We study the amount and distribution of dark matter substructures within dark matter
haloes, using a large set of high-resolution simulations ranging from group size to
cluster size haloes, and carried our within a cosmological model consistent with WMAP
7-year data. In particular, we study how the measured properties of subhaloes vary
as a function of the parent halo mass, the physical properties of the parent halo, and
redshift. The fraction of halo mass in substructures increases with increasing mass:
it is of the order of 5 per cent for haloes with M200 ∼ 1013 M⊙ and of the order
of 10 per cent for the most massive haloes in our sample, with M200 ∼ 1015 M⊙.
There is, however, a very large halo-to-halo scatter that can be explained only in
part by a range of halo physical properties, e.g. concentration. At given halo mass,
less concentrated haloes contain significantly larger fractions of mass in substructures
because of the reduced strength of tidal disruption. Most of the substructure mass is
located at the outskirts of the parent haloes, in relatively few massive subhaloes. This
mass segregation appears to become stronger at increasing redshift, and should reflect
into a more significant mass segregation of the galaxy population at different cosmic
epochs. When haloes are accreted onto larger structures, their mass is significantly
reduced by tidal stripping. Haloes that are more massive at the time of accretion
(these should host more luminous galaxies) are brought closer to the centre on shorter
time-scales by dynamical friction, and therefore suffer of a more significant stripping.
The halo merger rate depends strongly on the environment with substructure in more
massive haloes suffering more important mergers than their counterparts residing in
less massive systems. This should translate into a different morphological mix for
haloes of different mass.
