α_ox index as function of L_ν,UV (2500 Å) luminosity for our simulated ULX population (BH or NS primaries). The best fit to α_ox − L_ν,UV simulated data in our population (black solid line) and observed ULXs data in Sonbas et al. (2019) (black points) are also shown. The top, middle, and bottom panels correspond to metallicities of 0.025 Z_⊙, 0.20 Z_⊙, and 0.40 Z_⊙, respectively. We also show the α_ox − L_ν,UV fits for the BH-ULX and NS-ULX systems separately.