Radio-loud Quasars above Redshift 4: VLBI Imaging of an Extended Sample
  High-redshift radio sources provide plentiful opportunities for studying the
formation and evolution of early galaxies and supermassive black holes.
However, the number of known radio-loud active galactic nuclei (AGN) above
redshift 4 is rather limited. At high redshifts, it appears that blazars, with
relativistically beamed jets pointing towards the observer, are in majority
compared to radio-loud sources with jets misaligned with respect to the line of
sight. To find more of these misaligned AGN, milliarcsec-scale imaging studies
carried out with very long baseline interferometry (VLBI) are needed, as they
allow us to distinguish between compact core--jet radio sources and those with
more extended emission. Previous high-resolution VLBI studies revealed that
some of the radio sources among blazar candidates in fact show unbeamed radio
emission on milliarcsecond scales. The most accurate optical coordinates
determined with the Gaia astrometric space mission are also useful in the
classification process. Here, we report on dual-frequency imaging observations
of 13 high-redshift (4 < z < 4.5) quasars at 1.7 and 5 GHz with the European
VLBI Network. This sample increases the number of z>4 radio sources for which
VLBI observations are available by about a quarter. Using structural and
physical properties, such as radio morphology, spectral index, variability,
brightness temperature, as well as optical coordinates, we identified six
blazars and six misaligned radio AGNs, with the remaining one tentatively
identified as blazar.
