上下文很快就有可能利用许多常见的河外天体将最精确的无线电参考系与盖亚光学参考系直接联系起来。了解致密活动星系核（AGN）的射电位置和光学位置之间的重合程度是非常重要的。目标。使用目前最好的目录，我们调查有多少活动星系核显着大的光-无线电位置偏移存在，以及这些偏移的可能原因。方法.我们进行了一个案例研究，通过寻找光学对应的国际天体参考系（ICRF 2）在斯隆数字巡天（SDSS）数据发布9（DR 9）的无线电源。ICRF 2目录被用作参考，因为甚长基线干涉测量（VLBI）观测确定的无线电位置比光学位置精确两个数量级。结果我们发现ICRF 2和SDSS DR 9共有1297个对象。对光学-无线电差异的统计分析证实，SDSS DR 9的位置在两个坐标系中的精度都达到了~55 mas，相对于ICRF 2没有系统偏移。我们发现51个源（~4%的样本）的位置偏移超过170 mas（~3σ）。天体物理学对这些异常值的解释必定存在。其中有3个已知的强引力透镜。双活动星系核或反冲超大质量黑洞也是可能的。结论.最精确的盖亚-VLBI参考系链接将需要通过消除异常值来仔细选择一组公共对象。另一方面，显著的光-射位置不重合可能为寻找引力透镜或双重活动星系核候选者提供新的工具。详细的后续无线电干涉和光学光谱观测鼓励调查在这项研究中发现的异常源。
英文摘要：
Context. It will soon become possible to directly link the most accurate radio reference frame with the Gaia optical reference frame using many common extragalactic objects. It is important to know the level of coincidence between the radio and optical positions of compact active galactic nuclei (AGN). Aims. Using the best catalogues available at present, we investigate how many AGN with significantly large optical-radio positional offsets exist as well as the possible causes of these offsets. Methods. We performed a case study by finding optical counterparts to the International Celestial Reference Frame (ICRF2) radio sources in the Sloan Digital Sky Survey (SDSS) Data Release 9 (DR9). The ICRF2 catalogue was used as a reference because the radio positions determined by Very Long Baseline Interferometry (VLBI) observations are about two orders of magnitude more accurate than the optical positions. Results. We find 1297 objects in common for ICRF2 and SDSS DR9. Statistical analysis of the optical-radio differences verifies that the SDSS DR9 positions are accurate to ~55 mas in both coordinates, with no systematic offset with respect to ICRF2. We find 51 sources (~4% of the sample) for which the positional offset exceeds 170 mas (~3σ). Astrophysical explanations must exist for most of these outliers. There are 3 known strong gravitational lenses among them. Dual AGN or recoiling supermassive black holes may also be possible. Conclusions. The most accurate Gaia-VLBI reference frame link will require a careful selection of a common set of objects by eliminating the outliers. On the other hand, the significant optical-radio positional non-coincidences may offer a new tool for finding e.g. gravitational lenses or dual AGN candidates. Detailed follow-up radio interferometric and optical spectroscopic observations are encouraged to investigate the outlier sources found in this study.
Merger-driven Fueling of Active Galactic Nuclei: Six Dual and Offset Active Galactic Nuclei Discovered with Chandra and Hubble Space Telescope Observations
Kiloparsec-scale Spatial Offsets in Double-peaked Narrow-line Active Galactic Nuclei. I. Markers for Selection of Compelling Dual Active Galactic Nucleus Candidates
具有kpc尺度分离的双活动星系核（AGN）的合并残余星系应该是广泛存在的，这是星系合并和触发气体吸积到超大质量黑洞上的结果，但很少观察到双活动星系核。斯隆数字巡天中具有双峰窄活动星系核发射线的星系核是可能的双重活动星系核候选者，但它们的双峰轮廓也可能是气体运动学或活动星系核驱动的流出和喷流在小或大尺度上的结果。为了区分这两种情况，我们利用Lick、Palomar和MMT天文台对SDSS中81个双峰窄线活动星系核在0.03 < z < 0.36处进行了长缝光谱观测，获得了活动星系核辐射的空间分布。我们发现，所有81个系统都表现出双活动星系核的发射分量与~kpc的投影空间分离的天空，这表明他们是由kpc尺度的双重活动星系核或kpc尺度的外流，喷流，或旋转的气体盘。此外，我们发现，子样本（58%）的对象与空间紧凑的发射成分可能优先产生的双重活动星系核，而子样本（42%）的空间扩展的发射成分可能优先产生的活动星系核流出。我们还发现，对于32%的样本的两个活动星系核的发射组件优先与宿主星系主轴对齐，预期的双活动星系核在宿主星系的潜在轨道。我们的研究结果既缩小了产生双重活动星系核成分的可能物理机制的列表，并提出了从双峰窄线活动星系核的完整样本中选择最有希望的双重活动星系核候选者的几个观测标准。使用这些标准，我们确定了17个最引人注目的双重活动星系核候选者在我们的样本。
