The Shape of Dark Matter Haloes in the Aquarius Simulations: Evolution
  and Memory
  We use the high resolution cosmological N-body simulations from the Aquarius
project to investigate in detail the mechanisms that determine the shape of
Milky Way-type dark matter haloes. We find that, when measured at the
instantaneous virial radius, the shape of individual haloes changes with time,
evolving from a typically prolate configuration at early stages to a more
triaxial/oblate geometry at the present day. This evolution in halo shape
correlates well with the distribution of the infalling material: prolate
configurations arise when haloes are fed through narrow filaments, which
characterizes the early epochs of halo assembly, whereas triaxial/oblate
configurations result as the accretion turns more isotropic at later times.
Interestingly, at redshift z=0, clear imprints of the past history of each halo
are recorded in their shapes at different radii, which also exhibit a variation
from prolate in the inner regions to triaxial/oblate in the outskirts. Provided
that the Aquarius haloes are fair representatives of Milky Way-like 10^12 Msun
objects, we conclude that the shape of such dark matter haloes is a complex,
time-dependent property, with each radial shell retaining memory of the
conditions at the time of collapse.
