Evolution of Characteristic Quantities for Dark Matter Halo Density Profiles

We investigate the effect of an assembly history on the evolution of darkmatter (DM) halos of 10^{12} Msun/h using Constrained Realizations of randomGaussian fields. Five different realizations of a DM halo with distinct merginghistories were constructed and evolved. Our main results are: A halo evolvesvia a sequence of quiescent phases of a slow mass accretion intermitted byviolent episodes of major mergers. In the quiescent phases, the density is wellfitted by an NFW profile, the inner scale radius Rs and the mass enclosedwithin it remain constant, and the virial radius (Rvir) grows linearly with theexpansion parameter "a". Within each quiescent phase the concentrationparameter ("c") scales as "a", and the mass accretion history (Mvir) is welldescribed by the Tasitsiomi etal. fitting formula. In the violent phases thehalos are not in a virial equilibrium and both Rs and Rvir growdiscontinuously. The violent episodes drive the halos from one NFW dynamicalequilibrium to another. The final structure of a halo, including "c", dependson the degree of violence of the major mergers and on their number. Next, wefind a distinct difference between the behavior of various NFW parameters takenas averages over an ensemble of halos and those of individual halos. Moreover,the simple scaling relations c--Mvir do not apply to the entire evolution ofindividual halos, and so is the common notion that late forming halos are lessconcentrated than early forming ones. The entire evolution of the halo cannotbe fitted by single analytical expressions.Comment: 17 pages, 16 postscript figures. Accepted for publication by the Astrophysical Journa