The radial structure of galaxy groups and clusters

Simple self-consistent models of galaxy groups and clusters are tested against the results of high-resolution adiabatic gas-dynamical simulations. We investigate two models based on the existence of a 'universal' dark matter density profile and two versions of the beta-model. The mass distribution of relaxed clusters can be fitted by phenomenological formulae proposed in the literature. Haloes that have experienced a recent merging event are systematically less concentrated and show steeper profiles than relaxed objects near the centre. The hot X-ray emitting gas is found to be in approximate hydrostatic equilibrium with the dark matter potential, and it is well described by a polytropic equation of state. Analytical formulae for the gas density and temperature can be derived from these premises. Although able to reproduce the X-ray surface brightness, the beta-model is shown to provide a poor description of our numerical clusters. We find strong evidence of a 'universal' temperature profile that decreases by a factor of 2-3 from the centre to the virial radius. We claim that the spherically averaged profiles of all physical properties of galaxy groups and clusters can be fitted with only two free parameters. Numerical resolution and entropy conservation play a key role in the shapes of the simulated profiles at small radii.