2011A&A...531L...5F

There are about 60 quasars known at redshifts z>5.7 to date. Only three of them are detected in the radio above 1 mJy flux density at 1.4 GHz frequency. Among them, J1429+5447 (z=6.21) is the highest-redshift radio quasar known at present. These rare, distant, and powerful objects provide important insight into the activity of the supermassive black holes in the Universe at early cosmological epochs and into the physical conditions of their environment. We studied the compact radio structure of J1429+5447 on the milli-arcsecond (mas) angular scale to compare the structural and spectral properties with those of another two z∼6 radio-loud quasars, J0836+0054 (z=5.77) and J1427+3312 (z=6.12). We performed Very Long Baseline Interferometry (VLBI) imaging observations of J1429+5447 with the European VLBI Network (EVN) at 1.6GHz on 2010 June 8, and at 5GHz on 2010 May 27. Based on its observed radio properties, the compact but somewhat resolved structure on linear scales of <100pc, and the steep spectrum, the quasar J1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer the question whether the compact steep-spectrum radio emission is a ``universal'' feature of the most distant radio quasars, it is essential to study more yet to be discovered radio-loud active galactic nuclei at z>6.