The dark matter assembly of the Local Group in constrained cosmological
  simulations of a LambdaCDM universe
  We make detailed theoretical predictions for the assembly properties of the
Local Group (LG) in the standard LambdaCDM cosmological model. We use three
cosmological N-body dark matter simulations from the CLUES project, which are
designed to reproduce the main dynamical features of the matter distribution
down to the scale of a few Mpc around the LG. Additionally, we use the results
of an unconstrained simulation with a sixty times larger volume to calibrate
the influence of cosmic variance. We characterize the Mass Aggregation History
(MAH) for each halo by three characteristic times, the formation, assembly and
last major merger times. A major merger is defined by a minimal mass ratio of
10:1. We find that the three LGs share a similar MAH with formation and last
major merger epochs placed on average \approx 10 - 12 Gyr ago. Between 12% and
17% of the halos in the mass range 5 x 10^11 Msol/h < M_h < 5 x 10^12 Msol/h
have a similar MAH. In a set of pairs of halos within the same mass range, a
fraction of 1% to 3% share similar formation properties as both halos in the
simulated LG. An unsolved question posed by our results is the dynamical origin
of the MAH of the LGs. The isolation criteria commonly used to define LG-like
halos in unconstrained simulations do not narrow down the halo population into
a set with quiet MAHs, nor does a further constraint to reside in a low density
environment. The quiet MAH of the LGs provides a favorable environment for the
formation of disk galaxies like the Milky Way and M31. The timing for the
beginning of the last major merger in the Milky Way dark matter halo matches
with the gas rich merger origin for the thick component in the galactic disk.
Our results support the view that the specific large and mid scale environment
around the Local Group play a critical role in shaping its MAH and hence its
baryonic structure at present.
