arXiv subjects

K. Perger

Publications and source records attributed to K. Perger.

15 recordsLinked to original sources

arXiv2026

From compact jets to extended lobes: radio morphologies of distant quasars at z > 4

High-redshift quasars play an essential role in studying the growth and evolution of supermassive black holes and active galactic nuclei (AGN). Radio-loud quasars additionally enable us to investigate the interactions between the jets and their environment. We aimed to reveal the radio morphology of three radio quasars at redshifts z>4 that contain milliarcsecond (mas) scale compact radio features according to previous very long baseline interferometry (VLBI) observations, but show significant flux density at arcsecond scales, indicating the presence of extended radio structure that cannot be sampled by the highest-resolution observations. We analysed radio interferometric data obtained at various angular resolutions and multiple frequencies, including observations made by the international Low-Frequency Array and the enhanced Multi-Element Remotely-Linked Interferometer Network. We also re-imaged archival European VLBI Network observations of our targets. Two quasars (J0813+3508 and J1231+3816) exhibit complex radio structures with hotspots, extended to tens of kpc. They resemble Fanaroff--Riley II-type radio galaxies with extremely bent jet morphology. The third object (J1548+3335) shows a one-sided structure of $\lesssim 10$ kpc size. There was no sign of relativistic boosting at its previous mas-scale resolution radio observations. Its radio power and (inferred) linear size derived from the lower-resolution observations are similar to the known high-power compact steep-spectrum sources. We found that the previously detected mas-scale compact radio features are related to the centres of the AGN or in one case to one of the hotspots in an extended lobe.

astro-ph.GA↗

arXiv2026

SKA$-$VLBI view of AGN jets in the early Universe

Active Galactic Nuclei (AGN) are among the brightest sources in the Universe, and those that are also jetted are uniquely valuable at the earliest epochs, because their relativistic outflows can regulate the gas supply of their host galaxies, potentially affecting both early star formation and the rapid growth of supermassive black holes (SMBHs). Their compact, high-brightness-temperature radio cores provide the sharpest beacons for very long baseline interferometry (VLBI), enabling direct constraints on Doppler boosting, jet duty cycles, and jet$-$environment coupling at extreme redshifts. In this White Paper, we discuss how the SKA-VLBI will provide sub-$\mu$Jy sensitivity together with milliarcsecond (mas) angular resolution to image and characterise jetted AGN at $z>6$ across SKA-Mid and SKA-Low frequencies. These observations can directly test SMBHs ($>10^6$ M$_{\odot}$) formation/evolution models (including jet-assisted super-Eddington phases) and infer the geometry of the Universe, directly probing the cosmological framework at high precision. Synergies with current and next-generation multi-band facilities will also be crucial to fully understand their host galaxies and their environment, providing an unprecedented panchromatic knowledge of the first jetted AGN.

astro-ph.GA↗

arXiv2025

High-resolution radio imaging of TGSSJ1530+1049, a radio galaxy in a dense environment at z=4

High-redshift radio galaxies can provide insights into the structure formation and galaxy evolution at earlier cosmological epochs. TGSSJ1530+1049 was selected as a candidate high-redshift radio galaxy. Subsequent observations with the James Webb Space Telescope (JWST) presented in a companion paper (Saxena et al., 2026) have shown that it is located at a redshift z=4.0. The JWST data furthermore showed that the radio source is part of one of the densest structures of galaxies and ionized gas known at these redshifts. The complex system qualitatively resembles a massive (cluster) galaxy forming early through a rapid succession of mergers. TGSSJ1530+1049 is an unresolved source down to ~0.6" scale in multiple radio surveys. To reveal its high-resolution radio structure and allow for a detailed comparison with JWST observations, we studied its morphology at various angular scales with different radio interferometric instruments. We observed TGSSJ1530+1049 at milliarcsecond (mas) scale angular resolution with the European VLBI Network (EVN), and at ~100-mas scale resolution with the enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN). We recovered a complex north--south oriented structure with steep-spectrum radio-emitting features, which are associated with lobes and hot spots of a jetted active galactic nucleus. The centre of the radio galaxy proved to be too faint at cm wavelengths to be unambiguously detected in our observations. Nevertheless, the linear size (~5.5 kpc) and the radio power place it among the so-called medium-sized symmetric objects, a smaller and/or confined version of larger radio galaxies. Comparison between the radio morphology and that of the ionized gas observed by JWST shows that the two are closely aligned. However, the optical emission line gas extends out to ~25 kpc, which is well beyond the detected radio structures. (Abridged)

astro-ph.GA↗

arXiv2025

Radio emission from flaring stars and brown dwarfs

Aims. With the aim to reveal the effects of the magnetic field to the multi-band activity of dwarf stars, we search for associated radio emission for an extensive list of 14 915 brown dwarfs and 15 124 flaring stars. Methods. We utilised the first and second epoch catalogues and radio maps from all three epochs of the VLASS, supplemented with X-ray catalogues based on observations by the ROSAT, eROSITA, and XMM-Newton space telescopes, and 2-minute cadence optical light curves from the TESS mission. The radio-detected sub-sample was queried for concurrent TESS observations, and sources with coinciding light-curves were studied individually. Results. We found no associated radio emission for brown dwarfs, and found 55 radio counterparts for the sample of flaring stars, out of which seven have coincident TESS observations. The radio-detected sample follows both the radio-X-ray and the period-activity relations. We found a strong correlation between the radio powers and the stellar parameters of surface gravity, radius, and mass. We found no connection between the flare rate and the radio variability. For radio-detected stars with available effective temperatures and rotational periods, we estimated gyrochronological ages, which resulted in values of Tgyro < 1 Gyr, with the majority of the sample being younger than 150 Myr. We found no strong connection between the occurrence of optical flares and radio variability for the individually studied stars. Conclusions. We conclude that radio emission from intermediate-to-late type flaring stars is of synchrotron nature, and shares a common origin with X-ray processes. It is created by a predominantly young stellar population, and is the collective contribution of stellar flares, accretion, and coronal heating.

astro-ph.SR↗

arXiv2025

Zooming into the neutrino-associated blazar candidate J1718+4239

The radio quasar NVSS~J171822+423948 (J1718+4239) was proposed as the counterpart of the IceCube neutrino event IC-201221A. To reveal the nature of the source, we conducted new very long baseline interferometry (VLBI) observations of this blazar candidate with the Very Long Baseline Array (VLBA). The observations were carried out in dual-band mode between $4$ and $7$~GHz. Archival radio data from the literature were also collected for comparison. Our analysis revealed highly variable, Doppler-boosted radio emission of the source, with compact structure at both kpc and pc scales, a slightly inverted spectrum, and a maximum jet inclination angle of $\theta\le5\degr$. These results confirm J1718+4239 as a blazar-type object.

astro-ph.HE↗

arXiv2025

Solid Identification of Extragalactic Gamma-Ray Source Using High-Resolution Radio Interferometric Observation

The dominant fraction of the extragalactic $\gamma$-ray sources are blazars, active galactic nuclei with jets inclined at a small angle to the line of sight. Apart from blazars, a few dozen narrow-line Seyfert 1 galaxies (NLS1) and a number of radio galaxies are associated with $\gamma$-ray sources. The identification of $\gamma$-ray sources requires multiwavelength follow-up observations since several candidates could reside within the relatively large $\gamma$-ray localisation area. The $\gamma$-ray source 4FGL 0959.6+4606 was originally associated with a radio galaxy. However, follow-up multiwavelength work suggested a nearby NLS1 as the more probable origin of the $\gamma$-ray emission. We performed high-resolution very long baseline interferometry (VLBI) observation at 5 GHz of both proposed counterparts of 4FGL 0959.6+4606. We clearly detected the NLS1 source SDSS J095909.51+460014.3 with relativistically boosted jet emission. On the other hand, we did not detect milliarcsecond-scale compact emission in the radio galaxy 2MASX J09591976+4603515. Our VLBI imaging results suggest that the NLS1 object is the origin of the $\gamma$-ray emission in 4FGL 0959.6+4606.

astro-ph.HE↗

arXiv2024

Deep silence: radio properties of little red dots

To investigate the radio properties of the recently found high-redshift population, we collected a sample of $919$ little red dots (LRDs) from the literature. By cross-matching their coordinates with the radio catalogues based on the first- and second-epoch observations of the Very Large Array Sky Survey (VLASS) and the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey, we found no radio counterparts coinciding with any of the LRDs. To uncover possible sub-mJy level weak radio emission, we performed mean and median image stacking analyses of empty-field 'Quick Look' VLASS and FIRST image cutouts centred on the LRD positions. We found no radio emission above $3\sigma$ noise levels ($\sim11$ and $\sim18~\mu$Jy~beam$^{-1}$ for the VLASS and FIRST maps, respectively) in either of the stacked images for the LRD sample, while the noise levels of the single-epoch images are comparable to those found earlier in the stacking of high-redshift radio-quiet active galactic nuclei (AGNs). The non-detection of radio emission in LRDs suggests these sources host weaker (or no) radio AGNs.

astro-ph.GA↗

arXiv2023

Milliarcsecond-scale radio structure of the most distant BL Lac object candidate at redshift 6.57

The existence of accreting supermassive black holes up to billions of solar masses at early cosmological epochs (in the context of this work, redshifts z>=6) requires very fast growth rates which is challenging to explain. The presence of a relativistic jet can be a direct indication of activity and accretion status in active galactic nuclei (AGN), constraining the radiative properties of these extreme objects. However, known jetted AGN beyond z~6 are still very rare. The radio-emitting AGN J2331+1129 has recently been claimed as a candidate BL Lac object at redshift z=6.57, based on its synchrotron-dominated emission spectrum and the lack of ultraviolet/optical emission lines. It is a promising candidate for the highest-redshift blazar known to date. The aim of the observations described here was to support or refute the blazar classification of this peculiar source. We performed high-resolution radio interferometric imaging observations of J2331+1129 using the Very Long Baseline Array at 1.6 and 4.9 GHz in 2022 Feb. The images revealed a compact but slightly resolved, flat-spectrum core feature at both frequencies, indicating that the total radio emission is produced by a compact jet and originates from within a central 10-pc scale region. While these are consistent with the radio properties of a BL Lac object, the inferred brightness temperatures are at least an order of magnitude lower than expected from a Doppler-boosted radio jet, leaving the high-redshift BL Lac identification still an open question.

astro-ph.GA↗

arXiv2023

J2102+6015: an Intriguing Radio-loud Active Galactic Nucleus in the Early Universe

The powerful high-redshift quasar J2102+6015 (at z=4.575) may provide useful information for studying supermassive black hole growth, galaxy evolution and feedback in the early Universe. The source has so far been imaged with very long baseline interferometry (VLBI) at 2/8 GHz (S/X) bands only, showing complex compact structure. Its total radio spectrum peaks at ~6 GHz in the rest frame. There is no sign of Doppler-boosted jet emission, and the separation of the two major features in its east-west oriented structure spanning ~10 milliarcsec does not change significantly on a timescale longer than a decade. However, VLBI astrometric monitoring observations suggest quasi-periodic (~3 yr) variation in its absolute position. J2102+6015 is presumably a young radio source with jets misaligned with respect to the line of sight. Here we briefly report on our new high-resolution imaging observations made with the European VLBI Network (EVN) at 5 and 22 GHz frequencies in 2021 June, and give an overview of what is currently known about this peculiar distant jetted active galactic nucleus.

astro-ph.GA↗

arXiv2023

Very long baseline interferometry observations of the high-redshift blazar candidate J0141-5427

Active galactic nuclei (AGN) have been observed as far as redshift z~7. They are crucial in investigating the early Universe as well as the growth of supermassive black holes at their centres. Radio-loud AGN with their jets seen at a small viewing angle are called blazars and show relativistic boosting of their emission. Thus, their apparently brighter jets are easier to detect in the high-redshift Universe. DES J014132.4-542749.9 is a radio-luminous but X-ray weak blazar candidate at z = 5. We conducted high-resolution radio interferometric observations of this source with the Australian Long Baseline Array at 1.7 and 8.5 GHz. A single, compact radio emitting feature was detected at both frequencies with a flat radio spectrum. We derived the milliarcsecond-level accurate position of the object. The frequency dependence of its brightness temperature is similar to that of blazar sources observed at lower redshifts. Based on our observations, we can confirm its blazar nature. We compared its radio properties with those of two other similarly X-ray-weak and radio-bright AGN, and found that they show very different relativistic boosting characteristics.

astro-ph.GA↗

arXiv2022

VLBI observations of VIK J2318-3113, a quasar at z = 6.44

The nature of jets in active galactic nuclei (AGN) in the early Universe and their feedback to the host galaxy remain a highly topical question. Observations of the radio structure of high-redshift AGNs enabled by very long baseline interferometry (VLBI) provide indispensable input into studies of their properties and role in the galaxies' evolution. Up to now, only five AGNs at redshift $z > 6$ have been studied with the VLBI technique. VIKJ2318-3113 is a recently discovered quasar at z = 6.44 that has not been imaged with VLBI before the current work. Here we present the first VLBI imaging results of this high-redshift quasar, with the aim of corroborating its high-resolution appearance with the physical model of the object. We carried out VLBI phase-referencing observations of VIKJ2318-3113 using the Very Long Baseline Array at two frequencies, 1.6 and 4.7 GHz, and obtained the first view at the radio structure on the milliarcsecond scale. The source was clearly detected at 1.6 GHz. We found that almost all of its radio emission comes from the pc-scale core region. Our dual-frequency observations constrain the spectral index and brightness temperature of the radio core. Its properties are similar to those of other known high-redshift radio-loud AGNs.

astro-ph.CO↗

arXiv2022

Radio-loud Quasars above Redshift 4: VLBI Imaging of an Extended Sample

High-redshift radio sources provide plentiful opportunities for studying the formation and evolution of early galaxies and supermassive black holes. However, the number of known radio-loud active galactic nuclei (AGN) above redshift 4 is rather limited. At high redshifts, it appears that blazars, with relativistically beamed jets pointing towards the observer, are in majority compared to radio-loud sources with jets misaligned with respect to the line of sight. To find more of these misaligned AGN, milliarcsec-scale imaging studies carried out with very long baseline interferometry (VLBI) are needed, as they allow us to distinguish between compact core--jet radio sources and those with more extended emission. Previous high-resolution VLBI studies revealed that some of the radio sources among blazar candidates in fact show unbeamed radio emission on milliarcsecond scales. The most accurate optical coordinates determined with the Gaia astrometric space mission are also useful in the classification process. Here, we report on dual-frequency imaging observations of 13 high-redshift (4 < z < 4.5) quasars at 1.7 and 5 GHz with the European VLBI Network. This sample increases the number of z>4 radio sources for which VLBI observations are available by about a quarter. Using structural and physical properties, such as radio morphology, spectral index, variability, brightness temperature, as well as optical coordinates, we identified six blazars and six misaligned radio AGNs, with the remaining one tentatively identified as blazar.

astro-ph.GA↗

arXiv2019

Three little radio galaxies in the early Universe

Volonteri et al. (2011) found that the number of radio-loud quasars above redshift 4 calculated from the luminosity function (based upon Swift/BAT observations) is much smaller than the number estimated from the known high-redshift beamed sources, blazars, assuming that for every beamed source with a Lorentz factor of $\Gamma$, statistically $2 \Gamma^2$ non-beamed sources should exist. To explain the missing misaligned (non-beamed) population of high-redshift sources, they proposed various explanations, involving heavy optical obscuration and significantly different Lorentz factors at early cosmological epochs. Our EVN observations targeting high-redshift ($z>4$) blazar candidates revealed 3 sources not showing relativistic beaming, but rather kpc-scale double structures. These three sources have significant radio emission resolved out with the EVN, while they are compact on $\sim 5-10$ arcsec scale. Our dual-frequency ($1.5$ and $5$ GHz) e-MERLIN observations of these three sources revealed a rich morphology, bending jets, and hot spots with possible sites of interaction between the jets and the surrounding medium at intermediate scales.

astro-ph.GA↗

arXiv2018

High-Resolution Radio Image of a Candidate Radio Galaxy at z=5.72

Recently, Saxena et al. (2018) reported the discovery of a possible radio galaxy, J1530$+$1049 at a redshift of z=5.72. We observed the source with the European Very Long Baseline Interferometry Network at $1.7$ GHz. We detected two faint radio features with a separation of $\sim 400$ mas. The radio power calculated from the VLA flux density by Saxena et al. (2018), and the projected source size derived from our EVN data place J1530$+$1049 among the medium-sized symmetric objects (MSOs) which are thought to be young counterparts of radio galaxies (An and Baan 2012). Thus, our finding is consistent with a radio galaxy in an early phase of its evolution as proposed by Saxena et al. (2018).

astro-ph.GA↗

arXiv2018

Constraining the radio jet proper motion of the high-redshift quasar J2134-0419 at z=4.3

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 modeled its brightness distribution. By identifying a jet component at both epochs separated by 15.86 yr, a proper motion of mu=0.035 +- 0.023 mas/yr is found. It corresponds to an apparent superluminal speed of beta_a=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 deg, with a minimum bulk Lorentz factor Gamma=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 analyzed archival Very Large Array observations of J2143-0419 and found indication of a bent jet extending to ~30 kpc.

astro-ph.GA↗