Correlations of β with SFR, Age and I_e. Left panel: relationship between β and SFR. Each curve labeled M_B(T_G) is identified according to the color-coding adopted in the previous figures. The total baryonic mass is the homology parameter separating each curve from the others. More similar trends are found for the luminosity L_B and L_V, and for the velocity dispersion σ, which are not shown here for the sake of brevity. In all three relations β mirrors the behavior of the SFR, the luminosity in turn, and finally the velocity dispersion. The SFR is in M_⊙ yr⁻¹. Middle panel: relationship between β and age (in Gyr). Symbols and color-coding have the same meaning as in the left panel. Right panel: relationship between β and I_eB in L_⊙ pc⁻². The lines corresponding to different masses of galaxies have been shifted by 0.1; they collapse to a unique curve given by β = 3.159Log(I_eB)−2.003. The long dashed line is the best fit of the theoretical data. An identical relation can be found for I_eV with the same slope but slightly different zero point.