Studies of high redshift radio galaxies can shed light on the activity of active galactic nuclei (AGN) in massive elliptical galaxies, and on the assembly and evolution of galaxy clusters in the Universe. J1606+3124 has been tentatively identified as a radio galaxy at a redshift of 4.56, at an era of one-tenth of the current age of the Universe. Very long baseline interferometry (VLBI) images show a compact triple structure with a size of 68 parsecs. The radio properties of J1606+3124, including the edge-brightening morphology, peaked GHz radio spectrum, slow variability, and low jet speed, consistently indicate that it is a compact symmetric object (CSO). The radio source size and expansion rate of the hotspots suggest that J1606+3124 is a young (kinematic age of ∼3600 years) radio source. Infrared observations reveal a gas- and dust-rich host galaxy environment, which may hinder the growth of the jet; however, the ultra-high jet power of J1606+3124 gives it an excellent chance to grow into a large-scale double-lobe radio galaxy. If its redshift and galaxy classification can be confirmed by further optical spectroscopic observations, J1606+3124 will be the highest redshift CSO galaxy known to date.

We observed 20 Palomar-Green (PG) quasars at low redshift (z < 0.5) with total flux density > 1 mJy, including 4 radio-loud quasars (RLQs) and 16 radio-quiet quasars (RQQs), using the Very Long Baseline Array (VLBA) at 5 GHz. Ten RQQs are clearly detected in the VLBA images, and a compact radio core is identified in eight of them, indicating the prevalence of active galactic nucleus (AGN)-related radio emission in this flux-density-limited RQQ sample. The RQQs and RLQs in our sample have a division at ∼30 mJy. The radio emission from RQQs appears to be the result of a combination of star formation and AGN-associated activities. All RQQs in our sample have a 5 GHz flux density ratio of Very Large Array (VLA) A-array to D-array $f_{\rm c} = S_{\rm A}^{\rm VLA}/S_{\rm D}^{\rm VLA}$ above 0.2. The RQQs with fa (VLBA and VLA flux density ratio $S^{\rm VLBA}/S_{\rm A}^{\rm VLA}) > 0.2$ versus fa < 0.2 show significant differences in morphology, compactness and total flux density. fa of RQQs is systematically lower than that of RLQs, probably due to the extended jets or relic jets of RQQs on 10s to 100s parsecs which are resolved out in VLBA images. Future larger samples, especially with the addition of milli-arcsec resolution radio images of RQQs with total flux densities below 1 mJy, can test the conclusions of this paper and contributes to the understanding of the radio emission mechanism of RQQs, and the dichotomy and physical connection between RQQs and RLQs.

A tidal disruption event (TDE) involves the shredding of a star in the proximity of a supermassive black hole (SMBH). The nearby (≈230 Mpc) relatively radio-quiet, thermal-emission-dominated source AT2019dsg is the first TDE with a potential neutrino association. The origin of nonthermal emission remains inconclusive; possibilities include a relativistic jet or a subrelativistic outflow. Distinguishing between them can address neutrino production mechanisms. High-resolution very long baseline interferometry 5 GHz observations provide a proper motion of 0.94 ± 0.65 mas yr−1 (3.2 ± 2.2 c; 1σ). Modeling the radio emission favors an origin from the interaction between a decelerating outflow (velocity ≈0.1 c) and a dense circumnuclear medium. The transition of the synchrotron self-absorption frequency through the observation band marks a peak flux density of 1.19 ± 0.18 mJy at 152.8 ± 16.2 days. An equipartition analysis indicates an emission-region distance of ≥ 4.7 × 1016 cm, magnetic field strength ≥ 0.17 G, and number density ≥ 5.7 × 103 cm−3. The disruption involves a ≈2 M ⊙ star with a penetration factor ≈1 and a total energy output of ≤ 1.5 × 1052 erg. The outflow is radiatively driven by the accretion of stellar debris onto the SMBH. Neutrino production is likely related to the acceleration of protons to peta-electron-volt energies and the availability of a suitable cross section at the outflow base. The present study thus helps exclude jet-related origins for nonthermal emission and neutrino production, and constrains nonjetted scenarios.