Hierarchical clustering, the universal density profile, and the mass-temperature scaling law of galaxy clusters

The significance of hierarchical clustering on the density profile and mass-temperature scaling relation for galaxy clusters is examined using hydrodynamic N-body simulations. Clusters formed hierarchically are compared with clusters formed with the initial density fluctuations on sub-cluster scales removed via smoothing. The universal profile, as described by Navarro, Frenk, and White, is not a by-product of hierarchical clustering. It is found to fit the mean profiles of clusters formed both hierarchically and otherwise. The Hernquist profile is also found to fit the data well. The characteristic radius, r_s, moves outward from 0.1 R_200 to 0.2 R_200 when the initial substructure is eliminated. Interior to r_s, rho_DM is proportional to r^-1.8, regardless of initial smoothing. Exterior to this radius, the profile is marginally shallower in the non-hierarchical case, with rho_DM proportional to r^-2.4 compared with rho_DM proportional to r^-2.7. The mass-temperature scaling relation maintains the form T proportional to M^2/3, regardless of cluster formation method. The normalisation varies at the 20% level, which is at the level of the intrinsic scatter, with the non-hierarchical simulations producing the cooler clusters.