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

Bence Husvéth, Krisztina É. Gabányi, Tao An, Sándor Frey, Jun Yang, Iván Agudo, Yingkang Zhang

First listed 2026-08-20 | Last updated 2026-08-19

Abstract

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 $z_{\rm phot}=0.9\pm0.1$. 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 $T_{\mathrm{b,VLBI}}=(7.4\pm0.8)\cdot10^{10}$ K, allowing a mildly Doppler-boosted interpretation, while the young compact-source scenario also remains viable. The core of J1510+5702 has $T_{\mathrm{b,VLBI}}=(1.08\pm0.15) \cdot10^{12}$ K, implying a Doppler factor of ${\sim}22$ under the equipartition assumption. This component has a ${\sim}3.5$ % fractional polarization. These observations show that cm-wavelength VLBI can access rest-frame millimeter-band polarization in bright $z>4$ jets.

Short digest

Husvéth et al. present 22-GHz EVN total-intensity and polarimetric VLBI imaging of the secure z=4.31 quasar J1510+5702 and the redshift-uncertain compact source J1606+3124, sampling rest-frame frequencies above 118 GHz if both published high-redshift values apply. J1510+5702 contains a very compact, polarized core with Tb = (1.08 ± 0.15) × 10^12 K and about 3.5% fractional polarization, implying an equipartition Doppler factor of roughly 22 and directly probing its strongly beamed inner jet. J1606+3124 shows only low-level polarization and, conditional on z=4.56, a lower Tb of (7.4 ± 0.8) × 10^10 K that permits mild boosting but leaves a young compact-source interpretation viable; the key caveat is that its published spectroscopic redshift is uncertain, with a photometric estimate near z=0.9. Together, the measurements demonstrate that 22-GHz cm-VLBI can recover rest-frame millimeter polarization from bright z>4 jet systems.

Key figures to inspect

• Figure 1. This uv-coverage figure establishes the long-baseline EVN sampling behind the parsec-scale 22-GHz imaging of both targets, providing essential context for the angular-resolution and morphology claims.
• Figure 2. The paired 22-GHz total-intensity maps are the core structural result: they show the fitted Gaussian components, restoring beams, and linear-scale conversions used to identify the compact J1510+5702 core and characterize J1606+3124’s brightest feature.
• Figure 3. This is the conclusion-driving diagnostic because it overlays polarized intensity and EVPAs on the VLBI jet maps, making the detection of approximately 3.5% core polarization in J1510+5702 and the much weaker, relatively calibrated polarization in J1606+3124 immediately comparable.