The Evolution of X‐Ray Clusters in a Low‐Density Universe

We present results of N-body/gasdynamical simulations designed to investigatethe evolution of X-ray clusters in a flat, low-density, cold dark matter (CDM)cosmogony. The density profile of the dark matter component can be fittedrather 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 thecorresponding line-of-sight velocity dispersion profile are in very goodagreement with the observed profiles for galaxies in the CNOC sample ofclusters. The gas in our simulated clusters is less centrally concentrated thanthe dark matter, and its radial density profile is well described by thefamiliar $\beta$-model. The total mass and velocity dispersion of our clusterscan be accurately inferred (with $\sim 15%$ uncertainty) from their X-rayemission-weighted temperature. We generalize Kaiser's scaling relations forscale-free universes and show that the clusters in our simulations generallyfollow these relations. The agreement between the simulations and theanalytical results provides a convincing demonstration of the soundness of ourgasdynamical numerical techniques. The slope of the luminosity-temperaturerelation implied by the scaling relations, and obeyed by the simulations, is indisagreement with observations. This suggests that non-gravitational effectssuch as preheating or cooling must have played an important role in determiningthe properties of the observed X-ray emission from galaxy clusters.