Diagnostic diagrams comparing Pop III (heavy orange line) and several AGN photoionization models. The AGN model U-sequences shown here are: α = −1.5 (thick black line); α = −2.0 (thin black line); α = −1.5 with “front view” (“back-mirror”) (dot-dashed line); photoionization by our F_esc filtered AGN continuum (dashed line). All models shown in this figure use Z/Z_⊙ = 0.01. See main text for further details. The model loci cover the full range of ionization parameter in our model grid, i.e., −5.55 < log U < 0.25.