Top: as in Fig. 3, but on the y-axis we show the velocity dispersion along the line of sight (σ_los). Black points are σ_los at z = 0 as stored in the Millennium Database; orange points are σ_los computed using the redshift of the galaxies at z ~ 2.9 with the biweight method (see text for details); red symbols represent the median values of the orange points, in bin of 0.2 in log (M/M_⊙), and their bars show the 25% and 75% of the distribution. The green cross represents the velocity dispersion of our overdensity, as computed at the beginning of Sect. 3. Bottom: the same as in the top panel, but on the y-axis there is the maximum separation in redshift at z ~ 2.9 of the galaxies in each proto-cluster. The green cross represents the Δz_max of our overdensity (see Sect. 3).