光暗活动星系中辐射低效吸积的偏振光谱证据
Spectropolarimetric Evidence for Radiatively Inefficient Accretion in an Optically Dull Active Galaxy
摘要：
我们提出了斯巴鲁/FOCAS光谱偏振的两个活跃的星系在宇宙演化调查。这些物体被选择为光学暗淡，具有AGN的明亮X射线发射，但在我们以前的光谱学中缺少光学发射线。我们的新观测表明，一个目标具有与光学暗淡AGN一致的非常弱的发射线，而另一个目标具有典型的宿主稀释2型塞弗特星系的强发射线。在这两个源中，我们都没有观察到偏振发射线，P_BLR的3-sigma上限<2%。这意味着缺失的宽发射线（和较弱的窄发射线）不是由于简单的各向异性遮蔽，例如，由活动星系核环组成弱衬光学暗AGN在4400 A < lambda_rest < 7200 A时呈现出P = 0.78 +/- 0.07%的蓝色偏振连续谱（在4400 A < lambda_rest < 5050 A时P = 1.37 +/- 0.16%）。这种偏振通量的波长依赖性类似于一个未被遮蔽的活动星系核连续体，并代表活动星系核的内在发射，无论是作为同步辐射或外部的一个吸积盘反射的尘埃散射。由于活动星系核的发射缺乏发射谱线，因此这个源很可能有一个辐射效率低的吸积流。
英文摘要：
We present Subaru/FOCAS spectropolarimetry of two active galaxies in the Cosmic Evolution Survey. These objects were selected to be optically dull, with the bright X-ray emission of an AGN but missing optical emission lines in our previous spectroscopy. Our new observations show that one target has very weak emission lines consistent with an optically dull AGN, while the other object has strong emission lines typical of a host-diluted Type 2 Seyfert galaxy. In neither source do we observe polarized emission lines, with 3-sigma upper limits of P_BLR < 2%. This means that the missing broad emission lines (and weaker narrow emission lines) are not due to simple anisotropic obscuration, e.g., by the canonical AGN torus. The weak-lined optically dull AGN exhibits a blue polarized continuum with P = 0.78 +/- 0.07% at 4400 A < lambda_rest < 7200 A (P = 1.37 +/- 0.16% at 4400 A < lambda_rest < 5050 A). The wavelength dependence of this polarized flux is similar to that of an unobscured AGN continuum and represents the intrinsic AGN emission, either as synchrotron emission or the outer part of an accretion disk reflected by a clumpy dust scatterer. Because this intrinsic AGN emission lacks emission lines, this source is likely to have a radiatively inefficient accretion flow.
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