Milli-arcsecond imaging of the radio emission from the quasar with the
  most massive black hole at reionization
  We report Very Long Baseline Array (VLBA) observations of the 1.5 GHz radio
continuum 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, 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+/-9.0 micro-Jy/beam and a total flux density of 88+/-19
micro-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 $\rm (7.1+/-3.5) mas x (3.1+/-1.7) mas. This corresponds to a physical
scale of (40+/-20) pc x (18+/-10) pc. We estimate the intrinsic brightness
temperature of the VLBA source to be T_{B}=(1.6 +/- 1.2) x 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.
