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

We present results of N-body/gasdynamical simulations designed to investigate the evolution of X-ray clusters in a flat, low-density, Lambda-dominated cold dark matter (CDM) cosmogony. The simulations include self-gravity, pressure 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 describe the structure of clusters in a CDM universe with Omega = 1. In projection, the shape of the dark matter radial density profile and the corresponding line-of-sight velocity dispersion profile are in very good agreement with the observed profiles for galaxies in the Canadian Network for Observational Cosmology sample of clusters. This suggests that galaxies are not strongly segregated relative to the dark matter in X-ray luminous clusters. The gas in our simulated clusters is less centrally concentrated than the dark matter, and its radial density profile is well described by the familiar beta-model. As a result, the average baryon fraction within the virial radius (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 accurately inferred (with similar to 15% uncertainty) from their X-ray emission-weighted temperature. We generalize Kaiser's scale-free scaling relations to arbitrary power spectra and low-density universes and show that simulated clusters generally follow these relations. The agreement between the simulations and the analytical results provides a convincing demonstration of the soundness of our gasdynamical numerical techniques. Although our simulated clusters resemble observed clusters in several respects, the slope of the luminosity-temperature relation implied by the scaling relations, and obeyed by the simulations, is in disagreement with observations. This suggests that nongravitational effects such as preheating or cooling must have played an important role in determining the properties of the observed X-ray emission from galaxy clusters.