Vol. 643
1. Letters

Parsec-scale properties of the radio brightest jetted AGN at z >6

Blazars are powerful quasars with accretion disks that are nearly face-on. Additionally, their light beams and radio jets are approximately located along the line of sight and thus experience Doppler boosting. They tend to be under-abundant at high redshift with respect to the local universe. The authors report on the very long baseline interferometry of the most powerful jetted active galactic nuclei at z>6. Using milliarcsecond resolution, they discovered a bright one-sided jet about 500 pc long in projection that is resolved into multiple clumps. They derived its viewing angle as well as its Lorentz and Doppler factors. Surprisingly, its bulk Lorentz factor appears to be relatively low (less than 5), which might explain the paucity of high-z blazars with respect to current predictions. However, other scenarios have not been ruled out. More stringent constraints on this jet and other high-z blazars will be needed to form conclusions regarding the possible z-evolution of the blazar population.