Lyman-Werner photon-production rate (in the wavelength range 912 Å–1150 Å) as a function of the stellar-population age for the same GALSEVN models as in Fig. 1 (normalized to a total initial stellar mass of 1 M_⊙ integrated over 0.1–300 M_⊙). The models with Z = 0.0001 and 0.001, which are nearly indistinguishable from the Z = 0.0005, are omitted here for clarity. For reference, the predictions of Incatasciato et al. (2023) for populations of Pop II stars (using Z = 0.0005 BPASS models with the Chabrier 2003 IMF) and Pop III stars (using zero-metallicity models from Raiter et al. 2010 with a lognormal IMF over 1–500 M_⊙) are shown as dotted and solid black curves, respectively.