We report Very Long Baseline Array ( VLBA ) observations of the 1.5 GHz radio continuum emission of the z=6.326 quasar SDSS J010013.02+280225.8 ( hereafter J0100+2802 ) .
J0100+2802 is , by far the most optically luminous , and radio-quiet quasar with the most massive black hole known at z > 6 .
The VLBA observations 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 surface brightness of 64.6 \pm 9.0 \mu Jy beam ^ { -1 } and a total flux density of 88 \pm 19 \mu Jy .
The position of the radio peak is consistent with that from SDSS in the optical and Chandra in the X-ray .
The radio source is marginally resolved by the VLBA observations .
A 2-D Gaussian fit to the image constrains the source size to ( 7.1 \pm 3.5 ) mas \times ( 3.1 \pm 1.7 ) mas .
This corresponds to a physical scale of ( 40 \pm 20 ) pc \times ( 18 \pm 10 ) pc .
We estimate the intrinsic brightness temperature of the VLBA source to be T _ { B } = ( 1.6 \pm 1.2 ) \times 10 ^ { 7 } K. This is significantly higher than the maximum value in normal star forming galaxies , indicating an AGN origin for the radio continuum emission .
However , it is also significantly lower than the brightness temperatures found in highest redshift radio-loud quasars .
J0100+2802 provides a unique example to study the radio activity in optically luminous and radio quiet active galactic nuclei in the early universe .
Further observations at multiple radio frequencies will accurately measure the spectral index and address the dominant radiation mechanism of the radio emission .