The growth of disks and bulges during hierarchical galaxy formation. I:
  fast evolution vs secular processes
  We present a theoretical model for the evolution of mass, angular momentum
and size of galaxy disks and bulges, and we implement it into the semi-analytic
galaxy formation code SAGE. The model follows both secular and violent
evolutionary channels, including smooth accretion, disk instabilities, minor
and major mergers. We find that the combination of our recipe with hierarchical
clustering produces two distinct populations of bulges: merger-driven bulges,
akin to classical bulges and ellipticals, and instability-driven bulges, akin
to secular (or pseudo-)bulges. The model mostly reproduces the mass-size
relation of gaseous and stellar disks, the evolution of the mass-size relation
of ellipticals, the Faber-Jackson relation, and the magnitude-colour diagram of
classical and secular bulges. The model predicts only a small overlap of
merger-driven and instability-driven components in the same galaxy, and
predicts different bulge types as a function of galaxy mass and disk fraction.
Bulge type also affects the star formation rate and colour at a given
luminosity. The model predicts a population of merger-driven red ellipticals
that dominate both the low-mass and high-mass ends of the galaxy population,
and span all dynamical ages; merger-driven bulges in disk galaxies are
dynamically old and do not interfere with subsequent evolution of the
star-forming component. Instability-driven bulges dominate the population at
intermediate galaxy masses, especially thriving in massive disks. The model
green valley is exclusively populated by instability-driven bulge hosts.
Through the present implementation the mass accretion history is perceivable in
the galaxy structure, morphology and colours.
