Model predictions of emission-line flux ratios, i.e., [Ne v]λ3426/[O iii]λ5007 (left, a), [O ii]λ3727/[O iii]λ5007 (middle, b), and [Ne iii]λ3869/[O iii]λ5007 (right, c), as a function of NLR metallicities. Black solid, dashed, dot-dashed, and dotted lines are the photoionization model calculations with (log nH, log U) = (1.0, −1.5), (1.0, −3.5), (4.0, −1.5), and (4.0, −3.5), respectively. The left panel shows only two high-U models because [Ne v]λ3426 line fluxes in low-U models are fainter than the [O iii]λ5007 fluxes. Horizontal lines show the 3σ upper limits of our sample.
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