RGB composite colour images of the 1.4′ × 1.4′ field centred on the galaxy overdensity. The R channel of both images corresponds to the K_s band; the G and B channels corresponds to the J and z bands in the left image and to the B and z bands in the right one. The B and z images were taken using the Suprime-Cam instrument on the Subaru telescope while the J and K_s images are a composite of MOIRCS and ISAAC data, on the Subaru and VLT observatories. The BzK_s image shows how the red galaxies are basically unseen at optical wavelengths. On the left, the white overlapping contours show the 1, 2, and 3σ significance levels of the diffuse X-ray emission in the XMM-Newton image, after subtraction of a point source seen in the Chandra image and smoothing with a 8′′ radius PSF (as described in Sect. 5). On the right, they show the 10, 20, and 30σ levels above the background galaxy number density, computed using the Σ₅ estimator.