Radio-loud quasars at high redshift (z > 4) are rare objects in the Universe and rarely observed with Very Long Baseline Interferometry (VLBI). But some of them have flux density sufficiently high for monitoring of their apparent position. The instability of the astrometric positions could be linked to the astrophysical process in the jetted active galactic nuclei in the early Universe. Regular observations of the high-redshift quasars are used for estimating their apparent proper motion over several years. We have undertaken regular VLBI observations of several high-redshift quasars at 2.3 GHz (S band) and 8.4 GHz (X band) with a network of five radio telescopes: 40-m Yebes (Spain), 25-m Sheshan (China), and three 32-m telescopes of the Quasar VLBI Network (Russia) -- Svetloe, Zelenchukskaya, and Badary. Additional facilities joined this network occasionally. The sources have also been observed in three sessions with the European VLBI Network (EVN) in 2018--2019 and one Long Baseline Array (LBA) experiment in 2018. In addition, several experiments conducted with the Very Long Baseline Array (VLBA) in 2017--2018were used to improve the time sampling and the statistics. Based on these 37 astrometric VLBI experiments between 2017 and 2021, we estimated the apparent proper motions of four quasars: 0901+697, 1428+422, 1508+572, and 2101+600.
Search for quasar pairs with Gaia astrometric data. II. Photometric redshift prediction with machine learning for the MGQPC catalogue
Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University（北京师范大学前沿天文与天体物理研究院）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）
Testing the Cosmological Principle: Astrometric Limits on Systemic Motion of Quasars at Different Cosmological Epochs
构造了一个在8400平方度的椭圆天空区域内具有天体自行和光谱红移大于0.5的光学类星体的$60,410$样品。使用正交Zernike函数的极坐标，自行领域的加权最小二乘调整的整个样本和六个相等的仓排序红移。与37个Zernike函数的整体拟合揭示了一个统计学上显著的模式，这可能是工具的起源。这种模式的主要特征是一系列的高峰和低谷，主要在R. A。振幅为25~ $μ$的分量作为年$^{-1}$。这个场是从六个类星体的类似拟合中减去的，这些类星体按红移范围从0.5到7.03进行分组，中值为0.72，1.00，1.25，1.52，1.83，2.34。由此产生的残差模式是嘈杂的，与正式的不确定性高达8~ $μ$作为年$^{-1}$在该地区的中部。我们检测到一个单一的高置信Zernike项的RA。红移约为1.52的类星体的自行分量，代表天空上的总梯度为30 $μ$ as yr $^{-1}$ over $150\degr$。我们没有发现任何小规模或中等规模的系统变化的剩余自行场的红移以上的$2.5\,σ$的显着性水平的功能。
Search for quasar pairs with Gaia astrometric data IV. Confirmation of 17 dual quasars and 143 projected quasars
School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）；Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University（北京师范大学前沿交叉科学研究院）
A study of purely astrometric selection of extragalactic point sources with Gaia
选择河外点源，如类星体，往往受到显着的选择效应，导致现有的样本有相当复杂的选择功能。我们探讨是否一个纯粹的天体测量选择河外点源，如类星体，是可行的与正在进行的盖亚任务。这样的选择将是有趣的，因为它在目标的颜色方面是无偏的，因此也允许选择恒星序列中的颜色。我们已经分析了总共18个代表性的天空区域，使用GUMS，为欧空局盖亚任务准备的模拟器，无论是在范围内的$12\le G \le 20$马格和$12\le G \le 18$马格。对于每个区域，我们确定的密度显然是固定的恒星源，即盖亚不能测量一个显着的自行的来源。的密度与河外点源，如类星体的密度，以建立一个纯粹的天体测量的选择是可行的天体方向。当瞄准银纬$|b| \ge 30^\mathrm{o}$的区域时，类星体与表面上静止的恒星的比例超过50%，而当朝向两极观测时，类星体的比例上升到大约$\sim80$ %。我们表明，在大约20年的时间里，拟议中的盖亚后续任务的自行将大大改善所有纬度的这些结果。仅从零自行探测类星体，不受任何光谱假设的影响，可能会导致发现新类型的类星体或新类别的河外点源。
Search for Quasar Pairs with ${\it Gaia}$ Astrometric Data. III. Discovery of 9 dual and projected quasars
Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University（北京师范大学前沿天文与天体物理研究所）；Department of Physics, Nanchang University（南昌大学物理学院）；Center for Relativistic Astrophysics and High Energy Physics, Nanchang University（南昌大学相对论天体物理与高能物理中心）；Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）
Beyond UV: Rest-frame B-band and Apparent Luminosity Functions of z=5-9 Galaxies
National Astronomical Research Institute of Thailand (NARIT)（泰国国家天文研究所）；California Institute of Technology（加州理工学院）；Suranaree University of Technology（素拉那里理工大学）；University of Melbourne（墨尔本大学）
Systematic research of low redshift optically selected SDSS Type-2 AGN but with apparent long-term optical variabilities from Catalina Sky Survey, I: data sample and basic results
Search for Quasar Pairs with ${\it Gaia}$ Astrometric Data. I. Method and Candidates
Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University（北京师范大学前沿天文与天体物理研究所）；College of Intelligent Systems Science and Engineering, Hubei Minzu University（湖北民族大学智能系统科学与工程学院）；Department of Physics, Nanchang University（南昌大学物理系）；Center for Relativistic Astrophysics and High Energy Physics, Nanchang University（南昌大学相对论天体物理与高能物理中心）；Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）；School of Physics and Astronomy, Beijing Normal University（北京师范大学物理与天文学院）
On the kinematic nature of apparent discs at high redshifts: Local counterparts are not dominated by ordered rotation but by tangentially anisotropic random motion
