On the evolution of the entropy and pressure profiles in X-ray luminous galaxy clusters at z > 0.4

Context. Galaxy clusters are the most recent products of hierarchical accretion over cosmological scales. The gas accreted from the cosmic field is thermalized inside the cluster halo. Gas entropy and pressure are expected to have a self-similar behaviour with their radial distribution following a power law and a generalized Navarro-Frenk-White profile, respectively. This has also been shown in many different hydrodynamical simulations.
Aims: We derive the spatially resolved thermodynamical properties of 47 X-ray galaxy clusters observed with Chandra in the redshift range 0.4 <z< 1.2, which is one of the largest samples investigated so far with X-ray spectroscopy and masses reconstructed via the hydrostatic equilibrium equation, with particular care taken to reconstruct the gas entropy and pressure radial profiles. We search for deviation from the self-similar behaviour and look for possible evolution with redshift.
Methods: Under the assumption of a spherically symmetric distribution of the intracluster plasma, we combine the deprojected gas density and deprojected spectral temperature profiles via the hydrostatic equilibrium equation to constrain the concentration and scale radius, which are the parameters that describe a Navarro-Frenk-White profile for each of the clusters in our sample. The temperature profile, which combined with the observed gas density profile reproduces the best-fit mass model, is then used to reconstruct the profiles of entropy and pressure. These profiles cover a median radial interval of [0.04 R₅₀₀-0.76 R₅₀₀]. After interpolating on the same radial grid and partially extrapolating up to R₅₀₀, these profiles are then stacked in three independent redshift bins to increase the precision of the analysis. The gas mass fraction is then used to improve the self-similar behaviour of the profiles by reducing the scatter among the profiles by a factor 3.
Results: The entropy and pressure profiles lie very close to the baseline prediction from gravitational structure formation. We show that these profiles deviate from the baseline prediction as function of redshift, in particular at z> 0.75, where, in the central regions, we observe higher values of the entropy (by a factor of 2.2) and systematically lower estimates (by a factor of 2.5) of the pressure. The effective polytropic index, which retains information about the thermal distribution of the gas, shows a slight linear positive evolution with the redshift and concentration of the dark matter distribution. A prevalence of non-cool core, disturbed systems, as we observe at higher redshifts, can explain such behaviours.