PARSEC-SCALE DUST EMISSION FROM THE POLAR REGION IN THE TYPE 2 NUCLEUS OF NGC 424
PARSEC-SCALE DUST EMISSION FROM THE POLAR REGION IN THE TYPE 2 NUCLEUS OF NGC 424
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DOI：
10.1088/0004-637x/755/2/149
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发表时间：
2012-06
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通讯作者：
S. Hoenig;M. Kishimoto;R. Antonucci;A. Marconi;M. Prieto;K. Tristram;Gerd Weigelt Ucsb;Mpifr;U. Firenze;Iac
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作者：
S. Hoenig;M. Kishimoto;R. Antonucci;A. Marconi;M. Prieto;K. Tristram;Gerd Weigelt Ucsb;Mpifr;U. Firenze;Iac
红外线干涉测量技术的进步为在秒差距尺度上对活动星系核进行空间分辨和研究环核尘埃分布提供了可能性，环核尘埃分布通常被称为“尘埃环”，它是活动星系核1型/2型二分法的原因。我们使用中红外光束组合器，连同8米望远镜在甚大望远镜干涉仪观察塞弗特2星系NGC 424的核，实现了几乎完全覆盖的紫外线平面可用的望远镜配置。我们探测到扩展的中红外辐射，其相对基线和模型无关的中红外半光半径为（2.0 ± 0.2）pc ×（1.5 ± 0.3）pc（在8-13 μm波长范围内平均）。扩展的中红外光源显示出随着波长增加的尺寸。这些性质与尘埃与活动星系核处于热平衡状态的观点一致。位置角-27 °的长轴方向与光学偏振观察设定的系统轴紧密对齐。环面模型通常倾向于在中红外波长处沿中平面沿着延伸。因此，我们的结论是，在这2型活动星系核的秒差距尺度的中红外辐射（约60%）的大部分来自光学薄尘埃在活动星系核的极地地区，一个方案符合近远红外光谱能量分布。我们认为，一个辐射驱动的尘埃风，可能在一个膨胀的区域内热的一部分的环面，是负责极地尘埃。在这张照片中，环面主导了近红外辐射，直到约5 μm，而极地尘埃是中红外通量的主要贡献者。我们对NGC 424的研究结果与最近对圆规星系活动星系核的观测结果一致，并且与其他天体的大尺度特征相似。如果我们的研究结果反映了活动星系核的一般性质，那么目前用于解释和模拟活动星系核红外辐射的范式就必须进行修正。
Advancements in infrared (IR) interferometry open up the possibility to spatially resolve active galactic nuclei (AGNs) on the parsec-scale level and study the circumnuclear dust distribution, commonly referred to as the “dust torus,” that is held responsible for the type 1/type 2 dichotomy of AGNs. We used the mid-IR beam combiner MIDI together with the 8 m telescopes at the Very Large Telescope Interferometer to observe the nucleus of the Seyfert 2 galaxy NGC 424, achieving an almost complete coverage of the uv-plane accessible by the available telescope configurations. We detect extended mid-IR emission with a relatively baseline- and model-independent mid-IR half-light radius of (2.0 ± 0.2) pc × (1.5 ± 0.3) pc (averaged over the 8–13 μm wavelength range). The extended mid-IR source shows an increasing size with wavelength. These properties are in agreement with the idea of dust heated in thermal equilibrium with the AGN. The orientation of the major axis in position angle ∼ − 27° is closely aligned with the system axis as set by optical polarization observations. Torus models typically favor extension along the mid-plane at mid-IR wavelengths instead. Therefore, we conclude that the majority of the parsec-scale mid-IR emission (≳60%) in this type 2 AGN originates from optically thin dust in the polar region of the AGN, a scenario consistent with the near- to far-IR spectral energy distribution. We suggest that a radiatively driven dusty wind, possibly launched in a puffed-up region of the inner hot part of the torus, is responsible for the polar dust. In this picture, the torus dominates the near-IR emission up to about 5 μm, while the polar dust is the main contributor to the mid-IR flux. Our results of NGC 424 are consistent with recent observations of the AGN in the Circinus galaxy and resemble large-scale characteristics of other objects. If our results reflect a general property of the AGN population, the current paradigm for interpreting and modeling the IR emission of AGNs has to be revised.