The Total and Polarized Radio Emission from the Innermost Jets of a High-Redshift Quasar and a Candidate at Parsec-Scale Resolution

High-frequency very long baseline interferometry (VLBI) polarimetry probes synchrotron-emitting plasma closer to the central engines of radio-loud active galactic nuclei (AGNs), but observations above 43 GHz are technically demanding. We present 22-GHz European VLBI Network observations of the z=4.31 quasar J1510+5702 and J1606+3124, whose published spectroscopic redshift, z=4.56, is uncertain; a photometric estimate gives zphot=0.9±0.1. For the published z>4 redshifts, 22 GHz corresponds to rest-frame frequencies above 118 GHz. Polarized emission is detected in J1510+5702, and a low-level polarized signal is recovered from the brightest feature of J1606+3124. Adopting z=4.56, that feature has a brightness temperature of Tb,VLBI=(7.4±0.8)·1010 K, allowing a mildly Doppler-boosted interpretation, while the young compact-source scenario also remains viable. The core of J1510+5702 has Tb,VLBI=(1.08±0.15)·1012 K, implying a Doppler factor of ∼22 under the equipartition assumption. This component has a ∼3.5% fractional polarization. These observations show that cm-wavelength VLBI can access rest-frame millimeter-band polarization in bright z>4 jets.