The Origin of the Milky Way's Halo Age Distribution
  We present an analysis of the radial age gradients for the stellar halos of
five Milky Way mass-sized systems simulated as part of the Aquarius Project.
The halos show a diversity of age trends, reflecting their different assembly
histories. Four of the simulated halos possess clear negative age gradients,
ranging from approximately -7 to -19 Myr/kpc , shallower than those determined
by recent observational studies of the Milky Way's stellar halo. However, when
restricting the analysis to the accreted component alone, all of the stellar
halos exhibit a steeper negative age gradient with values ranging from $-$8 to
$-$32~Myr/kpc, closer to those observed in the Galaxy. Two of the
accretion-dominated simulated halos show a large concentration of old stars in
the center, in agreement with the Ancient Chronographic Sphere reported
observationally. The stellar halo that best reproduces the current observed
characteristics of the age distributions of the Galaxy is that formed
principally by the accretion of small satellite galaxies. Our findings suggest
that the hierarchical clustering scenario can reproduce the MW's halo age
distribution if the stellar halo was assembled from accretion and disruption of
satellite galaxies with dynamical masses less than ~10^9.5M_sun, and a minimal
in situ contribution.
