[论文解读] AGN Reddening and Ultraviolet Extinction Curves from Hubble Space Telescope Spectra

C. M. Gaskell, A. J. Benker|ArXiv.org|Nov 7, 2007

Photocathodes and Microchannel Plates被引用 25

一句话总结

本研究利用哈勃空间望远镜和FUSE的光谱数据，为14个AGN推导了个别消光曲线，发现大多数AGN的紫外消光曲线比银河系或SMC曲线更平坦，且普遍缺乏2175Å特征。结果表明，先前针对辐射明亮型AGN所构建的综合消光曲线可能因光度依赖性消光偏差而过于平坦，且AGN环境中的尘埃可能受到AGN辐射场的影响而发生改变。

ABSTRACT

We present intrinsic extinction curves for 14 AGNs. The AGNs have reddenings, E(B-V), of up to 0.36 mag. The majority (13 out of 14) of the extinction curves are not steep in the UV. Of the seven best determined extinction curves, five have extinction curves that are as flat as the standard Galactic curve in the optical and near UV, but flatter in the far UV, and without the 2175 Angstrom feature. One AGN, B3 0754+394, has a steep SMC-like extinction curve, and another, Mrk 304, has an LMC-like extinction curve, including a probable 2175 Ang. bump. The remaining seven, lower-quality, extinction curves have overall shapes that are consistent with an LMC-like shape or a flatter shape. Two have possible 2175 features, and one might be identical to the Galactic curve. The flatter curves that predominate in our best determined extinction curves are not as flat as the Gaskell et al. (2004) extinction curve for radio-loud AGNs. This suggests that the previous radio-loud extinction curve might be slightly too flat in the range 4 < 1/lambda < 6.5 inverse microns because of luminosity-dependent reddening biases in the composite spectra, but further investigation is needed. We present a parameterized average AGN extinction curve. Observed variations in the continuum properties of the AGNs are inconsistent with intrinsic object-to-object variations because observed differences are least in the far UV where changes in the accretion disk spectrum should be greatest. We suggest that the steepening of AGN spectra around Lyman alpha is the result of a small amount of SMC-like dust (E(B-V) ~ 0.03). We find the largest object-to-object differences in spectral shape to be in the Fe II emission of the "small blue bump".

研究动机与目标

• 推导单个AGN的本征消光曲线，避免因使用综合光谱而产生的偏差。
• 评估AGN中观测到的光谱变化是否由消光引起，或源于吸积盘发射的本征差异。
• 研究AGN中紫外消光曲线的形状与特性，特别是2175Å特征的存在与否。
• 评估光度依赖性消光偏差是否影响了先前从辐射明亮型AGN中推导出的综合消光曲线。
• 判断宿主星系光或本征光谱变化是否能解释AGN中观测到的光谱多样性。

提出的方法

• 使用哈勃空间望远镜和FUSE的光谱数据，覆盖静止系波长范围900至9000Å。
• 利用Schlegel, Finkbeiner, & Davis (1998)的消光估计值，并结合标准银河系消光曲线，对银河系消光进行校正。
• 通过将观测到的AGN光谱与参考类星体3C 273在1.8 µm⁻1波长处归一至相同通量水平后，推导出消光曲线。
• 分析聚焦于紫外与光学波段，排除Lyman-α区域因吸收特征的影响。
• 根据形状对消光曲线进行分类，特别关注2175Å谷峰的存在或缺失。
• 基于最佳确定的个体消光曲线，构建参数化平均AGN消光曲线。

实验结果

研究问题

• RQ1 单个AGN中紫外消光曲线的本征形状是什么？其与标准银河系和SMC曲线相比有何差异？
• RQ2 AGN中观测到的光谱差异在多大程度上由尘埃消光引起，而非吸积盘或宿主星系光的本征变化？
• RQ3 先前从辐射明亮型AGN中推导出的综合消光曲线是否因光度依赖性消光效应而存在偏差？
• RQ4 AGN消光曲线中是否仍存在2175Å消光特征？其缺失对AGN环境中尘埃处理过程有何含义？
• RQ5 AGN中Fe II发射线'小蓝 bump'的观测变化是否可由消光解释，还是其本身为AGN的本征特性？

主要发现

• 14个AGN中有13个在紫外波段的消光曲线不陡峭，大多数在远紫外波段比银河系曲线更平坦。
• 在七个最佳确定的消光曲线中，有五个在光学与近紫外波段与银河系曲线相当平坦，但在远紫外波段更平坦，且均缺乏2175Å特征。
• 一个AGN（B3 0754+394）表现出类似SMC的消光曲线，远紫外波段有陡峭上升，而另一个（Mrk 304）则表现出类似LMC的曲线，可能具有2175Å特征。
• 大多数消光曲线比Gaskell et al. (2004)为辐射明亮型AGN所构建的综合曲线更平坦，表明该综合曲线在4 < 1/λ < 6.5 µm⁻1波段可能因光度依赖性消光偏差而过于平坦。
• 观测到的AGN光谱形状变化与本征源间差异不一致，尤其是在远紫外波段，更可能由消光效应解释。
• AGN消光曲线中普遍缺乏2175Å特征，表明AGN环境中的尘埃可能已被AGN辐射场所改变。