The first estimate of radio jet proper motion at z>5
  The extremely high redshift (z=5.3) radio source SDSS J102623.61+254259.5
(J1026+2542) is among the most distant and most luminous radio-loud active
galactic nuclei (AGN) known to date. Its one-sided radio jet structure on
milli-arcsecond (mas) and ~10-mas scales typical for blazars was first imaged
at 5 GHz with very long baseline interferometry (VLBI) in 2006. Here we report
on our dual-frequency (1.7 and 5 GHz) imaging observations performed with the
European VLBI Network (EVN) in 2013. The prominent jet structure allows us to
identify individual components whose apparent displacement can be detected over
the time span of 7.33 yr. This is the first time when jet proper motions are
directly derived in a blazar at z>5. The small values of up to ~0.1 mas/yr are
consistent with what is expected in a relativistic cosmological model if
redshift is a measure of distance. The apparent superluminal jet speeds,
considered tentative because derived from two epochs only, exceed 10c for three
different components along the jet. Based on modeling its spectral energy
distribution, J1026+2542 is known to have its X-ray jet oriented close to the
line of sight, with significant Doppler boosting and a large bulk Lorentz
factor (Gamma~13). The new VLBI observations, indicating ~2.3 x 10^12 K lower
limit to the core brightness temperature, are consistent with this picture. The
spectral index in the core region is -0.35.
