We describe the results of a search for diffuse sources in the 0.4 deg 2 area of the XMM-Newton 770 ks survey of the Lockman Hole. An abundant population of sources is revealed, which is likely to stem from low-mass galaxy clusters at redshift near 1. We present the optical images in the K′ and R-bands for our candidates, suggesting a concentration of red galaxies at the position of the X-ray sources. Together with our improved understanding of the baryon physics in low-mass clusters these results reveal the important role that deep X-ray surveys will play in cosmological studies.
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