Galaxy surface density in groups as a function of EW of the associated absorbers (Mg II for C1, C6, and C7, and Al II for C3, respectively). Number density is computed by counting the number of galaxies (N_gal), and dividing for the effective area (A_eff) as described in Sect. 5.3. The associated uncertainties are obtained from Poisson statistics for low number counts (Gehrels 1986). The blue solid line and gray shaded area are the best fit linear model (N_gal/A_eff=a×(EW/Å)+b Mpc⁻², with a = 4±2, b = 4±3) and 1σ confidence interval, respectively. The Spearman's rank correlation coefficient and the corresponding p-value are also reported.