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We calculate by means of the Press-Schechter formalism the density profiledeveloped by dark-matter halos during accretion, i.e., the continuousaggregation of small clumps. We find that the shape of the predicted profile issimilar to that shown by halos in high-resolution cosmological simulations.Furthermore, the mass-concentration relation is correctly reproduced at anyredshift in all the hierarchical cosmologies analyzed, except for very largehalo masses. The role of major mergers, which can cause the rearrangement ofthe halo structure through violent relaxation, is also investigated. We showthat, as a result of the boundary conditions imposed by the matter continuouslyinfalling into the halo during the violent relaxation process, the shape of thedensity profile emerging from major mergers is essentially identical to theshape the halo would have developed through pure accretion. This resultexplains why, according to high-resolution cosmological simulations, relaxedhalos of a given mass have the same density profile regardless of whether theyhave had a recent merger or not, and why both spherical infall and hierarchicalassembly lead to very similar density profiles. Finally, we demonstrate thatthe density profile of relaxed halos is not affected either by the capture ofclumps of intermediate mass.Comment: 14 pages, 8 figures, accepted for publication in ApJ. Minor changes,  matches the published version. To appear in the ApJ August 10, 2003 issu

On the Origin of the Inner Structure of Halos