2D density plot showing the weights attributed to the SSP models by PPXF during the fit, in a log (age), [M/H] space (for simplicity, we also plot ages in linear scale on the top x-axis). Each column shows results for a different galaxy, while each row corresponds to the different results obtaining increasing the REGUL from 0 (top) to MAX_REGUL (bottom). The weight fractions are reported on the vertical colour-bars, where yellow indicates the maximum weight and dark blue indicates zero weight. The upper limit of the age axis is slightly higher (∼10%) than the age of the Universe at the redshift of the objects, in order to take into account possible uncertainties in the value computed from the stellar population analysis.