The impact of baryon physics on the structure of high-redshift galaxies
  We study the detailed structure of galaxies at redshifts z > 2 using
cosmological simulations with improved modeling of the interstellar medium and
star formation. The simulations follow the formation and dissociation of
molecular hydrogen, and include star formation only in cold molecular gas. The
molecular gas is more concentrated towards the center of galaxies than the
atomic gas, and as a consequence, the resulting stellar distribution is very
compact. For halos with total mass above 10^{11} Mo, the median half-mass
radius of the stellar disks is 0.8 kpc at z = 3. The vertical structure of the
molecular disk is much thinner than that of the atomic neutral gas. Relative to
the non-radiative run, the inner regions of the dark matter halo change shape
from prolate to mildly oblate and align with the stellar disk. However, we do
not find evidence for a significant dark disk of dark matter around the stellar
disk. The outer halo regions retain the orientation acquired during accretion
and mergers, and are significantly misaligned with the inner regions. The
radial profile of the dark matter halo contracts in response to baryon
dissipation, establishing an approximately isothermal profile throughout most
of the halo. This effect can be accurately described by a modified model of
halo contraction. The angular momentum of a fixed amount of inner dark matter
is approximately conserved over time, while in the dissipationless case most of
it is transferred outward during mergers. The conservation of the dark matter
angular momentum provides supporting evidence for the validity of the halo
contraction model in a hierarchical galaxy formation process.
