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

Abstract

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 been shown also 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, one of the largest sample investigated so far with X-ray spectroscopy and with mass reconstructed using hydrostatic equilibrium equation, with a particular care in reconstructing 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 the deprojected spectral temperature profiles via the hydrostatic equilibrium equation in order to constrain the concentration and the scale radius, which are the parameters that describe a Navarro-Frenk-White profile for each of the cluster in our sample. The temperature profile, that combined with the observed gas density profile reproduces the best-fit mass model, is then used to reconstruct the profiles of the 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 order to increase the precision of the analysis, also in 3 independent redshift bins. The gas mass fraction is then used in order 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 informations about the thermal distribution of the gas, shows a slight linear positive evolution with the redshift and the concentration of the dark matter distribution. A prevalence of non-cool-core, disturbed systems, as we observe at higher redshifts, can explain such behaviours.

X-ray analysis of thePlanck-detected triplet-cluster system PLCK G334.8-38

“…These observations indicate that cluster B falls rather into the class of disturbed noncool-core clusters. Our tentative classification for clusters A and B is also supported by a comparison with previous entropy measurements with large cluster samples (Cavagnolo et al 2009;Ghirardini et al 2017). For cluster C, it appears that the entropy profile flattens around ∼0.5 R 500 but due to the unconstrained core region, As for cluster C the core-model parameters were not properly determined, we show for n e,Core its 3σ upper limit ( 99.73%) and for T Core its fit boundaries.…”

X-ray analysis of the Planck-detected triplet-cluster system PLCK G334.8-38

“…These observations indicate that cluster B falls rather into the class of disturbed noncool-core clusters. Our tentative classification for clusters A and B is also supported by a comparison with previous entropy measurements with large cluster samples (Cavagnolo et al 2009;Ghirardini et al 2017). For cluster C, it appears that the entropy profile flattens around ∼ 0.5 R 500 but due to the unconstrained core region, we cannot assess how density and temperature change towards the cluster center and therefore we refrain from making any classification.…”

The Physics of Galaxy Cluster Outskirts

“…For 10 of the clusters these profiles extend out to r 200 , while the remaining 3 extend out to around 1.2r 200 . When clumping is taken into account using the azimuthal median technique (Eckert et al 2015), the entropy profiles are mostly consistent with the Voit baseline entropy profile except for Abell 2319, which remains flat at r 200 because of the high level of non-thermal pressure discussed in Ghirardini et al (2017). The entropy profile for Abell 644 also appears flat, but the errors are large enough to be consistent with the baseline entropy profile.…”

“…When clumping is taken into account, using the azimuthal median density profile following Eckert et al (2015), the resulting decrease in the outskirts density causes the entropy profile to continue to rise, in better agreement with the Voit et al (2005) baseline entropy profile. Ghirardini et al (2017) then presented results from Abell 2319 (the strongest Planck detection in X-COP with a signal to noise of 51). Again, when the nonclumping corrected density profile is combined with the Planck pressure profile, an entropy profile which flattens beyond r 500 is found.…”