Scaling laws in X - ray Galaxy Clusters

We present a study of the integrated physical properties of a sample of 28 X-ray galaxy clusters observed with Chandra at redshift between 0.4 and 1.3. In particular, we have twelve objects in the redshift range 0.4–0.6, five between 0.6 and 0.8, seven between 0.8 and 1 and four at z >1.0, compounding the largest sample available for such a study. We focus particularly on the properties and evolution of the X-ray scaling laws. We fit both a single and a double β−model with the former which provides a good representation of the observed surface brightness profiles, indicating that these clusters do not show any significant excess in their central brightness. By using the best-fit parameters of the β−model together with the measured emission-weighted temperature (in the range 3–11 keV), we recover gas luminosity, gas mass and total gravitating mass out to R500. We observe scaling relations steeper than expected from self-similar model by a significant (> 3σ) amount in the L− T and Mgas − T relations, by a marginal value in the Mtot − T and L−Mtot relations. The degree of evolution of the Mtot − T relation is found to be consistent with the expectation based on the hydrostatic equilibrium for gas within virialized dark matter halos. We detect hints of negative evolution in the L− T , Mgas − T and L−Mtot relations, thus suggesting that systems at higher redshift have lower X-ray luminosity and gas mass for fixed temperature. In particular, when the 16 clusters at z > 0.6 are considered, the evolution becomes more evident and its power-law modelization is a statistically good description of the data. In this subsample, we also find a significant evidence of positive evolution, such as (1 + z), in the E 4/3 z S − T relation, where the entropy S is defined as T/n 2/3 gas and is measured at 0.1R200. Such results point toward a scenario in which a relatively lower gas density is present in high–redshift objects, thus implying a suppressed X–ray emission, a smaller amount of gas mass and a higher entropy level. This represents a non–trivial constraint for models aiming at explaining the thermal history of the intra–cluster medium out to the highest redshift reached so far.