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The evolution of the properties of the hot gas that fills the potential well of galaxy clusters is poorly known, since models are unable to give robust predictions, and observations lack a sufficient redshift leverage and are affected by selection effects. Here, with just two high‐redshift,   z ≈ 1.8 , clusters avoiding selection biases, we obtain a significant extension of the redshift range and we begin to constrain the possible evolution of the X‐ray luminosity versus temperature relation. The two clusters, JKCS 041 at   z = 2.2  and ISCS J1438+3414 at   z = 1.41 , are, respectively, the most distant and the second most distant clusters, overall, that can be used for studying scaling relations. Their location in the X‐ray luminosity versus temperature plane, with an X‐ray luminosity five times lower than expected, suggests at the 95 per cent confidence level that the evolution of the intracluster medium has not been self‐similar in the last three‐quarters of the age of the Universe. Our conclusion is reinforced by data on a third, X‐ray‐selected, high‐redshift cluster, too faint for its temperature when compared to a sample of similarly selected objects. Our data suggest that non‐gravitational effects, such as the baryon physics, influence the evolution of galaxy clusters. Precise knowledge of evolution is central for using galaxy clusters as cosmological probes in planned X‐ray surveys, such as  WFXT  or  JDEM .

Cluster X‐ray luminosity–temperature relation at z ≳ 1.5