A two-phase scenario for bulge assembly in LCDM cosmologies
  We analyze and compare the bulges of a sample of L* spiral galaxies in
hydrodynamical simulations in a cosmological context, using two different
codes, P-DEVA and GASOLINE. The codes regulate star formation in very different
ways, with P-DEVA simulations inputing low star formation efficiency under the
assumption that feedback occurs on subgrid scales, while the GASOLINE
simulations have feedback which drives large scale outflows. In all cases, the
marked knee-shape in mass aggregation tracks, corresponding to the transition
from an early phase of rapid mass assembly to a later slower one, separates the
properties of two populations within the simulated bulges. The bulges analyzed
show an important early starburst resulting from the collapse-like fast phase
of mass assembly, followed by a second phase with lower star formation, driven
by a variety of processes such as disk instabilities and/or mergers.
Classifying bulge stellar particles identified at z=0 into old and young
according to these two phases, we found bulge stellar sub-populations with
distinct kinematics, shapes, stellar ages and metal contents. The young
components are more oblate, generally smaller, more rotationally supported,
with higher metallicity and less alpha-element enhanced than the old ones.
These results are consistent with the current observational status of bulges,
and provide an explanation for some apparently paradoxical observations, such
as bulge rejuvenation and metal-content gradients observed. Our results suggest
that bulges of L* galaxies will generically have two bulge populations which
can be likened to classical and pseudo-bulges, with differences being in the
relative proportions of the two, which may vary due to galaxy mass and specific
mass accretion and merger histories.
