Correlations between the absolute magnitude of the BCG (top left panel), the magnitude gap (top right panel), the X-ray luminosity (bottom left panel), and redshift (bottom right) vs. the density of galaxies brighter than −22 within 50 Mpc. The area is computed using Pick’s theorem, so footprint incompleteness is properly taken into account. Colour coding and symbols are the same as in Fig. 2. Big open circles represent FGs with central galaxies brighter than −22.