射电活动星系核高红移鉴别的一种检验方法
A method for checking high-redshift identification of radio AGNs
摘要：
在大尺度光谱测量中，由于发射谱线的弱性和使用的自动识别算法的不同，许多天体被发现具有多次红移测量。其中包括一些可疑的高红移（z >= 5）活动星系核（AGN）。在这里，我们提出了一种方法，检查高红移识别的源，他们是无线电响亮，并有很长的基线干涉（VLBI）成像观测的毫角秒（mas）尺度的喷流结构，在多个时代。该方法是基于确定喷流组件自行，并结合喷流物理学（所观察到的最大值的体积洛伦兹因子）和宇宙学（在早期宇宙中观察到的现象的时间膨胀）约束的可能值的表观自行的事实。作为一个例子，我们提出了类星体J2346+0705的情况下，有两个不同的红移，$z_{1} = 5.063$和$z_{2} = 0.171$，在文献中报告。我们利用2014年至2018年拍摄的VLBI数据测量了其射电喷流中识别出的三个分量的视自行（$μ$）。我们获得了$μ_{J1} = 0.334 \pm 0.099$ mas yr $^{-1}$、$μ_{J2} = 0.116 \pm 0.029$ mas yr $^{-1}$和$μ_{J3} = 0.060 \pm 0.005$ mas yr $^{-1}$。最大自行被转换为$β_{\rm app} = 41.2\pm12.2\,c$的视在横向速度。如果源的红移是5.063这个数值超过了迄今已知的耀变体喷流速度。这个和其他论点表明J2346+0705位于一个低红移星系中。我们的方法可能适用于其他高红移AGN候选人缺乏明确的光谱红移测定或光度红移估计，但显示突出的射电喷流允许VLBI测量快速喷流自行。
英文摘要：
In large-scale optical spectroscopic surveys, there are many objects found to have multiple redshift measurements due to the weakness of their emission lines and the different automatic identification algorithms used. These include some suspicious high-redshift (z >= 5) active galactic nuclei (AGNs). Here we present a method for inspecting the high-redshift identification of such sources provided that they are radio-loud and have very long baseline interferometry (VLBI) imaging observations of their milli-arcsec (mas) scale jet structure available at multiple epochs. The method is based on the determination of jet component proper motions, and the fact that the combination of jet physics (the observed maximal values of the bulk Lorentz factor) and cosmology (the time dilation of observed phenomena in the early Universe) constrain the possible values of apparent proper motions. As an example, we present the case of the quasar J2346+0705 that was reported with two different redshifts, $z_{1} = 5.063$ and $z_{2} = 0.171$, in the literature. We measured the apparent proper motions ($μ$) of three components identified in its radio jet by utilizing VLBI data taken from 2014 to 2018. We obtained $μ_{J1} = 0.334 \pm 0.099$ mas yr$^{-1}$, $μ_{J2} = 0.116 \pm 0.029$ mas yr$^{-1}$, and $μ_{J3} = 0.060 \pm 0.005$ mas yr$^{-1}$. The maximal proper motion is converted to an apparent transverse speed of $β_{\rm app} = 41.2\pm12.2\,c$. if the source is at redshift 5.063. This value exceeds the blazar jet speeds known to date. This and other arguments suggest that J2346+0705 is hosted by a low-redshift galaxy. Our method may be applicable for other high-redshift AGN candidates lacking unambiguous spectroscopic redshift determination or having photometric redshift estimates only, but showing prominent radio jets allowing for VLBI measurements of fast jet proper motions.
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