[Mg IV]/Hu-12 ratio for an instantaneous burst of star formation as a function of stellar age. The circles, diamonds, and stars indicate the ratios for 0.4 Z_⊙, Z_⊙, and 2 Z_⊙. The filled (empty) symbols connected by a solid (dashed) line corresponds to a standard (top heavy) IMF. The grid with n_H = 10² cm⁻³ is plotted. The color of the lines indicates the ionization parameter, U₀.