Constrained cosmological simulations of dark matter halos

The formation and structure of dark matter halos are investigated by means of constrained realizations of Gaussian fields using N-body simulations. Experiments in the formation of a 10¹² h^-1 M _⊙ halo are designed to study the dependence of the density profile on its merging history. We find that (1) the halo growth consists of several violent and quiescent phases, with the density well approximated by the Navarro-Frenk-White (NFW) profile at most times; (2) the NFW scale radius R _s stays constant during the quiet phases and grows abruptly during the violent ones, while the virial radius grows linearly during the quiet phases and grows abruptly during the violent phases; (3) the value of R_s, reflects the violent merging history of the halo; (4) the central density stays unchanged during the quiet phases while dropping abruptly during the violent ones (its value does not reflect the formation time of the halo); and (5) the clear separation of the evolution of an individual halo into series of quiescent and violent phases explains the inability to fit its entire evolution by simple scaling relations, in agreement with previous studies.