Multi-frequency VLBA Polarimetry of the high-redshift GPS Quasar OQ172
  Multi-frequency Very Long Baseline Array (VLBA) polarimetry observation of
the GHz-Peaked Spectrum (GPS) quasar OQ172 (J1445+0958) has been performed at
1.6, 2.2, 4.8, 8.3 and 15.3 GHz in 2005. Core-jet structures are detected in
all bands with the jet strongly bent at about 3 mas from the core. The radio
emission of the source is polarised at all five bands. We study the Faraday
Rotation in the core and jet components at all five bands, and find good linear
fits of Faraday Rotation in the core and jet components at 4.8 and 8.3 GHz. At
these two frequencies, the Rotation Measure (RM) is $\sim 2000~\rm rad~m^{-2}$
in the core and $\sim 700~\rm rad~m^{-2}$ in the inner jet components and
continues to decrease at the outer jet parts. We find that the depolarisation
at 4.8 and 8.3 GHz might be caused by the internal medium in the source. We
investigate consistency of the turnover spectra of VLBI components with the
Synchrotron Self-Absorption (SSA) and Free-Free Absorption (FFA) models.
Although these two models can not be easily distinguished due to the lack of
low-frequency data, the physical parameters can be constrained for each model.
We find that the large width of the $\rm [OIII]_{5007}$ line is likely caused
by a jet interaction with a Narrow Line Region (NLR) medium. The jet bending,
significant RM variations, Faraday depolarisation, spectral turnover, and broad
line width of $\rm [OIII]_{5007}$ could be closely related, likely caused by
the same nucleus medium, presumably NLR.
