Evolution at z 0.5 of the X-ray properties of simulated galaxy clusters : comparison with observational constraints

We analyse the X-ray properties of a sample of local and high-redshift galaxy clusters extracted from a large cosmological hydrodynamical simulation. This simulation has been realized using the tree + SPH (smoothed particle hydrodynamic) code GADGET-2 for a A-cold dark matter (ACDM) model. It includes radiative cooling, star formation and supernova feedback and allows the thermodynamic structure of clusters to be resolved radially up to redshift z = 1 in a way that is not yet completely accessible to observations. We consider only objects with T ew > 2 keV to avoid the large scatter in the physical properties present at the scale of groups and compare their properties to recent observational constraints. In our analysis, we adopt an approach that mimics observations, associating with each measurement an error comparable with recent observations and providing best-fitting results via robust techniques. Within the clusters, baryons are distributed among (i) a cold neutral phase, with a relative contribution that increases from less than 1 per cent to 3 per cent at higher redshift, (ii) stars, which contribute about 20 per cent, and (iii) the X-ray-emitting plasma, which contributes 80 (76) per cent at z = 0 (1) to the total baryonic budget. A depletion of the cosmic baryon fraction of ∼7 per cent (at z = 0) and 5 per cent (at z = 1) is measured at the virial radius, R vir , in good agreement with adiabatic hydrodynamical simulations. We confirm that, also at redshift z > 0.5, power-law relations hold between the gas temperature T, the bolometric luminosity L, the central entropy S, the gas mass M gas and the total gravitating mass M tot , and that these relations are steeper than predicted by simple gravitational collapse. A significant negative evolution in the L-T and L-M tot relations and positive evolution in the S-T relation are detected at 0.5 < z < 1 in this set of simulated galaxy clusters. This is partially consistent with recent analyses of the observed properties of z ≥ 0.5 X-ray galaxy clusters. By fixing the slope to the values predicted by simple gravitational collapse, at high redshift we measure normalizations lower than the observed estimates by 10-40 per cent in the L-T, M tot -T, M gas -T, f gas -T and L-M tot relations. This suggests either that the amount of hot X-ray-emitting plasma measured in the central regions of simulated systems is smaller than the observed one or that systematically higher values of gas temperatures and total masses than actually measured are recovered in the present simulated data set.