Clusters of galaxies at high redshift (z>1) are vitally important to understand the evolution of the large scale structure of the Universe, the processes shaping galaxy populations and the cycle of the cosmic baryons, and to constrain cosmological parameters. After 13 years of operation of the Chandra and XMM-Newton satellites, the discovery and characterization of distant X-ray clusters is proceeding at a slow pace, due to the low solid angle covered so far, and the time-expensive observations needed to physically characterize their intracluster medium (ICM). At present, we know that at z>1 many massive clusters are fully virialized, their ICM is already enriched with metals, strong cool cores are already in place, and significant star formation is ongoing in their most massive galaxies, at least at z>1.4. Clearly, the assembly of a large and well characterized sample of high-z X-ray clusters is a major goal for the future. We argue that the only means to achieve this is a survey-optimized X-ray mission capable of offering large solid angle, high sensitivity, good spectral coverage, low background and angular resolution as good as 5 arcsec.
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