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 \ga 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 ( \mu ) of three components identified in its radio jet by utilizing VLBI data taken from 2014 to 2018 .
We obtained \mu _ { \mathrm { J 1 } } = 0.334 \pm 0.099 mas yr ^ { -1 } , \mu _ { \mathrm { J 2 } } = 0.116 \pm 0.029 mas yr ^ { -1 } , and \mu _ { \mathrm { J 3 } } = 0.060 \pm 0.005 mas yr ^ { -1 } .
The maximal proper motion is converted to an apparent transverse speed of \beta _ { app } = 41.2 \pm 12.2 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 .