Stacked spectra between 1000 and 2500 Å in three bins of increasing β_median: −1.98 (blue line), −1.51 (gray line), and −1.10 (red line; see also Table 3). The absorption complexes used in this work to estimate the metallicity, located at 1501 and 1719 Å, are highlighted with yellow vertical bands. Top two panels: three stacks are plotted together, and we zoom-in around the two metallicity indexes. A thicker line specifies both the width of each absorption complex, while shaded gray areas indicate the spectral ranges for the estimation of the pseudo-continuum. Spectral stacks of galaxies with redder UV slopes have deeper absorption features, hence higher metallicity.