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Context: There are about 60 quasars known at redshifts z>5.7 to date. Onlythree of them are detected in the radio above 1 mJy flux density at 1.4 GHzfrequency. Among them, J1429+5447 (z=6.21) is the highest-redshift radio quasarknown at present. These rare, distant, and powerful objects provide importantinsight into the activity of the supermassive black holes in the Universe atearly cosmological epochs, and on the physical conditions in their environment.Aims: We studied the compact radio structure of J1429+5447 on themilli-arcsecond (mas) angular scale, in order to compare the structural andspectral properties with those of other two z~6 radio-loud quasars, J0836+0054(z=5.77) and J1427+3312 (z=6.12). Methods: We performed Very Long BaselineInterferometry (VLBI) imaging observations of J1429+5447 with the European VLBINetwork (EVN) at 1.6 GHz on 2010 June 8, and at 5 GHz on 2010 May 27. Results:Based on its observed radio properties, the compact but somewhat resolvedstructure on linear scales of <100 pc, and the steep spectrum, the quasarJ1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer thequestion whether the compact steep-spectrum radio emission is a "universal"feature of the most distant radio quasars, it is essential to study more, yetto be discovered radio-loud active galactic nuclei at z>6.

Into the central 10 pc of the most distant known radio quasar. VLBI imaging observations of J1429+5447 at z=6.21