High‐Resolution Simulations of Cluster Formation

The formation history of rich clusters is investigated using a hybrid N-bodysimulation in which high spatial and mass resolution can be achievedself-consistently within a small region of a very large volume. The evolutionof three massive clusters is studied via mass accretion, spherically-averageddensity profiles, three-dimensional and projected shapes, and degree ofsubstructure. Each cluster consists of order $4\times 10^5$ particles at thepresent epoch and in the case that rich cluster evolution is well-described bya 1-parameter family, the simulations have sufficient resolution to demonstratethis. At $z=0$ the clusters have similar masses, $M(r \le 1.5h^{-1} Mpc) \sim2\times 10^{15} h^{-1} M_\odot$, and similar spherically-averaged densityprofiles, however markedly different formation histories are observed. Nosingle, dominant pattern is apparent in the time variation of the massaccretion rate, the cluster shape, or the degree of substructure. Although nota statistically large sample, these objects suggest that the detailed formationhistory of rich clusters cannot be characterized by a simple 1-parameterfamily. These results suggest that the use of observations of rich clustersover a wide range of redshifts to constrain cosmological parameters may not beentirely straightforward.Comment: 28 pages, 8 figures, plain Te