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High-redshift radio sources provide plentiful opportunities for studying theformation and evolution of early galaxies and supermassive black holes.However, the number of known radio-loud active galactic nuclei (AGN) aboveredshift 4 is rather limited. At high redshifts, it appears that blazars, withrelativistically beamed jets pointing towards the observer, are in majoritycompared to radio-loud sources with jets misaligned with respect to the line ofsight. To find more of these misaligned AGN, milliarcsec-scale imaging studiescarried out with very long baseline interferometry (VLBI) are needed, as theyallow us to distinguish between compact core--jet radio sources and those withmore extended emission. Previous high-resolution VLBI studies revealed thatsome of the radio sources among blazar candidates in fact show unbeamed radioemission on milliarcsecond scales. The most accurate optical coordinatesdetermined with the Gaia astrometric space mission are also useful in theclassification process. Here, we report on dual-frequency imaging observationsof 13 high-redshift (4 < z < 4.5) quasars at 1.7 and 5 GHz with the EuropeanVLBI Network. This sample increases the number of z>4 radio sources for whichVLBI observations are available by about a quarter. Using structural andphysical properties, such as radio morphology, spectral index, variability,brightness temperature, as well as optical coordinates, we identified sixblazars and six misaligned radio AGNs, with the remaining one tentativelyidentified as blazar.

Radio-loud Quasars above Redshift 4: VLBI Imaging of an Extended Sample