The distribution of the β parameter. Upper panel: histogram of the β solutions derived from Eq. (14) for the real ETGs (black line) compared with the distribution derived for the galaxies of the Illustris-1 simulation when β is calculated looking at the variation in luminosity and velocity dispersion in two close redshift epochs (z = 0.2 and z = 0). Lower panel: histogram of the β solutions derived from Eq. (14) for the real ETGs (black line) and the Illustris-1 galaxies (red line) that are close to the virial equilibrium. The solid black line gives the average value of β.