The evolution of X-ray clusters in a low-density universe

We present results of N-body/gasdynamical simulations designed to investiga te the evolution of X-ray clusters in a flat, low-density, Lambda-dominated cold dark matter (CDM) cosmogony. The simulations include self-gravity, pr essure gradients, and hydrodynamical shocks, but neglect radiative cooling. The density profile of the dark matter component can be fitted accurately by the simple formula originally proposed by Navarro, Frenk, & White to des cribe the structure of clusters in a CDM universe with Omega = 1. In projec tion, the shape of the dark matter radial density profile and the correspon ding line-of-sight velocity dispersion profile are in very good agreement w ith the observed profiles for galaxies in the Canadian Network for Observat ional Cosmology sample of clusters. This suggests that galaxies are not str ongly segregated relative to the dark matter in X-ray luminous clusters. Th e gas in our simulated clusters is less centrally concentrated than the dar k matter, and its radial density profile is well described by the familiar beta-model. As a result, the average baryon fraction within the virial radi us (r(vir)) is only 85%-90% of the universal value and is lower nearer the center. The total mass and velocity dispersion of our clusters can be accur ately inferred (with similar to 15% uncertainty) from their X-ray emission- weighted temperature. We generalize Kaiser's scale-free scaling relations t o arbitrary power spectra and low-density universes and show that simulated clusters generally follow these relations. The agreement between the simul ations and the analytical results provides a convincing demonstration of th e soundness of our gasdynamical numerical techniques. Although our simulate d clusters resemble observed clusters in several respects, the slope of the luminosity-temperature relation implied by the scaling relations, and obey ed by the simulations, is in disagreement with observations. This suggests that nongravitational effects such as preheating or cooling must have playe d an important role in determining the properties of the observed X-ray emi ssion from galaxy clusters.