence driving, and shortly
after that, energetic feedback by young stars should become important. The star
formation rate equilibrates at the accretion rate after 1 to 2 t_acc, depending
on the star formation efficiency per dynamical time. The fast turbulence that
is observed in high redshift starburst disks is not likely to be driven by
accretion because the initial t_acc phase is over by the time the starburst is
present. However, the high turbulent speeds that must have been present
earlier, when the observed massive clumps first formed, could have been driven
by accretion energy. The combined observations of a high relative velocity
dispersion in the gas of z~2 clumpy galaxies and a gas mass comparable to the
stellar mass suggests that either the star formation efficiency is fairly high,
perhaps 10x higher than in local galaxies, or the observed turbulence is
powered by young stars.
