ALMA Polarimetry Measures Magnetically Aligned Dust Grains in the Torus of NGC 1068
ALMA Polarimetry Measures Magnetically Aligned Dust Grains in the Torus of NGC 1068
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DOI：
10.3847/1538-4357/ab8013
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发表时间：
2019-05
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影响因子：
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通讯作者：
E. Lopez-Rodriguez;A. Alonso-Herrero;S. Garc'ia-Burillo;M. Gordon;K. Ichikawa;M. Imanishi;S. Kameno;N. Levenson;R. Nikutta;C. Packham
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文献类型：
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作者：
E. Lopez-Rodriguez;A. Alonso-Herrero;S. Garc'ia-Burillo;M. Gordon;K. Ichikawa;M. Imanishi;S. Kameno;N. Levenson;R. Nikutta;C. Packham
活跃星系核（AGN）周围的模糊结构可以解释为尘埃和气体流动循环，从根本上将AGN与其宿主星系连接起来。这种结构被认为与辐射压力驱动的尘埃风有关。然而，在几乎所有超大质量黑洞吸积和流出的模型中，磁场的作用还没有得到彻底的研究。在这里，我们报告了利用ALMA Cycle 4偏振尘埃连续体观测，通过NGC 1068尘埃环中的磁性排列尘埃颗粒首次探测到偏振热发射。″07.4.2 pc；348.5 GHz, 860 μm)。极化环面沿赤道轴呈不对称变化，极化峰为3.7%±0.5%，在距地核以东约8pc处的位置角为109°±2°（b矢量）。我们计算了磁排列尘埃颗粒的合成极化观测，假设一个环向磁场和均匀的颗粒排列。我们得出结论，测量到的860 μm连续极化是由磁排列的尘埃颗粒在环面的光学薄区域引起的。环面赤道轴上的不对称极化是由(1)不均匀的光学深度和(2)速度色散的变化引起的，即从环面东部到西部的亚秒差距尺度上的磁场湍流的变化。这些观测和模型限制了环面性质，超出了光谱能量分布的结果。这项研究有力地支持了高达几个秒差距的磁场对活动核的吸积流的贡献。
The obscuring structure surrounding active galactic nuclei (AGN) can be explained as a dust and gas flow cycle that fundamentally connects the AGN with their host galaxies. This structure is believed to be associated with dusty winds driven by radiation pressure. However, the role of magnetic fields, which are invoked in almost all models for accretion onto a supermassive black hole and outflows, has not been thoroughly studied. Here we report the first detection of polarized thermal emission by means of magnetically aligned dust grains in the dusty torus of NGC 1068 using ALMA Cycle 4 polarimetric dust continuum observations (0.″07, 4.2 pc; 348.5 GHz, 860 μm). The polarized torus has an asymmetric variation across the equatorial axis with a peak polarization of 3.7% ± 0.5% and position angle of 109° ± 2° (B-vector) at ∼8 pc east from the core. We compute synthetic polarimetric observations of magnetically aligned dust grains assuming a toroidal magnetic field and homogeneous grain alignment. We conclude that the measured 860 μm continuum polarization arises from magnetically aligned dust grains in an optically thin region of the torus. The asymmetric polarization across the equatorial axis of the torus arises from (1) an inhomogeneous optical depth and (2) a variation of the velocity dispersion, i.e., a variation of the magnetic field turbulence at subparsec scales, from the eastern to the western region of the torus. These observations and modeling constrain the torus properties beyond spectral energy distribution results. This study strongly supports that magnetic fields up to a few parsecs contribute to the accretion flow onto the active nuclei.