Simulations of X-ray clusters

We present simulations of the formation and evolution of galaxy clusters in the cold dark matter cosmogony. Clusters with a wide range of mass were selected from previous N-body models, and were resimulated at higher resolution using a combined N-body/Smooth Particle hydrodynamics code. The effects of radiative cooling on the gas are neglected. While many present-day clusters are predicted to be undergoing mergers, the density profiles of those that are approximately in equilibrium are all very similar, both for the gas and for the dark matter. These profiles show no sign of a uniform-density core and steepen gradually from the centre outwards. The standard β-model is a reasonable fit over most of the radius range observable in real clusters. However, the value obtained for the slope parameter βf increases with the outermost radius of the fit. Temperature profiles of different simulated clusters are also similar. Typically the temperature is almost uniform in the regions that emit most of the X-ray flux but drops at larger radii. The gas temperature and dark matter velocity dispersion in equilibrium clusters give values of βT≡ μmp σ2DM/KT which are consistent with unity provided an X-ray emission-weighted temperature is used. Larger values of βT are found in merging objects where there is a transient boost in the velocity dispersion of the system. Thus βT> 1 may be an observational indicator of merging in real clusters. The similar structure of clusters of differing mass results in scaling relations between the X-ray and dynamical properties of clusters identified at any given redshift. These scalings are inconsistent with the observed slope of the luminosity-temperature relation or the observed sense of evolution of the cluster luminosity function. This suggests that the central properties of intracluster medium are determined by non-gravitational process such as radiative cooling or substantial pre-heating at high redshift.