[论文解读] Dust in active nuclei. I. Evidence for "anomalous" properties

R. Maiolino, A. Marconi+5 more|ArXiv.org|Sep 30, 2000

Astrophysics and Star Formation Studies参考文献 51被引用 234

一句话总结

本文提供了观测证据，表明活动星系核（AGNs）喷流区的尘埃表现出‘异常’特性，其消光程度相对于气体柱密度显著低于银河系星际介质中的水平。作者发现，AGNs中的E_B-V/N_H与A_V/N_H比率系统性地低于银河系值，降幅达3至100倍，表明尘埃组成以大颗粒为主，导致消光曲线变平，抑制了特征吸收谱线。

ABSTRACT

We present observational evidences that dust in the circumnuclear region of AGNs has different properties than in the Galactic diffuse interstellar medium. By comparing the reddening of optical and infrared broad lines and the X-ray absorbing column density we find that the E(B-V)/N_H ratio is nearly always lower than Galactic by a factor ranging from ~3 up to ~100. Other observational results indicate that the Av/N_H ratio is significantly lower than Galactic in various classes of AGNs including intermediate type 1.8-1.9 Seyferts, hard X-ray selected and radio selected quasars, broad absorption line QSOs and grism selected QSOs. The lack of prominent absorption features at 9.7um (silicates) and at 2175A (carbon dip) in the spectra of Seyfert 2s and of reddened Seyfert 1s, respectively, add further evidence for dust in the circumnuclear region of AGNs being different from Galactic. These observational results indicate that the dust composition in the circumnuclear region of AGNs could be dominated by large grains, which make the extinction curve flatter, featureless and are responsible for the reduction of the E(B-V)/N_H and Av/N_H ratios. Regardless of the physical origin of these phenomena, the reduced dust absorption with respect to what expected from the gaseous column density should warn about a mismatch between the optical and the X-ray classification of the active galactic nuclei in terms of their obscuration.

研究动机与目标

• 调查AGNs中的尘埃消光是否偏离银河系标准消光曲线及尘气比。
• 确定观测到的X射线吸收柱密度（N_H）是否与银河系模型中预期的光学/红外红化（E_B-V或A_V）相关。
• 检查AGN光谱中关键尘埃特征（如9.7 µm硅酸盐吸收、2175 Å碳吸收谷）的缺失是否意味着尘埃颗粒群体不同。
• 评估这些异常现象对统一AGN模型的影响，特别是光学与X射线分类之间的解耦问题。
• 确立低E_B-V/N_H与A_V/N_H比率并非选择效应所致，而是AGN尘埃的系统性特征。

提出的方法

• 通过Hα/Hβ及其他发射线比值作为红化指标，比较光学与红外宽线比值以推导E_B-V。
• 利用X射线谱分析，通过1–10 keV能段内的冷吸收特征测量吸收柱密度N_H。
• 分析ISO中红外光谱中Seyfert 2星系，以搜寻9.7 µm硅酸盐吸收特征。
• 检查红化Seyfert 1星系的紫外光谱中是否存在2175 Å碳吸收谷。
• 计算20个AGNs样本的E_B-V/N_H与A_V/N_H比率，包括中间类型、射电与X射线选的类星体及宽吸收线QSO。
• 将观测到的比率与银河系标准值（E_B-V/N_H = 3.1，A_V/N_H = 3.1）进行统计比较，以识别系统性偏差。

实验结果

研究问题

• RQ1 AGNs中的尘埃消光是否与银河系消光曲线及尘气比一致？
• RQ2 AGNs中观测到的X射线柱密度（N_H）在多大程度上与光学/红外红化（E_B-V或A_V）相符？
• RQ3 在显著红化的AGN光谱中，9.7 µm与2175 Å的特征尘埃吸收特征是否存在？
• RQ4 尘埃颗粒的哪些物理特性（如尺寸分布）可解释观测到的低E_B-V/N_H与A_V/N_H比率？
• RQ5 这些异常现象如何影响统一模型中AGN的分类，特别是光学与X射线遮蔽之间的对应关系？

主要发现

• AGNs中的E_B-V/N_H比率系统性地低于银河系值3至100倍，仅有三个低光度AGNs表现出银河系类似比率。
• A_V/N_H比率在各类AGNs中也显著低于银河系值，包括中间类型1.8–1.9 Seyfert星系、硬X射线选、射电选及宽吸收线QSO。
• Seyfert 2星系的中红外光谱中虽存在显著尘埃吸收，却未检测到9.7 µm硅酸盐吸收特征，表明小颗粒缺失。
• 红化Seyfert 1星系的紫外光谱中未观测到明显的2175 Å碳吸收谷，与银河系消光曲线预期相悖。
• 这些异常现象最合理的解释是尘埃颗粒群体以大颗粒为主，导致消光曲线变平，抑制了特征吸收谱线。
• 这些结果意味着AGNs中光学与X射线分类出现解耦，即显著的X射线遮蔽可在光学消光极低的情况下发生。