Star formation history of the four extreme relics (M_⋆≥99% at t_{BB=3 Gyr}). The systems are ordered in descending order of degree of relicness. Upper plots of each panel: cumulative stellar mass fraction formed in time, starting from the BB and up to the age of the Universe at the redshift of each system, which is plotted as a vertical dashed line. The other vertical line shows instead the 2 Gyr threshold used in the text to classify an object as an extreme relic. Black lines show the result obtained from the unregularised PPXF fit with the best value of [α/Fe] estimated from line indices. The shadow regions show the uncertainties on such estimates (±0.1). Finally, the green lines show the results of the fit performed with the regularisation set to the MAX_REGUL. Bottom plots of each panel: 2D density plot of the weights attributed to the SSP models by PPXF in the log (age)–[M/H] space. The colour bar shows the weight fractions of the SSP models with the corresponding age and metallicity used in the fit.