Average intrinsic spectrum from ULX populations assuming different CO masses (color) and spectrum models (line style), as well as stellar populations from BPASS (black line for solar metallicity and gray for one-tenth of the solar metallicity). All spectra were normalized by the SFR and the metallicity (Z); since we assumed that the XLF scales with Z_⊙/Z, there is no variation in the ULX spectra, contrary to the stellar populations. The units in the y axis are the same as the provided data (see Table 1). The ionization potentials of H I, He II, and Ne V are indicated with vertical lines. The shaded areas denote the regions in the spectrum that we do not consider as ionizing when calculating Q_HI, Q_HeII, and Q_NeV (see Figs. 2 and 3).