Searching for cool core clusters at high redshift

Aims. We investigate the detection of Cool Cores (CCs) in the distant galaxy cluster population with the purpose of measuring the CC fraction out to redshift 0.7 <= z < 1.4. Using a sample of nearby clusters spanning a wide range of morphologies, we define criteria to characterize cool cores, which are applicable to the high-redshift sample. Methods. We analyzed azimuthally-averaged surface brightness (SB) profiles with the known scaling relations, and we fitted single/double beta models to the data. Additionally, we measured a surface brightness concentration, c(SB), as the ratio of the peak over the ambient SB. To verify that this is an unbiased parameter as a function of redshift, we developed a model independent cloning technique to simulate the nearby clusters as they would appear at the same redshifts and luminosities as those in the distant sample. This method is based on the application of the cosmological surface brightness dimming to high-resolution Chandra images, assuming no intrinsic cluster evolution. We obtained a more physical parameterization of the CC presence by computing the cooling time at a radius of 20 kpc from the cluster center. Results. The distribution of the SB concentration and the stacked radial profiles of the low-z sample, combined with published information on the CC properties of these clusters, show 3 degrees of SB cuspiness: non-CC, moderate, and strong CC. The same analysis applied to the high-z clusters reveals two regimes: non-CC and moderate CC. The cooling time distribution corroborates this result by showing a strong negative correlation with cSB. Conclusions. Our analysis indicates a significant fraction of distant clusters harboring a moderate CC out to z = 1.4, similar to those found in the local sample. The absence of strong cooling is likely linked with a higher merger rate expected at redshift z > 0.7, and should also be related to the shorter age of distant clusters, implying less time to develop a cool core.