VLBI observations of four radio quasars at $z>4$: blazars or not?
  Blazars are active galactic nuclei (AGN) whose relativistic jets point nearly
to the line of sight. Their compact radio structure can be imaged with very
long baseline interferometry (VLBI) on parsec scales. Blazars at extremely high
redshifts provide a unique insight into the AGN phenomena in the early
Universe. We observed four radio sources at redshift $z>4$ with the European
VLBI Network (EVN) at 1.7 and 5 GHz. These objects were previously classified
as blazar candidates based on X-ray observations. One of them, J2134$-$0419 is
firmly confirmed as a blazar with our VLBI observations, due to its
relativistically beamed radio emission. Its radio jet extended to $\sim$10
milli-arcsec scale makes this source a promising target for follow-up VLBI
observations to reveal any apparent proper motion. Another target, J0839+5112
shows a compact radio structure typical of quasars. There is evidence for flux
density variability and its radio "core" has a flat spectrum. However, the EVN
data suggest that its emission is not Doppler-boosted. The remaining two blazar
candidates (J1420+1205 and J2220+0025) show radio properties totally unexpected
from radio AGN with small-inclination jet. Their emission extends to arcsec
scales and the Doppler factors of the central components are well below 1.
Their structures resemble that of double-lobed radio AGN with large inclination
to the line of sight. This is in contrast with the blazar-type modeling of
their multi-band spectral energy distributions. Our work underlines the
importance of high-resolution VLBI imaging in confirming the blazar nature of
high-redshift radio sources.
