Two-dimensional diagnostic diagram displaying the D4000 continuum break vs. log₁₀(EW([O III])²), the DO3 diagnostic diagram. Within this plot we observe the spatial distribution of galaxies predominantly influenced by a single principal excitation mechanism (pure-activity classes), as well as the positions of those with mixed-activity classes. The black lines are the boundaries separating the different classes (see Sect. 4.6; for the equation of each boundary see Appendix B). The contours representing each pure-activity class are computed using a kernel density function and illustrate the 68% (inner contour) and 90% (outer contour) population density levels. Labels: pure SB (pure starburst), pure AGN (pure active galactic nucleus), pure Pas (pure passive).