Observed relation between D_n4000 and log₁₀(−EW_[OII]) for individual galaxies (small symbols) and median per stellar mass bin (large symbols), colour-coded by their stellar mass. The small purple dots show the values for galaxies with log₁₀(M_⋆/M_⊙) < 10. Different symbols correspond to different over-densities. The dashed lines are linear fits to the data of individual galaxies in two stellar mass sub-samples (see text for details): the steeper slope is the fit for galaxies with 10 <  log₁₀(M_⋆/M_⊙) < 11, and the shallower slope is the fit for galaxies with log₁₀(M_⋆/M_⊙) > 11. This highlights the underlying anti-correlation between the observed strength of the [OII] emission and the strength of the 4000 Å break. We show as a black contour the location of 85% of the zCOSMOS sample at 0.48 <  z <  1.2 with stellar masses greater than 10¹⁰ M_⊙ (Vergani et al. 2010).