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 { ( J 1 ) } = 0.017 \pm 0.002 mas yr ^ { -1 } and \mu ( { J 2 } ) =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 .