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The extreme synchronicity of stellar ages of red galaxies in the JKCS041 cluster at z=2.2

Above redshift z ~ 1.4, we known little or nothing about the stellar ages of red galaxies in clusters, yet at these high redshifts important changes are predicted by current renditions of galaxy formation models embedded in the standard hierarchical paradigm of structure formation. Red-sequence galaxies in the cluster JKCS041 at z=2.2 show a tight distribution in colour that indicates a star formation history that is highly synchronized across galaxies. Specifically, we measure a spread in stellar age of 160+/-30 Myr, in marked disagreement with the current understanding of how massive red galaxies form in clusters, i.e. if they are produced somewhat stochastically in merging episodes that sometimes involve gas, hence star formation. The existence of a tight distribution in colour when the universe was at one quarter of its current age implies that mechanisms that have not yet been implemented in current galaxy formation scenarios long ago began to shape the star formation history of red cluster galaxies.