The narrow-line region (NLR) consists of gas clouds ionized by the strong radiation from the active galactic nucleus (AGN), distributed in the spatial scale of AGN host galaxies. The strong emission lines from the NLR are useful to diagnose physical and chemical properties of the interstellar medium in AGN host galaxies. However, the origin of the NLR is unclear; the gas clouds in NLRs may originally be in the host and photoionized by the AGN radiation, or they may be transferred from the nucleus with AGN-driven outflows. In order to study the origin of the NLR, we systematically investigate the gas density and velocity dispersion of NLR gas clouds using a large spectroscopic data set taken from the Sloan Digital Sky Survey. The [S ii] emission-line flux ratio and [O iii] velocity width of 9571 type-2 Seyfert galaxies and 110041 star-forming galaxies suggest that the gas density and velocity dispersion of NLR clouds in Seyfert galaxies (ne ≈ 194 cm−3 and $\sigma _{\rm [O\,{\small III}]} \approx 147\:$km s−1) are systematically larger than those of clouds in H ii regions of star-forming galaxies (ne ≈ 29 cm−3 and $\sigma _{\rm [O\,{\small III}]} \approx 58\:$km s−1). Interestingly, the electron density and velocity dispersion of NLR gas clouds are larger for Seyfert galaxies with a higher [O iii]/Hβ flux ratio, i.e., with a more active AGN. We also investigate the spatially-resolved kinematics of ionized gas clouds using the Mapping Nearby Galaxies at the Apache Point Observatory (MaNGA) survey data for 90 Seyfert galaxies and 801 star-forming galaxies. We find that the velocity dispersion of NLR gas clouds in Seyfert galaxies is larger than that in star-forming galaxies at a fixed stellar mass, at both central and off-central regions. These results suggest that gas clouds in NLRs come from the nucleus, probably through AGN outflows.

中文翻译：

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Seyfert 星系窄线区域的气体云是否来自它们的原子核？

窄线区域（NLR）由来自活动星系核（AGN）的强辐射电离的气体云组成，分布在AGN宿主星系的空间尺度上。NLR 的强发射线有助于诊断 AGN 宿主星系中星际介质的物理和化学性质。然而，NLR 的起源尚不清楚。NLR 中的气体云可能最初在宿主中并被 AGN 辐射光离子化，或者它们可能通过 AGN 驱动的流出物从原子核转移。为了研究 NLR 的起源，我们使用从斯隆数字巡天获取的大型光谱数据集系统地研究了 NLR 气体云的气体密度和速度分散。9571个2型Seyfert星系和110041个恒星形成星系的[S ii]发射线通量比和[O iii]速度宽度表明NLR云在Seyfert星系（ne≈194 cm- 3 和 $\sigma _{\rm [O\,{\small III}]} \约 147\:$km s−1) 系统地大于恒星形成星系 H ii 区域中的云（ne ≈ 29 cm−3 和 $\sigma _{\rm [O\,{\small III}]} \约 58\:$km s−1)。有趣的是，对于具有较高[O iii]/Hβ通量比的Seyfert星系，即具有更活跃的AGN，NLR气体云的电子密度和速度色散更大。我们还使用 Apache Point 天文台 (MaNGA) 的 90 个 Seyfert 星系和 801 个恒星形成星系的调查数据研究了电离气体云的空间分辨运动学。我们发现，在中心和非中心区域，Seyfert 星系中 NLR 气体云的速度弥散大于恒星形成星系中固定恒星质量的速度弥散。这些结果表明，NLR 中的气体云可能来自原子核，可能是通过 AGN 流出。