Median number of satellites as a function of the surface brightness (μ_r^cen) for all central galaxies in the analysed mass range. The galaxies are split by stellar mass in low-mass ($10\le log(M_{\star }^{\mathrm{cen}}/{\mathrm{M}}_{\odot })<10.36$, indigo line) and high-mass ($10.36\le log(M_{\star }^{\mathrm{cen}}/{\mathrm{M}}_{\odot })\le 11.0$, pink line), while the base sample is presented in grey. The shadowed areas indicate the median bootstrap error.