Structure of high-redshift galaxies in cosmological simulations
  We investigate the structure of galaxies formed in a suite of high-resolution
cosmological simulations. Consistent with observations of high-redshift
galaxies, our simulated galaxies show irregular, prolate shapes with thick
stellar disks, which are dominated by turbulent motions instead of rotation.
Yet molecular gas and young stars are restricted to relatively thin disks. We
examine the accuracy of applying the Toomre linear stability analysis to
predict the location and amount of gas available for star formation. We find
that the Toomre criterion still works for these irregular galaxies, after
correcting for multiple gas and stellar components: the $Q$ parameter in
$\rm{H_2}$ rich regions is in the range $0.5-1$, remarkably close to unity. Due
to the violent stellar feedback from supernovae and strong turbulent motions,
young stars and molecular gas are not always spatially associated. Neither the
$Q$ map nor the $\rm{H_2}$ surface density map coincide with recent star
formation exactly. We argue that the Toomre criterion is a better indicator of
future star formation than a single $\rm{H_2}$ surface density threshold
because of the smaller dynamic range of $Q$. The depletion time of molecular
gas is below 1~Gyr on kpc scale, but with large scatter. Centering the aperture
on density peaks of gas/young stars systematically biases the depletion time to
larger/smaller values and increases the scatter.
