High-resolution simulations of cluster formation

The formation history of rich clusters is investigated using a hybrid N-bod y simulation in which high spatial and mass resolution can be achieved self -consistently within a small region of a very large volume. The evolution o f three massive dusters is studied via mass accretion, spherically averaged density profiles, three-dimensional and projected shapes, and degree of su bstructure. Each duster is resolved well and consists of similar to 4 x 10( 5) particles at the present epoch. Although the clusters have similar masse s, M(r = 1.5 h(-1) Mpc) similar to 2 x 10(15) h(-1) M., and similar spheric ally averaged density profiles at the end of the simulations, markedly diff erent formation histories are observed. No single, dominant pattern is appa rent in the time variation of the mass accretion rate, the cluster shape, o r the degree of substructure. Individually, the density profiles of the clu sters are fitted well by Navarro, Frenk, & White (NFW) profiles over the co urse of the simulations. The values of the NFW concentration parameter that best reproduce the cluster profiles are, however, lower than the values pr edicted for halos with masses identical to those of the simulated clusters. The largest discrepancy between the observed and predicted concentration p arameters is of order a factor of 2.5 and is most likely caused by substruc ture in the clusters on scales less than the "virial radius," r(200).