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There is still a limited number of high-redshift (z > 3) active galactic nuclei (AGNs) whose jet kinematics have been studied with very long baseline interferometry (VLBI). Without a dedicated proper motion survey, regularly conducted astrometric VLBI observations of bright radio-emitting AGN with sensitive arrays can be utilized to follow changes in the jets, by means of high-resolution imaging and brightness distribution modelling. Here, we present a first-time VLBI jet kinematic study of NVSS J080518 + 614423 (z = 3.033) and NVSS J165844 − 073918 (z = 3.742), two flat-spectrum radio quasars that display milliarcsecond-scale jet morphology. Archival astrometric observations carried out mainly with the Very Long Baseline Array, supplemented by recent data taken with the European VLBI Network, allowed us to monitor changes in their radio structure in the 7.6−8.6 GHz frequency band, covering almost two decades. By identifying individual jet components at each epoch, we were able to determine the apparent proper motion for multiple features in both sources. Apparent superluminal motions range |$(1-14)\, c$|⁠ , and are found to be consistent with studies of other high-redshift AGN targets. Using the physical parameters derived from the brightness distribution modelling, we estimate the Doppler-boosting factors (δ ≈ 11.2 and δ ≈ 2.7), the Lorentz factors (Γ ≈ 7.4 and Γ ≈ 36.6), and the jet viewing angles (θ ≈ 4.4° and θ ≈ 8.0°), for NVSS J080518 + 614423 and NVSS J165844 − 073918, respectively. The data revealed a stationary jet component with negligible apparent proper motion in NVSS J165844 − 073918. [ABSTRACT FROM AUTHOR]

Using empty-field 'Quick Look' images from the first two epochs of the VLA Sky Survey (VLASS) observations, centred on the positions of ∼3700 individually radio-non-detected active galactic nuclei (AGNs) at z ≥ 4, we performed image stacking analysis to examine the sub-mJy emission at 3 GHz. We found characteristic monochromatic radio powers of P char = (2–13) × 1024 W Hz−1, indicating that AGN-related radio emission is widespread in the sample. The signal-to-noise ratios of the redshift-binned median stacked maps are between 4–6, and we expect that with the inclusion of the yet to be completed third-epoch VLASS observations, the detection limit defined as signal-to-noise ratio SNR ≥ 6 could be reached, and the redshift dependence can be determined. To obtain information on the general spectral properties of the faint radio emission in high-redshift AGNs, we confined the sample to ∼3000 objects covered by both the VLASS and the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey. We found that the flux densities from the median stacked maps show a characteristic spectral index of α* = −0.30 ± 0.15, which is in agreement with the median spectral index of the radio-detected z ≥ 4 AGNs from our high-redshift AGN catalogue. The three-band mid-infrared colour–colour diagram based on Wide-field Infrared Survey Explorer observations provides further support regarding the AGN contribution to the radio emission in the sub-mJy sample. [ABSTRACT FROM AUTHOR]

Less than 200 radio‐loud active galactic nuclei are known above redshift 4. Around 40 of them have been observed at milliarcsecond (mas) scale resolution with very long baseline interferometry (VLBI) technique. Some of them are unresolved, compact, relativistically beamed objects, and blazars with jets pointing at small angles to the observer's line of sight. But there are also objects with no sign of relativistic beaming possibly having larger jet inclination angles. In a couple of cases, X‐ray observations indicate the presence of relativistic beaming contrary to the VLBI measurements made with the European VLBI Network. J1420+1205 is a prominent example, where our 30–100 mas‐scale enhanced multi element remotely linked interferometer network radio observations revealed a rich structure reminiscent of a small radio galaxy. It shows a bright hotspot that might be related to the denser interstellar medium around a young galaxy at an early cosmological epoch. [ABSTRACT FROM AUTHOR]

We applied image stacking on empty field Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey maps centred on optically identified high-redshift quasars at |$z\ge 4$| to uncover the hidden |$\mu$| Jy radio emission in these active galactic nuclei (AGN). The median stacking procedure for the full sample of 2229 optically identified AGN uncovered an unresolved point source with an integrated flux density of 52  |$\mu$| Jy, with a signal-to-noise ratio |${\sim} 10$|⁠. We co-added the individual image centre pixels to estimate the characteristic monochromatic radio power at 1.4 GHz considering various values for the radio spectral index, revealing a radio population with |$P_\mathrm{1.4\, GHz}\sim 10^{24}$|  W Hz |$^{-1}$|⁠. Assuming that the entire radio emission originates from star-forming (SF) activity in the nuclear region of the host galaxy, we obtained an upper limit on the characteristic star formation rate, |${\sim} 4200$|  M |$_\odot$|  yr |$^{-1}$|⁠. The angular resolution of FIRST images is insufficient to distinguish between the SF and AGN origin of radio emission at these redshifts. However, a comparison with properties of individual sources from the literature indicates that a mixed nature is likely. Future very long baseline interferometry radio observations and ultradeep Square Kilometre Array surveys are expected to be sensitive enough to detect and resolve the central |$1\!-\!10$|  kpc region in the host galaxies, and thus discriminate between SF and AGN related emission. [ABSTRACT FROM AUTHOR]

To date, PMN J2134-0419 (at a redshift z = 4.33) is the second most distant quasar known with a milliarcsecond-scale morphology permitting direct estimates of the jet proper motion. Based on two-epoch observations, we constrained its radio jet proper motion using the very long baseline interferometry (VLBI) technique. The observations were conducted with the European VLBI Network (EVN) at 5 GHz on 1999 November 26 and 2015 October 6. We imaged the central 10-pc scale radio jet emission and modelled its brightness distribution. By identifying a jet component at both epochs separated by 15.86 yr, a proper motion of μ = 0.035 ± 0.023 mas yr-1 is found. It corresponds to an apparent superluminal speed of βa = 4.1 ± 2.7 c. Relativistic beaming at both epochs suggests that the jet viewing angle with respect to the line of sight is smaller than 20°, with a minimum bulk Lorentz factor Γ = 4.3. The small value of the proper motion is in good agreement with the expectations from the cosmological interpretation of the redshift and the current cosmological model. Additionally we analysed archival Very Large Array observations of J2143-0419 and found indication of a bent jet extending to ∼30 kpc. [ABSTRACT FROM AUTHOR]

Radio-loud high-redshift quasars (HRQs), although only a few of them are known to date, are crucial for studies of the growth of supermassive black holes (SMBHs) and the evolution of active galactic nuclei at early cosmological epochs. Radio jets offer direct evidence of SMBHs, and their radio structures can be studied with the highest angular resolution using very long baseline interferometry (VLBI). Here we report on the observations of three HRQs (J0131-0321, J0906+6930 and J1026+2542) at z > 5 using the Korean VLBI Network and VLBI Exploration of Radio Astrometry Arrays (together known as KaVA) with the purpose of studying their pc-scale jet properties. The observations were carried out at 22 and 43 GHz in 2016 January among the first-batch open-use experiments of KaVA. The quasar J0906+6930 was detected at 22 GHz but not at 43 GHz. The other two sources were not detected and upper limits to their compact radio emission are given. Archival VLBI imaging data and the singledish 15-GHz monitoring light curve of J0906+6930 were also acquired as complementary information. J0906+6930 shows a moderate-level variability at 15 GHz. The radio image is characterized by a core-jet structure with a total detectable size of ~5 pc in projection. The brightness temperature, ≳1.9 × 1011 K, indicates relativistic beaming of the jet. The radio properties of J0906+6930 are consistent with a blazar. Follow-up VLBI observations will be helpful for determining its structural variation. [ABSTRACT FROM AUTHOR]

High-redshift quasars are important to study galaxy and active galactic nuclei evolution, test cosmological models and study supermassive black hole growth. Optical searches for high-redshift sources have been very successful, but radio searches are not hampered by dust obscuration and should be more effective at finding sources at even higher redshifts. Identifying high-redshift sources based on radio data is, however, not trivial. Here we report on newmultifrequency Giant MetrewaveRadio Telescope observations of eight z >4.5 sources previously studied at high angular resolution with very long baseline interferometry (VLBI). Combining these observations with those from the literature, we construct broad-band radio spectra of all 30 z > 4.5 sources that have been observed with VLBI. In the sample we found flat, steep and peaked spectra in approximately equal proportions. Despite several selection effects, we conclude that the z > 4.5 VLBI (and likely also non-VLBI) sources have diverse spectra and that only about a quarter of the sources in the sample have flat spectra. Previously, the majority of high-redshift radio sources were identified based on their ultrasteep spectra. Recently, a new method has been proposed to identify these objects based on their megahertzpeaked spectra. No method would have identified more than 18 per cent of the high-redshift sources in this sample. More effective methods are necessary to reliably identify complete samples of high-redshift sources based on radio data. [ABSTRACT FROM AUTHOR]

High-redshift radio-loud quasars are used to, among other things, test the predictions of cosmological models, set constraints on black hole growth in the early Universe and understand galaxy evolution. Prior to this paper, 20 extragalactic radio sources at redshifts above 4.5 have been imaged with very long baseline interferometry (VLBI). Here, we report on observations of an additional ten z > 4.5 sources at 1.7 and 5 GHz with the European VLBI Network, thereby increasing the number of imaged sources by 50 per cent. Combining our newly observed sources with those from the literature, we create a substantial sample of 30 z > 4.5 VLBI sources, allowing us to study the nature of these objects. Using spectral indices, variability and brightness temperatures, we conclude that of the 27 sources with sufficient information to classify, the radio emission from one source is from star formation, 13 are flat-spectrum radio quasars and 13 are steep-spectrum sources. We also argue that the steep-spectrum sources are off-axis (unbeamed) radio sources with rest-frame self-absorption peaks at or below GHz frequencies and that these sources can be classified as gigahertz peaked-spectrum and megahertz peaked-spectrum sources. [ABSTRACT FROM AUTHOR]