Spectral region around the ISM+stellar Si IV absorption doublet. In all the panels, the blue line is the stacked spectrum of all star-forming galaxies with C III] or He II emission selected in this work, where the error bars at each pixel represent the 1σ error. The orange line is the best-fit Gaussian made with MPFIT, assuming two Gaussian components for each side of the doublet (i.e. four Gaussians in total), as explained in the text. The red lines represent the best-fit models from Starburst99 (S99), Beagle (i.e. BC03 models), and BPASS, from top to bottom, respectively. The best-fit models have the closest stellar metallicity Z_⋆ to the median of our galaxies (Calabrò et al. 2021), that is, log (Z_⋆/Z_⊙) = − 0.7 for S99, −0.82 for BC03, and −0.85 for BPASS. We reiterate the fact that the measured Z_⋆ using BPASS calibrations is 0.1 dex lower than S99.