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We report the discovery of an X-ray-luminous galaxy cluster at z = 1.26. RX J0848.9 + 4452 was selected as an X-ray cluster candidate in the ROSAT Deep Cluster Survey on the basis of its spatial extent. Deep optical and near-IR imaging have revealed a galaxy overdensity around the peak of the X-ray emission, with a significant excess of red objects with J-K colors typical of elliptical galaxies at z > 1. Spectroscopic observations at the Reck II Telescope have secured six galaxy redshifts in the range 1.257 1.
Rosati et al. (Thu,) studied this question.