High-resolution simulations of cluster formation

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