Evolution of the strength of the Mg_UV index as function of time for four metallicities: Z_⊙ (solar, black solid line), 0.2 × Z_⊙ (red dashed line), 0.4 × Z_⊙ (green dot-dashed line), and 2.5 × Z_⊙ (solid blue line). The index is computed from SEDs created from the BC03 models (Bruzual & Charlot 2003) with an exponentially delayed SFH with an e-folding time of 0.1 Gyr and no dust extinction. The index was computed directly on the synthetic spectra.