  We consider the infall of a massive clump into a dark-matter halo as a simple
and extreme model for the effect of baryonic physics (neglected in gravity-only
simulations of large-scale structure formation) on the dark-matter. We find
that such an infalling clump is extremely efficient in altering the structure
of the halo and reducing its central density: a clump of 1% the mass of the
halo can remove about twice its own mass from the inner halo and transform a
cusp into a core or weaker cusp. If the clump is subsequently removed,
mimicking a galactic wind, the central halo density is further reduced and the
mass removed from the inner halo doubled. Lighter clumps are even more
efficient: the ratio of removed mass to clump mass increases slightly towards
smaller clump masses. This process is the more efficient the more radially
anisotropic the initial dark-matter velocities. While such a clumpy infall may
be somewhat unrealistic, it demonstrates that the baryons need to transfer only
a small fraction of their initial energy to the dark matter via dynamical
friction to explain the discrepancy between predicted dark-matter density
profiles and those inferred from observations of dark-matter dominated
galaxies.

Cosmology and Nongalactic Astrophysics