Spectral energy distributions of the ionizing continuum adopted in our photoionization models, which are starburst99 constant star-formation model spectra (Leitherer et al. 1999) with an age of 1 Myr and an α = 2.35 IMF (M_low = 1 M_⊙ and M_up = 100 M_⊙). Although most of our analysis adopts these spectra, models with different ages (3, 10, and 30 Myr) are investigated in Sect. 3.3. The blue, cyan, green, orange, and red spectra denote a model with Z_star = 0.05 Z_⊙, 0.2 Z_⊙, 0.4 Z_⊙, 1.0 Z_⊙, and 2.0 Z_⊙, respectively. The Z_star = 2.0 Z_⊙ spectrum is used also for our Z_gas = 3.0 Z_⊙ calculations (see the main text). All spectra are normalized at 5000 Å (≃ 0.18 Ryd).