  We use cosmological N-body simulations from the Aquarius Project to study the
tidal effects of a dark matter halo on the shape and orientation of its
substructure. Although tides are often assumed to enhance asphericity and to
stretch subhaloes tangentially, these effects are short lived: as in earlier
work, we find that subhaloes affected by tides become substantially more
spherical and show a strong radial alignment toward the centre of the host
halo. These results, combined with a semi-analytic model of galaxy formation,
may be used to assess the effect of Galactic tides on the observed population
of dwarf spheroidal (dSph) satellites of the Milky Way and Andromeda galaxies.
If, as the model suggests, the relatively low dark matter content of luminous
dSphs such as Fornax and Leo I is due to tidal stripping, then their
gravitational potential must be substantially more spherical than that of more
heavily dark matter-dominated systems such as Draco or Carina. The model also
predicts a tidally-induced statistical excess of satellites whose major axis
aligns with the direction to the central galaxy. We find tantalizing evidence
of this in the M31 satellite population, which suggests that tides may have
played an important role in its evolution.
