Rate of He II (left) and Ne V (right) ionizing photons from ULX populations in galaxies as a function of the gas-phase metallicity (solar value indicated with vertical magenta lines). The black lines show the prediction from stellar populations using BPASS models. The colored solid lines depict the mean value from ULXs assuming three different combinations of CO masses and spectrum models (see legend), covering the full range of our (nine) estimates. The dashed lines show the upper 99% limit to give a sense of the scatter due to the stochastic nature of ULXs.