Data and theoretical models for the β − I_eV plane. Data from the different sources are plotted: (i) Burstein et al. (1997, light blue). Three sequences are seen: GCGs and GCs, ETGs (no evidence of star formation), LTGs and DGs (evidence of ongoing star formation). By construction, these data are well behaved with no evidence of dispersion. (ii) WINGS data (red squares) showing large dispersion in both coordinates, log I_e is always positive, and β can be very large both positive and negative. (iii) ETGs of Bernardi et al. (2010, open green circles), but limited to z ≤ 0.02. The Illustris-1 model galaxies are indicated by the blue dots; their distribution closely mimics that of the observational data. Finally, the long dashed red line shows the present-day position on the β − I_e plane of our models for the reference case (with τ = 1 Gyr, z_f = 10, and no galactic winds). This line coincides with the lower border of the Illustris-1 distribution in the β > 0 hemi-plane.