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Several lines of evidence suggest that as dark matter haloes grow their scaleradius increases, and that the density in their central region drops. Majormergers seem an obvious mechanism to explain both these phenomena, and theresulting patterns in the concentration--mass--redshift relation. To test thispossibility, we have simulated equal-mass mergers between haloes with a varietyof cosmological density profiles, placed on various different orbits. Theremnants typically have higher densities than the initial conditions, butdiffer only slightly from self-similar scaling predictions. They are reasonablywell fit by Einasto profiles, but have parameters distinct from those of theinitial conditions. The net internal energy available to the merger remnant,relative to the internal energy of the initial conditions, $\kappa$, has thegreatest influence on the properties of the final mass distribution. Asexpected, energetic encounters produce more extended remnants while mergers ofstrongly bound systems produce compact remnants. Surprisingly, however, thescale radius of the density profile shows the opposite trend, {\it increasing}in the remnants of low-energy encounters relative to energetic ones. Also evenin the most energetic encounters, the density within the scale radius decreasesonly slightly (by 10--20\%), while for very low-energy systems it increasessignificantly after the merger. We conclude that while major mergers canproduce remnants that are more diffuse at large radii, they are relativelyineffective at changing the central densities of haloes, and seem unlikely toexplain the mean trends in the concentration--mass--redshift relation.

Major mergers between dark matter haloes -- II. Profile and concentration changes