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We report Very Long Baseline Array (VLBA) observations of the 1.5 GHz radiocontinuum emission of the {\it z=6.326} quasar SDSS J010013.02+280225.8(hereafter J0100+2802). J0100+2802 is, by far the most optically luminous, andradio-quiet quasar with the most massive black hole known at z>6. The VLBAobservations have a synthesized beam size of 12.10 mas $\times$5.36 mas (FWHM),and detected the radio continuum emission from this object with a peak surfacebrightness of 64.6+/-9.0 micro-Jy/beam and a total flux density of 88+/-19micro-Jy. The position of the radio peak is consistent with that from SDSS inthe optical and Chandra in the X-ray. The radio source is marginally resolvedby the VLBA observations. A 2-D Gaussian fit to the image constrains the sourcesize to $\rm (7.1+/-3.5) mas x (3.1+/-1.7) mas. This corresponds to a physicalscale of (40+/-20) pc x (18+/-10) pc. We estimate the intrinsic brightnesstemperature of the VLBA source to be T_{B}=(1.6 +/- 1.2) x 10^{7} K. This issignificantly higher than the maximum value in normal star forming galaxies,indicating an AGN origin for the radio continuum emission. However, it is alsosignificantly lower than the brightness temperatures found in highest redshiftradio-loud quasars. J0100+2802 provides a unique example to study the radioactivity in optically luminous and radio quiet active galactic nuclei in theearly universe. Further observations at multiple radio frequencies willaccurately measure the spectral index and address the dominant radiationmechanism of the radio emission.

Milli-arcsecond imaging of the radio emission from the quasar with the most massive black hole at reionization