Nebular ionizing-photon production efficiency ${\xi }_{\mathrm{ion}}^{\mathrm{HII}}$ as a function of the stellar-population age for the same GALSEVN models as in Fig. 1. The time evolution is followed up to 40 Myr, but the H II regions surrounding the young stellar populations are expected to have been disrupted before at most 10 Myr (Murray 2011; Ma et al. 2015). The vertical dashed line marks the time of appearance the first SN in the lowest-metallicity models, corresponding to the beginning of chemical enrichment. The models with top-heavy IMFs are plotted until their absolute UV magnitude reaches M_UV = −14, marked by cyan stars. The blue and green-shaded regions correspond to ${\xi }_{\mathrm{ion}}^{\mathrm{HII}}$ constraints derived from observations at low to intermediate redshifts, from different works. The gray-shaded region corresponds to the canonical values usually adopted for ${\xi }_{\mathrm{ion}}^{\ast }$ in simulations.