Variation of the EW₀ of He II 1640 Å emission (y-axis) with the absorption component of the N Vλ1240 P-Cygni line profile (x-axis) on the left and the same with the absorption component of C IVλ1550 P-Cygni line profile (x-axis) on the right as a function of age assuming a continuous star formation history. Measurements of our UV-bright galaxies are marked with solid circles. Different tracks represent models with different IMF upper mass limits (M_up). Original BPASS models with M_up = 100 M_⊙ and 300 M_⊙ (using the wind prescription from Vink et al. 2001) are shown in red and orange, respectively, while BPASS models with the new VMS template spectra of Martins & Palacios (2022) are shown in yellow, green, blue, pink, and brown for M_up = 175 M_⊙, 225 M_⊙, 275 M_⊙, 325 M_⊙, and 475 M_⊙ respectively.