Structural and Dynamical Properties of Galaxies in a Hierarchical
  Universe: Sizes and Specific Angular Momenta
  We use a state-of-the-art semi-analytic model to study the size and the
specific angular momentum of galaxies. Our model includes a specific treatment
for the angular momentum exchange between different galactic components. Disk
scale radii are estimated from the angular momentum of the gaseous/stellar
disk, while bulge sizes are estimated assuming energy conservation. The
predicted size--mass and angular momentum--mass relations are in fair agreement
with observational measurements in the local Universe, provided a treatment for
gas dissipation during major mergers is included. Our treatment for disk
instability leads to unrealistically small radii of bulges formed through this
channel, and predicts an offset between the size--mass relations of central and
satellite early-type galaxies, that is not observed. The model reproduces the
observed dependence of the size--mass relation on morphology, and predicts a
strong correlation between specific angular momentum and cold gas content. This
correlation is a natural consequence of galaxy evolution: gas-rich galaxies
reside in smaller halos, and form stars gradually until present day, while
gas-poor ones reside in massive halos, that formed most of their stars at early
epochs, when the angular momentum of their parent halos is low. The dynamical
and structural properties of galaxies can be strongly affected by a different
treatment for stellar feedback, as this would modify their star formation
history. A higher angular momentum for gas accreted through rapid mode does not
affect significantly the properties of massive galaxies today, but has a more
important effect on low-mass galaxies at higher redshift.
