Density map, based on u,  g,  r,  i, and z images, of the CFHT/MEGACAM 1° ×1° field around CL1301.7−1139. The colour-coding indicates the log₁₀ of the density of galaxies averaged over the ten nearest neighbours. Black contours are at 1 and 3σ above the field mean density. The grey points identify all galaxies with a z_spec within 5σ_cl of the cluster redshift. The inner and outer white circles are positioned at a R₂₀₀ and 5R₂₀₀ radius, respectively. The grey bands indicate the gaps between the MEGACAM CCDs. The green squares show our ALMA targets. The pink outline shows the position of the star-forming galaxies with low gas fractions (see Fig. 8 and Sect. 4.1).