Stacked spectra of the metal-poor (top panel, [ − 2.2, −1.4]), metal-intermediate (central panel, [ − 1.4, −0.6]) and metal-rich (bottom panel, [ − 0.6, 0.2]) subsamples, in comparison with a 4 Gyr (in blue) and 13 Gyr old (in red) synthetic spectra. The values of mass, dust absorption and velocity dispersion of the synthetic spectra are set to the median literature values of each subgroup. All the spectra are normalised in the window 4500–5000 Å. At the bottom of each panel the residuals of the stacked spectra with respect to the synthetic ones are shown with corresponding colours. The Balmer series is indicated with dashed lines, other relevant spectral features are highlighted with grey boxes. It can be noticed how the different sub-groups show different continuum and spectral features properties, and how the models are able to reproduce more accurately the high-metallicity ones.