Fast jet proper motion discovered in a blazar at z = 4.72
  High-resolution observations of high-redshift ($z>4$) radio quasars offer a
unique insight into jet kinematics at early cosmological epochs, as well as
constraints on cosmological model parameters. Due to the general weakness of
extremely distant objects and the apparently slow structural changes caused by
cosmological time dilation, only a couple of high-redshift quasars have been
studied with parsec-scale resolutions, and with limited number of observing
epochs. Here we report on very long baseline interferometry (VLBI) observations
of a high-redshift blazar J1430+4204 ($z=4.72$) in the 8 GHz frequency band at
five different epochs spanning 22 years. The source shows a compact core--jet
structure with two jet components being identified within 3 milli-arcsecond
(mas) scale. The long time span and multiple-epoch data allow for the kinematic
studies of the jet components. That results in a jet proper motion of $\mu {\rm
(J1)}$ = 0.017$\pm$0.002~mas\,yr$^{-1}$ and $\mu({\rm
J2})$=0.156$\pm$0.015~mas\,yr$^{-1}$, respectively. For the fastest-moving
outer jet component J2, the corresponding apparent transverse speed is $19.5
\pm 1.9 \,c$. The inferred bulk jet Lorentz factor $\Gamma = 14.6 \pm 3.8$ and
viewing angle $\theta = 2.2^{\circ} \pm 1.6^{\circ}$ indicate highly
relativistic beaming. The Lorentz factor and apparent proper motion are the
highest measured to date among the $z>4$ jetted radio sources, while the jet
kinematics is still consistent with the cosmological interpretation of quasar
redshifts.
