Line-index inference on the [α/Fe] abundance of the relic candidates. In this index-index plot, the classical Mg_b index (Trager et al. 2008) is plotted against the Fe₂₄ index, which is the mean of the 24 iron lines present in the spectral range we considered here (3500−6500). The grid represents the strengths of MILES SSP models with varying metallicity (dotted) and [α/Fe] (solid; we colour-coded – in black – the original models and – in green – the interpolated ones), as indicated in the plot but with a fixed age of 10 Gyr. To show the effect of a varying age, which does not affect the inference on [α/Fe], we also plot younger models with an age of 5 Gyr with less thickness. Black arrows in the top left of each panel indicate the direction in which the model grid will move if the corresponding parameters increase. Both galaxies and SSP line strengths are calculated after convolving the spectra to a final, common resolution of σ = 270 km s⁻¹.