Major mergers between dark matter haloes -- II. Profile and
  concentration changes
  Several lines of evidence suggest that as dark matter haloes grow their scale
radius increases, and that the density in their central region drops. Major
mergers seem an obvious mechanism to explain both these phenomena, and the
resulting patterns in the concentration--mass--redshift relation. To test this
possibility, we have simulated equal-mass mergers between haloes with a variety
of cosmological density profiles, placed on various different orbits. The
remnants typically have higher densities than the initial conditions, but
differ only slightly from self-similar scaling predictions. They are reasonably
well fit by Einasto profiles, but have parameters distinct from those of the
initial conditions. The net internal energy available to the merger remnant,
relative to the internal energy of the initial conditions, $\kappa$, has the
greatest influence on the properties of the final mass distribution. As
expected, energetic encounters produce more extended remnants while mergers of
strongly bound systems produce compact remnants. Surprisingly, however, the
scale radius of the density profile shows the opposite trend, {\it increasing}
in the remnants of low-energy encounters relative to energetic ones. Also even
in the most energetic encounters, the density within the scale radius decreases
only slightly (by 10--20\%), while for very low-energy systems it increases
significantly after the merger. We conclude that while major mergers can
produce remnants that are more diffuse at large radii, they are relatively
ineffective at changing the central densities of haloes, and seem unlikely to
explain the mean trends in the concentration--mass--redshift relation.
