  We use the Aquarius simulation series to study the imprint of assembly
history on the structure of Galaxy-mass cold dark matter halos. Our results
confirm earlier work regarding the influence of mergers on the mass density
profile and the inside-out growth of halos. The inner regions that contain the
visible galaxies are stable since early times and are significantly affected
only by major mergers. Particles accreted diffusely or in minor mergers are
found predominantly in the outskirts of halos. Our analysis reveals trends that
run counter to current perceptions of hierarchical halo assembly. For example,
major mergers (i.e. those with progenitor mass ratios greater than 1:10)
contribute little to the total mass growth of a halo, on average less than 20
per cent for our six Aquarius halos. The bulk is contributed roughly equally by
minor mergers and by "diffuse" material which is not resolved into individual
objects. This is consistent with modeling based on excursion-set theory which
suggests that about half of this diffuse material should not be part of a halo
of any scale. Interestingly, the simulations themselves suggest that a
significantly fraction is not truly diffuse, since it was ejected from earlier
halos by mergers prior to their joining the main system. The Aquarius
simulations resolve halos to much lower mass scales than are expected to retain
gas or form stars. These results thus confirm that most of the baryons from
which visible galaxies form are accreted diffusely, rather than through
mergers, and they suggest that only relatively rare major mergers will affect
galaxy structure at later times.
