Resolving the Radio Emission from the Quasar P172+18 at $z = 6.82$
  We present high angular resolution imaging of the quasar PSO
J172.3556+18.7734 at $z=6.82$ with the Very Long Baseline Array (VLBA). This
source currently holds the record of being the highest redshift radio-loud
quasar. These observations reveal a dominant radio source with a flux density
of $398.4 \pm 61.4~\mu$Jy at 1.53 GHz, a deconvolved size of $9.9 \times 3.5$
mas ($52.5 \times 18.6$ pc), and an intrinsic brightness temperature of ($4.7
\pm 0.7) \times 10^7$ K. A weak unresolved radio extension from the main source
is also detected at $\sim~3.1\sigma$ level. The total flux density recovered
with the VLBA at 1.53 GHz is consistent with that measured with the Very Large
Array (VLA) at a similar frequency. The quasar is not detected at 4.67 GHz with
the VLBA, suggesting a steep spectral index with a limit of
$\alpha^{1.53}_{4.67} < -$1.55. The quasar is also not detected with the VLBA
at 7.67 GHz. The overall characteristics of the quasar suggest that it is a
very young radio source similar to lower redshift Gigahertz Peaked Spectrum
radio sources, with an estimated kinematic age of $\sim~10^3$ years. The VLA
observations of this quasar revealed a second radio source in the field
$23\rlap{.}{''}1$ away. This radio source, which does not have an optical or IR
counterpart, is not detected with the VLBA at any of the observed frequencies.
Its non-detection at the lowest observed VLBA frequency suggests that it is
resolved out, implying a size larger than ~$0\rlap{.}{''}17$. It is thus likely
situated at lower redshift than the quasar.
