The distribution of galaxies in optical diagnostic diagrams can provide information about their physical parameters when compared with ionization models under proper assumptions. By using a sample of central emitting regions from the MaNGA survey, we find evidence of the existence of upper boundaries for narrow-line regions (NLRs) of active galactic nuclei (AGN) in optical BPT diagrams, especially in the diagrams involving [S II]$\lambda \lambda$6716, 6731/H$\alpha$. Photoionization models can well reproduce the boundaries as a consequence of the decrease of [S II]$\lambda \lambda$6716, 6731/H$\alpha$ and [O III]$\lambda$5007/H$\beta$ ratios at very high metallicity. Whilst the exact location of the upper boundary in the [S II] BPT diagram only weakly depends on the electron density of the ionized cloud and the secondary nitrogen prescription, its dependence on the shapes of the input SEDs is much stronger. This allows us to constrain the power-law index of the AGN SED between 1 Ryd and $\sim100$ Ryd to be less than or equal to $-1.40\pm 0.05$. The coverage of the photoionization models in the [N II] BPT diagram has a stronger dependence on the electron density and the secondary nitrogen prescription. With the density constrained by the [S II] doublet ratio and the input SED constrained by the [S II] BPT diagram, we find that the extent of the data in the [N II] BPT diagram favors those prescriptions with high N/O ratios. Although shock-ionized cloud can produce similar line ratios as those by photoionization, the resulting shapes of the upper boundaries, if exist, would likely be different from those of a photoionizing origin.

中文翻译：

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光学诊断图中活动星系核区域的上边界

与适当假设下的电离模型相比，光学诊断图中的星系分布可以提供有关其物理参数的信息。通过使用来自 MaNGA 调查的中心发射区样本，我们发现光学 BPT 图中活动星系核 (AGN) 的窄线区 (NLR) 存在上边界的证据，特别是在涉及 [S II ]$\lambda\lambda$6716, 6731/H$\alpha$。由于 [S II]$\lambda\lambda$6716, 6731/H$\alpha$ 和 [O III]$\lambda$5007/H$\beta$ 比率的降低，光电离模型可以很好地再现边界高金属度。虽然 [S II] BPT 图中上边界的确切位置仅微弱地取决于电离云的电子密度和二次氮处方，它对输入 SED 形状的依赖要强得多。这允许我们将 AGN SED 的幂律指数限制在 1 Ryd 和 $\sim100$ Ryd 之间，使其小于或等于 $-1.40\pm 0.05$。[N II] BPT 图中光电离模型的覆盖范围对电子密度和二次氮处方有更强的依赖性。由于密度受 [S II] 双峰比约束，输入 SED 受 [S II] BPT 图约束，我们发现 [N II] BPT 图中的数据范围有利于那些具有高 N/O 的处方比率。尽管激波电离云可以产生与光电离相似的线比，但由此产生的上边界形状（如果存在）可能与光电离起源的形状不同。这允许我们将 AGN SED 的幂律指数限制在 1 Ryd 和 $\sim100$ Ryd 之间，使其小于或等于 $-1.40\pm 0.05$。[N II] BPT 图中光电离模型的覆盖范围对电子密度和二次氮处方有更强的依赖性。由于密度受 [S II] 双峰比约束，输入 SED 受 [S II] BPT 图约束，我们发现 [N II] BPT 图中的数据范围有利于那些具有高 N/O 的处方比率。尽管激波电离云可以产生与光电离相似的线比，但由此产生的上边界形状（如果存在）可能与光电离起源的形状不同。这允许我们将 AGN SED 的幂律指数限制在 1 Ryd 和 $\sim100$ Ryd 之间，使其小于或等于 $-1.40\pm 0.05$。[N II] BPT 图中光电离模型的覆盖范围对电子密度和二次氮处方有更强的依赖性。由于密度受 [S II] 双峰比约束，输入 SED 受 [S II] BPT 图约束，我们发现 [N II] BPT 图中的数据范围有利于那些具有高 N/O 的处方比率。尽管激波电离云可以产生与光电离相似的线比，但由此产生的上边界形状（如果存在）可能与光电离起源的形状不同。