Relative output of hydrogen ionizing photons to UV continuum light, measured at 1500 Å restframe, Q_H / L₁₅₀₀, as a function of metallicity for constant star formation over 1 Gyr (left panel) and very young bursts (right panel). Q_H / L₁₅₀₀ is given in L_λ units on the left side of each panel, and in L_ν units on the right. Results for different IMFs are shown using the colour codes and labels summarised in Table 1. The shaded area indicates the critical metallicity range where the IMF is expected to change from a “normal” Salpeter-like regime to a more massive IMF (see text).