Diagnostic diagrams proposed by Nakajima & Maiolino (2022) to differentiate ionization by Pop III stars from that by other high-energy sources. The markers represent zero-age, pure binary-star GALSEVN models with different metallicities and IMFs, color-coded as indicated on the bottom. The gray-shaded area denotes the region identified by Nakajima & Maiolino (2022) as populated by Pop III stars, while the other colored regions correspond to areas populated by primeval DCBHs, evolved (Pop II) galaxies, and AGNs, as indicated. In panel (b), the horizontal arrows for Z = 10⁻¹¹ and 10⁻⁶ indicate the positions of corresponding models with n_H = 10² cm⁻³ (left-pointing) and 10⁴ cm⁻³ (right-pointing).