Radio Structure Research Articles

Black hole jets on the scale of the cosmic web.

When sustained for megayears (refs. 1,2), high-power jets from supermassive black holes (SMBHs) become the largest galaxy-made structures in the Universe3. By pumping electrons, atomic nuclei and magnetic fields into the intergalactic medium (IGM), these energetic flows affect the distribution of matter and magnetism in the cosmic web4-6 and could have a sweeping cosmological influence if they reached far at early epochs. For the past 50 years, the known size range of black hole jet pairs ended at 4.6-5.0 Mpc (refs. 7-9), or 20-30% of a cosmic void radius in the Local Universe10. An observational lack of longer jets, as well as theoretical results11, thus suggested a growth limit at about 5 Mpc (ref. 12). Here we report observations of a radio structure spanning about 7 Mpc, or roughly 66% of a coeval cosmic void radius, apparently generated by a black hole between and 6.3 Gyr after the Big Bang. The structure consists of a northern lobe, a northern jet, a core, a southern jet with an inner hotspot and a southern outer hotspot with a backflow. This system demonstrates that jets can avoid destruction by magnetohydrodynamical instabilities over cosmological distances, even at epochs when the Universe was 7 to times denser than it is today. How jets can retain such long-lived coherence is unknown at present.

A global view on star formation: The GLOSTAR Galactic plane survey

Context. The GLObal view on STAR formation in the Milky Way (GLOSTAR) survey studies star formation with the Very Large Array (VLA) and the Effelsberg 100 meter radio telescope in the Galactic plane between −2° < ℓ < 60° and |b| < 1°, and the Cygnus X region (76° < ℓ < 83° and −1° < b < 2°), with unprecedented sensitivity in both flux density (1σ ~50 μJy beam−1) and the capability of detecting emission with angular scales in the range from 1.″0 to the largest radio structures in the Galaxy on the order of a few degrees in size. Aims. Here, we provide a complete GLOSTAR-VLA D-configuration radio source catalog for the part of the Galactic disk covered. A catalog for the “pilot region” between 28° < ℓ < 36° has been published in a previous paper and here we present the complementary catalog for the area within 2° < ℓ < 28°, 36° < ℓ < 60° and |b| < 1°. Methods. Observations were taken with the VLA in a 4–8 GHz band to image 100 square degrees of the inner Galactic disk at a reference frequency of 5.8 GHz, using a total of 260 h of telescope time. We determined spectral indices (α; Sν ∝ να) inside the observed band and in the frequency range of 1.4–5.8 GHz by complementing our results with those from The HI/OH/Recombination line survey of the inner Milky Way (THOR), which covers 1–2 GHz. Results. The final images have an angular resolution of 18″ and an average sensitivity of 123 μJy beam−1. The sensitivity is better (~60 μJy beam−1) in areas free of extended emission. The complementary Galactic disk catalog presented in this work consists of 11 211 radio sources. Of these, 1965 are known large-scale structure sources such as star-forming region complexes, well-known supernova remnants (SNRs), SNR candidates, or parts thereof. The remaining 9227 are discrete individual sources. Source parameters–namely flux densities, sizes, spectral indices, and classifications-are reported. We identify 769 H II region candidates, 359 of which have been newly classified as such. The mean value of spectral indices of 225 H II regions is +0.14 ± 0.02, consistent with most of them emitting optically thin thermal radio emission. Combining our results with the previously published catalog of the pilot region, the final GLOSTAR-VLA D-configuration catalog contains 12 981 radio sources.

ASKAP reveals the radio tail structure of the Corkscrew Galaxy shaped by its passage through the Abell 3627 cluster

ABSTRACT Among the bent-tail radio galaxies common in galaxy clusters are some with long, collimated tails (so-called head-tail galaxies) shaped by their interactions with the intracluster medium (ICM). Here, we report the discovery of intricate filamentary structure in and beyond the ∼28 arcmin (570 kpc) long, helical radio tail of the Corkscrew Galaxy (1610–60.5, ESO 137-G007), which resides in the X-ray bright cluster Abell 3627 (D = 70 Mpc). Deep radio continuum data were obtained with wide-field phased array feeds on the Australian Square Kilometer Array Pathfinder (ASKAP) at 944 MHz and 1.4 GHz. While the bright (young) part of the Corkscrew Galaxy tail is highly collimated, the faint (old) part shows increasing oscillation amplitudes, break-ups, and filaments. We find a stunning set of arc-shaped radio filaments beyond and mostly orthogonal to the collimated Corkscrew tail end, forming a partial bubble. This may be the first detection of a ‘proto-lobe’ previously seen in 3D MHD simulations, formed by the face-on impact of the Corkscrew Galaxy with a shock front in the cluster outskirts. Interactions of the radio galaxy tail with the ICM are likely responsible for the tail collimation and shear forces within the ICM for its increasingly filamentary structure. We also report the discovery of small (∼20–30 kpc) ram-pressure stripped radio tails in four Abell 3627 cluster galaxies, located between the Corkscrew Galaxy and its prominent neighbour, the wide-angle tail (WAT) radio galaxy 1610–60.8 (ESO 137-G006).

Discovery of a 100 kpc Narrow Curved Twin Jet in the S-shaped Giant Radio Galaxy J0644+1043

We report the discovery of an S-shaped morphology of the radio galaxy J0644+1043 imaged with a 30 μJy beam−1 sensitive 525 MHz broadband (bands 3 + 4) uGMRT map. Dedicated spectroscopic observations of the host galaxy carried out with the 2 m Rozhen telescope yielded a redshift of 0.0488, giving a projected linear size of the peculiar radio structure of over 0.7 Mpc. This giant radio galaxy is powered by a black hole of mass 4.1−2.87+9.39×108 M ⊙, from the vicinity of which emanate well-collimated and knotty jets, each ∼100 kpc long. The entire radio structure, presumably due to the effective jet precession, is less than 50 Myr old, has a power of ∼6 × 1024 W Hz−1 at 1.4 GHz, and has observed morphological characteristics that do not strictly conform to the traditional Fanaroff–Riley (FR) FR I or FR II categories.

Results of a long-term optical variability study of 11 quasars and VRI photometry of comparison stars

ABSTRACT We present the results of a 15-yr long-term optical monitoring of 11 quasars conducted with the 2-m Ritchey–Chretien–Coude and the 50/70 cm Schmidt telescopes at the Rozhen National Astronomical Observatory, Bulgaria. Our observations are performed with standard Johnson-Cousins $VRI$ band filters and for each quasar we present a set of comparison standard stars that can be used for monitoring of objects in these fields (including finding charts for the stars identification). The variability and periodicity of each quasar are analysed individually and discussed. The physical properties of each quasar, such as their classification, redshift, and radio structures, are also discussed based on previous literature. Damped random walk model shows the best or the second best fit to the light curves of all objects. However, in six cases periodic models provide comparably good fits and make these six objects a valuable addition to the growing sample of quasars with periodic flux variation. They will be suitable for further investigation of the hitherto unclear mechanisms that give rise to this variability pattern. Our results provide important insights into the long-term variability and physical properties of quasars, which can further deepen our understanding of the nature and evolution of active galaxy nuclei.

Sloshing and spiral structures breeding a putative radio mini-halo in the environment of a cool-core cluster, Abell 795

ABSTRACT Spiral structures and cold fronts in X-rays are frequently observed in cool-core galaxy clusters. However, studies on radio mini-haloes associated with such spirals and their physical connections are rare. Here, we present the detection of an extended diffuse radio emission entrained in the X-ray spiral structure in a known cool-core cluster, Abell 795. Though the cool core is a sign of the relaxed nature, our re-analysed 30-ks Chandra X-ray data of Abell 795 confirm the presence of an interesting log spiral structure of an X-ray deficit region complemented by an X-ray excess counter spiral in the residual map, exposing its dynamical activity. Our new analysis of 150- and 325-MHz Giant Metrewave Radio Telescope archival data confirms the detection of a ∼180-kpc ultra-steep (α ∼ −2.7) diffuse radio structure, previously reported as a candidate radio mini-halo from low-sensitive survey maps. This emission spans the entire spiral structure, enclosed by two previously reported cold fronts. Furthermore, optical spectra from the Sloan Digital Sky Survey Data Release 13 and far-ultraviolet data from the Galaxy Evolution Explorer show a considerably low total star formation rate of 2.52 M⊙ yr−1 with no significant variation in metallicity distribution. We argue that the two-phase (hot and cold) plasma at the core with differential velocity has plausibly caused the spiral formation and has redistributed the secondary electrons from the brightest cluster galaxy or the pre-accelerated electrons, which have been (re-)accelerated by the sloshing turbulence to form the observed candidate radio mini-halo structure. This is supported by a few previous studies indicating that spiral formation and sloshing turbulence quenches star formation and facilitates smooth metallicity distribution by mixing the gas in the core.

Identification of multicomponent LOFAR sources with multimodal deep learning

ABSTRACT Modern high-sensitivity radio telescopes are discovering an increased number of resolved sources with intricate radio structures and fainter radio emissions. These sources often present a challenge because source detectors might identify them as separate radio sources rather than components belonging to the same physically connected radio source. Currently, there are no reliable automatic methods to determine which radio components are single radio sources or part of multicomponent sources. We propose a deep-learning classifier to identify those sources that are part of a multicomponent system and require component association on data from the LOFAR Two-Metre Sky Survey. We combine different types of input data using multimodal deep learning to extract spatial and local information about the radio source components: a convolutional neural network component that processes radio images is combined with a neural network component that uses parameters measured from the radio sources and their nearest neighbours. Our model retrieves 94 per cent of the sources with multiple components on a balanced test set with 2683 sources and achieves almost 97 per cent accuracy in the real imbalanced data (323 103 sources). The approach holds potential for integration into pipelines for automatic radio component association and cross-identification. Our work demonstrates how deep learning can be used to integrate different types of data and create an effective solution for managing modern radio surveys.

Understanding the impact of persistence and propagation on the Age of Information of broadcast traffic in 5G NR-V2X sidelink communications

The current Cellular V2X (C-V2X) standard for direct communication between vehicles is based on the so called Semi-Persistent Scheduling (SPS) algorithm. It is based on the multiple access structure of 5G New Radio (NR) and exploits sensing and persistence to realize a randomized multiple access. We consider the application of SPS to support periodic broadcast traffic where no acknowledgments are provided. Persistence is a key feature, which consists of a node using the same resource for its transmissions for multiple frames. The specific resource used is randomly selected from among those that are perceived to be idle, and reselected anew after a randomized number of frames. We define a model to gain insight into the interplay between persistence and key performance metrics for the type of traffic considered, namely probability of successful delivery and Age of Information (AoI). A core version of the model is validated against simulations and used to show that there exists an optimal level that minimizes AoI. The model is then extended to account for a distance-dependent propagation model, allowing further insight into the effects of the interplay between sender–receiver distance and persistence. Finally, an even more detailed model is investigated, using ns-3-based simulations. This further analysis confirms the qualitative behaviors revealed by the analytical model and provides more insight into the complex interactions of system parameters and channel characteristics. The obtained results help to identify the limits of SPS, opening the way to system-principled parameter optimization and design of more powerful variants of the multiple access scheme.

Possible jet contribution to the gamma-ray luminosity in NGC 1068

NGC\,1068 is a nearby, widely studied Seyfert II galaxy presenting radio, infrared, X-ray, and gamma -ray emission, along with strong evidence for high-energy neutrino emission. Recently, the evidence for neutrino emission was explained in a multimessenger model, whereby the neutrinos originate from the corona of the active galactic nucleus (AGN). In this environment, gamma -rays are strongly absorbed, so that an additional contribution is necessar, for instance, from the circumnuclear starburst ring. In this work, we discuss whether the radio jet can be an alternative source of the gamma -rays between about $0.1$ and $100$ GeV, as observed by Fermi-LAT. In particular, we include both leptonic and hadronic processes, namely,\ accounting for inverse Compton emission and signatures from $pp$ as well as $p interactions. In order to constrain our calculations, we used VLBA and ALMA observations of the radio knot structures, which are spatially resolved at different distances from the supermassive black hole (SMBH). Our results show that the best leptonic scenario for the prediction of the Fermi-LAT data is provided by the radio knot closest to the central engine. For that to be the case, a magnetic field strength of $ mG $ is needed as well as a strong spectral softening of the relativistic electron distribution at $(1-10)\ GeV $. However, we show that neither such a weak magnetic field strength, nor such a strong softening is expected for that knot. A possible explanation for the sim 10 GeV gamma -rays could potentially be provided by hadronic pion production in case of a gas density $ cm $. Nonetheless, this process is not found to contribute significantly to the low-energy end of the Fermi-LAT range. We conclude that the emission sites in the jet are not sufficient to explain the gamma -rays across the whole Fermi-LAT energy band.

Abell 746: A Highly Disturbed Cluster Undergoing Multiple Mergers

We present deep XMM-Newton, Karl G. Jansky Very Large Array, and upgraded Giant Metrewave Radio Telescope observations of Abell 746, a cluster that hosts a plethora of diffuse emission sources that provide evidence for the acceleration of relativistic particles. Our new XMM-Newton images reveal a complex morphology of the thermal gas with several substructures. We observe an asymmetric temperature distribution across the cluster: the southern regions exhibit higher temperatures, reaching ∼9 keV, while the northern regions have lower temperatures (≤4 keV), likely due to a complex merger. We find evidence of three surface brightness edges and one candidate edge, of which three are merger-driven shock fronts. Combining our new data with published LOw-Frequency ARray observations has unveiled the nature of diffuse sources in this system. The bright NW relic shows thin filaments and a high degree of polarization with aligned magnetic field vectors. We detect a density jump, aligned with the fainter relic to the north. To the south, we detect high-temperature regions, consistent with the shock-heated regions and a density jump coincident with the northern tip of the southern radio structure. Its integrated spectrum shows a high-frequency steepening. Lastly, we find that the cluster hosts large-scale radio halo emission. A comparison of the thermal and nonthermal emission reveals an anticorrelation between the bright radio and X-ray features at the center. Our findings suggest that Abell 746 is a complex system that involves multiple mergers.

Identification of the optical emission detected by Gaia with radio structures in parsec-scale active galactic nucleus jets

Aims. We investigated the location of the optical emission with respect to the parsec-scale radio structure and attempted to identify whether the optical centroid is coincident with the radio core or a radio component of the jet. Methods. We used optical Gaia DR3 and 8 GHz ICRF3 positions and a model fitting of 15 GHz imaging by the MOJAVE VLBA survey for 422 sources, mostly blazars. We searched for possible associations between optical centroids and radio components. We also compared the astrometric and photometric properties of each category, looked for trends between the various source parameters, and discussed possible biases. Results. Sources can be separated into four categories based on whether their optical centroid (i) falls onto the radio core (category C), (ii) lies close to the base of the jet (category B), (iii) coincides with a radio component downstream in the jet (category J), or (iv) is not found to coincide with a detected radio component (category O). Due to a number of random and systematic errors, the number of sources falling into each category remains approximate but close to 32%, 36%, 22%, and 10%, respectively. The family of quasars, mostly flat spectrum radio quasars (FSRQs), clearly dominates the C and B categories, with their percentage decreasing from categories C to O. Conversely, the family of BL Lacs is spread over the four classes and dominates the category O. Radio galaxies are mainly in classes O and J, and the radio-loud narrow-line Seyfert 1 all belong to class C. An individual analysis of O sources shows that, despite the absence of a direct association with a specific radio component, the optical Gaia centroid is globally related to the radio VLBI jet structure. Conclusions. Our study shows that the spatial distribution of the optical counterparts seen by Gaia below the 0.2-arcsec scale around the radio cores appears in the vast majority of cases to be related to the AGN core or to its jet, with most of them located downstream in the jet. Those associated to the core also exhibit a bluer color index, suggesting a possible contribution of the accretion disk to the optical emission, while those associated to a radio knot in the jet appear redder and tend on average to be found in more polarized radio sources. Most BL Lacs have their optical emission coincident with the jet base or a knot in the jet, while sources with an optical emission on or close to the radio core are mostly FSRQs. Radio components associated with the optical centroid at the jet base or along the jet are mainly stationary or quasi-stationary features, with low apparent MOJAVE velocities. There are indications that the apparent proper motion of the Gaia centroids may be higher than the speed of the associated radio components, but the significance of this trend requires further investigation.

European VLBI Network observations of the peculiar radio source 4C 35.06 overlapping with a compact group of nine galaxies

Context. According to the hierarchical structure formation model, brightest cluster galaxies (BCGs) evolve into the most luminous and massive galaxies in the Universe through multiple merger events. The peculiar radio source 4C 35.06 is located at the core of the galaxy cluster Abell 407, overlapping with a compact group of nine galaxies. Low-frequency radio observations have revealed a helical, steep-spectrum, kiloparsec-scale jet structure and inner lobes with less steep spectra, compatible with a recurring active galactic nucleus (AGN) activity scenario. However, the host galaxy of the AGN responsible for the detected radio emission remained unclear. Aims. We aim to identify the host of 4C 35.06 by studying the object at high angular resolution and thereby confirm the recurrent AGN activity scenario. Methods. To reveal the host of the radio source, we carried out very long baseline interferometry (VLBI) observations with the European VLBI Network of the nine galaxies in the group at 1.7 and 4.9 GHz. Results. We detected compact radio emission from an AGN located between the two inner lobes at both observing frequencies. In addition, we detected another galaxy at 1.7 GHz, whose position appears more consistent with the principal jet axis and is located closer to the low-frequency radio peak of 4C 35.06. The presence of another radio-loud AGN in the nonet sheds new light on the BCG formation and provides an alternative scenario in which not just one but two AGNs are responsible for the complex large-scale radio structure.

Magnetically arrested discs in FR I radio galaxies

ABSTRACT A sample of 17 Fanaroff–Riley type I (FR I) radio galaxies constructed from the 3CR catalogue, which is characterized by edge-darkened radio structures, is studied. The optical core luminosities derived from Hubble Space Telescope observation are used to estimate the Eddington ratios which are found to be below 10−3.4 for this sample. This is supported by the Baldwin–Phillips–Terlevich optical diagnostic diagrams derived with the spectroscopic observation of Telescopio Nazionale Galileo, suggesting that these sources are of low-ionization nuclear emission-line regions. It implies that the accretion in these FR I sources can be modelled as advection-dominated accretion flows (ADAFs). Given the low accretion rate, the predicted jet power with a fast-spinning black hole (BH) a = 0.95 in the Blandford–Znajek mechanics is lower than the estimated one for almost all the sources in our sample. Such powerful jets indicate the presence of magnetically arrested discs (MAD) in the inner region of the ADAF, in the sense that the magnetic fields in the inner accretion zone are strong. Moreover, we show that, even in the MAD scenario, the BH spins in the sample are most likely moderate and/or fast with a ≳ 0.5.

ALMA reveals a compact and massive molecular outflow driven by the young AGN in a nearby ULIRG

ABSTRACT The ultraluminous infrared galaxy F13451+1232 is an excellent example of a galaxy merger in the early stages of active galactic nucleus (AGN) activity, a phase in which AGN-driven outflows are expected to be particularly important. However, previous observations have determined that the mass outflow rates of the warm ionized and neutral gas phases in F13451+1232 are relatively modest, and there has been no robust detection of molecular outflows. Using high-spatial resolution Atacama Large Millimeter/submillimeter Array CO(1–0) observations, we detect a kiloparsec-scale circumnuclear disc, as well as extended (r ∼ 440 pc), intermediate-velocity (300 < |v| < 400 km s−1) cold molecular gas emission that cannot be explained by rotational disc motions. If interpreted as AGN-driven outflows, the mass outflow rates associated with this intermediate-velocity gas are relatively modest ($\dot{M}_\mathrm{out}=22$–27 M⊙ yr−1); however, we also detect a compact (rout < 120 pc), high-velocity (400 < v < 680 km s−1) cold molecular outflow near the primary nucleus of F13451+1232, which carries an order of magnitude more mass ($\dot{M}_\mathrm{out}$ ∼ 230 M⊙ yr−1) than (and several times the kinetic power of) the previously detected warmer phases. Moreover, the similar spatial scales of this compact outflow and the radio structure indicate that it is likely accelerated by the small-scale (r ∼ 130 pc) AGN jet in the primary nucleus of F13451+1232. Considering the compactness of the nuclear outflow and intermediate-velocity non-rotating gas that we detect, we argue that high-spatial resolution observations are necessary to properly quantify the properties of AGN-driven outflows and their impacts on host galaxies.

Typing supernova remnant G352.7−0.1 using XMM–Newton X-ray observations

ABSTRACT G352.7−0.1 is a mixed-morphology (MM) supernova remnant (SNR) with multiple radio arcs and has a disputed supernova origin. We conducted a spatially resolved spectroscopic study of the remnant with XMM–Newton X-ray data to investigate its explosion mechanism and explain its morphology. The global X-ray spectra of the SNR can be adequately reproduced using a metal-rich thermal plasma model with a temperature of ∼2 keV and ionization time-scale of ∼3 × 1010 cm−3 s. Through a comparison with various supernova nucleosynthesis models, we found that observed metal properties from Mg to Fe can be better described using core-collapse supernova models, while thermonuclear models fail to explain the observed high Mg/Si ratio. The best-fit supernova model suggests a ∼13 M⊙ progenitor star, consistent with previous estimates using the wind bubble size. We also discussed the possible mechanisms that may lead to SNR G352.7−0.1 being an MMSNR. By dividing the SNR into several regions, we found that the temperature and abundance do not significantly vary with regions, except for a decreased temperature and abundance in a region interacting with molecular clouds. The brightest X-ray emission of the SNR spatially matches with the inner radio structure, suggesting that the centrally filled X-ray morphology results from a projection effect.

Explaining supernova remnant G352.7-0.1 as a peculiar type Ia supernova inside a planetary nebula

I identify a point-symmetric morphology of the supernova remnant (SNR) G352.7-0.1 and propose that the outer axially-symmetric structure is the remnant of a common envelope evolution (CEE) of the progenitor system, while the inner structure is the ejecta of a thermonuclear explosion triggered by the merger of a white dwarf (WD) and the core of an asymptotic giant branch (AGB) star. The main radio structure of SNR G352.7-0.1 forms an outer (large) ellipse. The bright X-ray emitting gas forms a smaller ellipse with a symmetry axis inclined to the symmetry axis of the large radio ellipse. The high abundance of iron and the energy of its X-ray lines suggest a type Ia supernova (SN Ia). The massive swept-up gas suggests a relatively massive progenitor system. I propose a scenario with progenitors of initial masses of MZAMS,1≃5–7M⊙ and MZAMS,2≃4–5M⊙. At a later phase, the WD remnant of the primary star and the AGB secondary star experience a CEE that ejects the circumstellar material that swept up more ISM to form the large elliptical radio structure. An explosion during the merger of the WD with the core of the AGB star triggered a super-Chandrasekhar thermonuclear explosion that formed the inner structure that is bright in X-ray. A tertiary star in the system caused the misalignment of the two symmetry axes. This study adds to the wide variety of evolutionary routes within the scenarios of normal and peculiar SNe Ia.

Proper motion of the radio jets in two blazars at redshift above 3

ABSTRACT 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.

Superluminal Motion and Jet Parameters in the Gamma-ray-Emitting Narrow-Line Seyfert 1 Galaxy TXS 1206+549

Narrow-line Seyfert 1 (NLS1) galaxies are a peculiar subclass of active galactic nuclei (AGN). Among them, TXS 1206+549 belongs to a small group of radio-loud and γ-ray-emitting NLS1 galaxies. We focus on the radio properties of this galaxy by analysing archival, high-resolution, very long baseline interferometry (VLBI) imaging observations taken at 8 GHz frequency in six epochs between 1994 and 2018. Using the milliarcsecond-scale radio structure, we can resolve a core and a jet component whose angular separation increases by (0.055±0.006) mas yr−1. This corresponds to an apparent superluminal jet component motion of (3.5±0.4)c. From the core brightness temperature and the jet component proper motion, we determine the characteristic Doppler-boosting factor, the bulk Lorentz factor, and the jet viewing angle. We find no compelling evidence for a very closely aligned blazar-type jet. The parameters for TXS 1206+549 resemble those of radio-loud quasar jets with a moderate Lorentz factor (Γ≈4) and ϑ≈24∘ inclination to the line of sight.

Nuclear regions as seen with LOFAR international baselines

Context. Radio galaxies dominate the sky at radio wavelengths and represent an essential piece in the galaxy evolution puzzle. High-resolution studies focussed on statistical samples of radio galaxies are expected to shed light on the triggering mechanisms of the active galactic nucleus in their centre, alternating between the phases of activity and quiescence. Aims. For this work, we zoomed in on the sub-arcsec radio structures in the central regions of the 35 radio galaxies in the area covering 6.6 deg2 of the Lockman Hole region. The sources studied here were previously classified as active, remnant, and candidate restarted radio galaxies based on the LOw Frequency ARray (LOFAR) observations at 150 MHz. We examined the morphologies and studied the spectral properties of their central regions to explore their evolutionary stages and to revise the morphological and spectral criteria used to select the initial sample. Methods. We used the newly available LOFAR 150 MHz image obtained using international baselines, yielding a resolution of 0.38″ × 0.30″, making this the first systematic study of the nuclear regions at such a high resolution and low frequency. We used publicly available images from the Faint Images of the Radio Sky at Twenty-cm survey at 1.4 GHz and the Karl G. Jansky Very Large Array (VLA) Sky Survey at 3 GHz to achieve our goals. In addition, for one of the restarted candidates, we present new dedicated observations with the VLA at 3 GHz. Results. We characterised the central regions of the radio galaxies in our sample and found various morphologies, some even mimicking well-known double-double radio galaxies but on a smaller scale, that is, a few tens of kiloparsecs for the size of the restarted activity. We also see the beginnings of active jets or distinct detections unrelated to the large-scale structure. Furthermore, we found a variety of radio spectra characterising the sources in our sample, such as flat, steep, or peaked in the frequency range between 150 MHz and 3 GHz, indicative of the different life-cycle phases of the sources in our sample. Based on these analyses, we confirm five out of six previously considered restarted candidates and identify three more restarted candidates from the active sample. As the number of restarted candidates still exceeds that of remnant candidates, this is consistent with previous results suggesting that the restarted phase can occur after a relatively short remnant phase (i.e. a few tens of millions of years).

Compact Symmetric Objects. III. Evolution of the High-luminosity Branch and a Possible Connection with Tidal Disruption Events

We use a sample of 54 compact symmetric objects (CSOs) to confirm that there are two unrelated CSO classes: an edge-dimmed, low-luminosity class (CSO 1), and an edge-brightened, high-luminosity class (CSO 2). Using blind tests, we show that CSO 2s consist of three subclasses: CSO 2.0, having prominent hot spots at the leading edges of narrow jets and/or narrow lobes; CSO 2.2, without prominent hot spots and with broad jets and/or lobes; and CSO 2.1, which exhibit mixed properties. Most CSO 2s do not evolve into larger jetted active galactic nuclei (AGN), but spend their whole life cycle as CSOs of size ≲500 pc and age ≲5000 yr. The minimum energies needed to produce the radio luminosity and structure in CSO 2s range from ∼10−4 M ⊙ c 2 to ∼7 M ⊙ c 2. We show that the transient nature of most CSO 2s, and their birth rate, can be explained through ignition in the tidal disruption events of stars. We also consider possibilities of tapping the spin energy of the supermassive black hole, and tapping the energy of the accretion disk. Our results demonstrate that CSOs constitute a large family of AGN in which we have thus far studied only the brightest. More comprehensive CSO studies, with higher sensitivity, resolution, and dynamic range, will revolutionize our understanding of AGN and the central engines that power them.