Figure showing the results of simulations that we made to test the influence of the S/N on the measured EW for the two absorption features used in this work as metallicity indicators. Top: Relation between the measured EW (and recovered stellar metallicity on the left y-axis, using the calibrations in Equations 1) and input spectrum S/N for the 1501 Å index. This is obtained through Monte Carlo simulations where Gaussian noise is added to Starburst99 templates with two different metallicities Z_⋆ = 0.2 and 0.4 × Z_⊙. The vertical green line highlights the S/N range of the stacks that we analyze in our work. Bottom: Same as the upper panel, but for the 1719 Å index.