High - resolution double morphology of the most distant known radio quasar at z = 6 . 12

Context. The highest redshift quasars at z > ∼ 6 receive considerable attention since they provide strong constraints on the growth of the earliest supermassive black holes. They also probe the epoch of reionisation and serve as " lighthouses " to illuminate the space between them and the observer. The source J1427+3312 (z = 6.12) has recently been identified as the first and so far the only known radio-loud quasar at z > 6. Aims. We investigated the compact radio structure of J1427+3312 on milli-arcsecond (mas) angular scales, to compare it with that of the second most distant radio-loud quasar J0836+0054 (z = 5.77) and with lower-redshift radio quasars in general. Methods. We observed J1427+3312 in phase-reference mode with ten antennas of the European Very Long Baseline Interferometry Results. The source was clearly detected at both frequencies. At 1.6 GHz, it shows a prominent double structure. The two components are separated by 28.3 mas, corresponding to a projected linear distance of ∼ 160 pc. Both components with sub-mJy flux densities appear resolved. In the position of the brightest component at 1.6 GHz, we detected mas-scale radio emission at 5 GHz as well. The radio spectrum of this feature is steep. The double structure and the separation of the components of J1427+3312 are similar to those of the young (< ∼ 10 4 yr) compact symmetric objects (CSOs).