[论文解读] Interpretation of the emission line spectra of Seyfert 2 galaxies by multi-component photoionization models

S. Komossa, Hartmut Schulz|arXiv (Cornell University)|Jan 1, 1997

Astronomy and Astrophysical Research参考文献 2被引用 6

一句话总结

本文提出了包含窄线区（NLR）中密度非均质性的多组分光离化模型，成功再现了从紫外到近红外（NIR）的发射线谱，包括难以捉摸的高电离态线和[ SIII]线。关键结果是，亚太阳金属丰度和可变的光谱能量分布（包括一个热黑体成分）提供了自洽的拟合，解决了[ OIII]4363/5007比值长期存在的“温度问题”。

ABSTRACT

We present multi-component photoionization models allowing for local density inhomogeneities in the NLR to interpret the emission line spectra of Seyfert 2 galaxies. This approach leads to a successful match of a large set of line intensities from the UV to the NIR. In particular, the hitherto elusive NIR features [SIII]9062+9531 as well as high-ionization lines like [FeVII] are consistently fitted. The predictions of CIII] and CIV are considerably improved. From the detailed analysis of single-component photoionization models we derive the minimal radial extent of the NLR and the necessary span in density. Furthermore, we determine constraints on suggestions made about the role of matter-bounded clouds, and on proposed explanations for large [OIII]4363/5007 ratios (the so-called `temperature problem'), and assess the usability of some emission-line ratios as indicators of the ionization parameter. We find that a systematic variation of the cloud column densities in a population of matter-bounded clouds is inconsistent with the trends and correlations exhibited by the emission lines in the diagnostic diagrams. Concerning the temperature problem, the only possibility that leads to an overall consistency with the strengths of all other observed emission lines is subsolar metal abundances (as compared to e.g. the presence of dust, the existence of a high-density component, or matter-bounded clouds). In addition, the consequences of the presence of (Galactic-ISM-like) dust internal to the clouds were investigated. These models alleviate the [OIII]-ratio problem but did not lead to overall consistent fits. In our final model series, the NLR is composed of a mixture of metal-depleted (0.5 x solar) clouds with a radius-independent range in densities (10^2 to 10^5 cm^-3) distributed over a range of distances from the nucleus (galactocentric radii from at least $\\sim$ 10^{20} cm to 10^{21.5} cm, for $Q_{tot} = 10^{54}$ s^{-1}). In order to encompass the observed range of each line intensity relative to H$\\beta$, it turns out to be necessary to vary the spectral energy distribution incident on the clouds, qualitatively confirming the findings of Ho et al. (1993). We found a successful continuum sequence by adding an increasing contribution of a hot black body ($T \\approx 200000$ K) to a steep powerlaw ($\\alpha_{uv-x} \\approx -2$). These continua imply that low and high-excitation objects differ in the strength but not in the basic shape of the EUV bump.

研究动机与目标

• 为解决赛弗特2型星系发射线谱中的差异，特别是涉及高[ OIII]4363/5007比值的“温度问题”。
• 检验物质受限云团及云内尘埃作为观测线比值解释的可行性。
• 利用光离化模型确定NLR的径向范围和密度结构。
• 评估发射线比值在复杂NLR环境中作为电离参数指标的可靠性。
• 改进CIII]和CIV线的预测，这些线在以往的单组分模型中匹配效果较差。

提出的方法

• 采用包含NLR中局部密度非均质性的多组分光离化模型，模拟发射线谱。
• 使用云团密度范围（10²–10⁵ cm⁻³）和星系中心半径范围（≥10²⁰ cm至10²¹.⁵ cm），总离化光度Q_tot = 10⁵⁴ s⁻¹。
• 变化云团所受的光谱能量分布（SED），引入一个热黑体（T ≈ 200,000 K）成分，叠加在陡峭幂律（α_UV-X ≈ -2）之上。
• 测试云内尘埃（类银河系ISM）对线比值影响，评估其作用。
• 将模型预测与从紫外到近红外（NIR）的观测线强度进行比较，包括[ SIII]9062+9531和[ FeVII]。
• 评估模型趋势与诊断图及观测到的发射线行为相关性的自洽性。

实验结果

研究问题

• RQ1 包含密度非均质结构的多组分光离化模型能否再现赛弗特2型星系中观测到的全部发射线，包括高电离态和近红外特征？
• RQ2 为解决[ OIII]4363/5007比值中的“温度问题”，需要哪些物理条件——如金属丰度、云团几何结构和SED？
• RQ3 在物质受限云团群体中，柱密度的系统性变化是否与观测到的发射线趋势一致？
• RQ4 云内尘埃颗粒如何影响观测到的线比值？它们能否解决[ OIII]比值的不一致问题？
• RQ5 与单组分模型相比，使用多组分模型在CIII]和CIV线预测方面改善的程度如何？

主要发现

• 仅亚太阳金属丰度（0.5×太阳）是能够一致再现所有观测发射线（包括[ OIII]4363/5007比值）的可行解释。
• 在星系中心半径10²⁰至10²¹.⁵ cm范围内，密度为10²至10⁵ cm⁻³的金属丰度降低（0.5×太阳）云团混合，可提供一致的NLR结构。
• 在SED中引入热黑体成分（T ≈ 200,000 K），叠加在陡峭幂律（α_UV-X ≈ -2）之上，是匹配从紫外到近红外观测线强度的必要条件。
• 在物质受限云团群体中，柱密度的系统性变化与观测到的发射线趋势和相关性不一致。
• 云内尘埃颗粒（类银河系ISM）可缓解[ OIII]比值问题，但无法在所有发射线中实现整体一致的拟合。
• 模型证实，低激发与高激发赛弗特2型星系的主要区别在于电离辐射的强度，而非电离辐射谱形。