Search

We characterise the co-evolution of radio-loud AGN and their galaxies by mapping the dependence of radio-loud AGN activity on stellar mass and star-formation rate (SFR) across cosmic time (out to $z \sim 1.5$). Deep LOFAR radio observations are combined with large galaxy samples to study the incidence of radio-loud AGN across the galaxy population; the AGN are further split into low-excitation radio galaxies (LERGs) and high-excitation radio galaxies (HERGs). We find that LERG activity occurs over a wide range of SFRs, whereas HERGs are typically found in galaxies with ongoing star formation. The LERGs are then split based on their SFRs relative to the main sequence, across redshift. Within quiescent galaxies, LERG activity shows a steep stellar mass dependence with the same normalisation across the past $\sim$ 10 Gyr; this indicates that hot gas fuels LERGs in quiescent galaxies across cosmic time. In massive galaxies ($\log_{10}(M/\rm{M_{\odot}}) \gtrsim 11$), the incidence of LERGs is roughly constant across the galaxy population, suggesting that LERGs in massive galaxies may be fuelled by hot gas regardless of the star-formation activity. At lower masses, however, LERG activity is significantly more enhanced (by a factor of up to 10) in star-forming galaxies compared to quiescent galaxies; this suggests that an additional fuelling mechanism, likely associated with cold gas, may fuel the LERGs in galaxies with higher SFRs. We find that HERGs typically accrete above 1 per cent of the Eddington-scaled accretion rate, and the LERGs typically accrete below this level., Comment: Accepted for publication in MNRAS, 18 pages, 10 figures

Merging galaxy clusters often host spectacular diffuse radio synchrotron sources. These sources can be explained by a non-thermal pool of relativistic electrons accelerated by shocks and turbulence in the intracluster medium. The origin of the pool and details of the cosmic ray transport and acceleration mechanisms in clusters are still open questions. Due to the often extremely steep spectral indices of diffuse radio emission, it is best studied at low frequencies. However, the lowest frequency window available to ground-based telescopes (10-30 MHz) has remained largely unexplored, as radio frequency interference and calibration problems related to the ionosphere become severe. Here, we present LOFAR observations from 16 to 168 MHz targeting the famous cluster Abell 2256. In the deepest-ever images at decametre wavelengths, we detect and resolve the radio halo, radio shock and various steep spectrum sources. We measure standard single power-law behaviour for the radio halo and radio shock spectra and find significant spectral index and curvature fluctuations across the radio halo, indicating an inhomogeneous emitting volume. In contrast to the straight power-law spectra of the large-scale diffuse sources, the various AGN-related sources often show extreme steepening towards higher frequencies and flattening towards low frequencies. We also discover a new fossil plasma source with a steep spectrum between 23 and 144 MHz, with $\alpha=-1.9\pm 0.1$. Finally, by comparing radio and gamma-ray observations, we rule out purely hadronic models for the radio halo origin in Abell 2256, unless the magnetic field strength in the cluster is exceptionally high, which is unsupportable by energetic arguments and inconsistent with the knowledge of other cluster magnetic fields., Comment: Accepted for publication in Astronomy & Astrophysics

The largely unexplored decameter radio band (10-30 MHz) provides a unique window for studying a range of astronomical topics, such as auroral emission from exoplanets, inefficient cosmic ray acceleration mechanisms, fossil radio plasma, and free-free absorption. The scarcity of low-frequency studies is mainly due to the severe perturbing effects of the ionosphere. Here we present a calibration strategy that can correct for the ionosphere in the decameter band. We apply this to an observation from the Low Frequency Array (LOFAR) between 16 to 30 MHz . The resulting image covers 330 square degrees of sky at a resolution of 45", reaching a sensitivity of 12 mJy/beam. Residual ionospheric effects cause additional blurring ranging between 60 to 100". This represents an order of magnitude improvement in terms of sensitivity and resolution compared to previous decameter band observations. In the region we surveyed, we have identified four fossil plasma sources. These rare sources are believed to contain old, possibly re-energised, radio plasma originating from previous outbursts of active galactic nuclei. At least three of them are situated near the center of low-mass galaxy clusters. Notably, two of these sources display the steepest radio spectral index among all the sources detected at 23 MHz. This indicates that fossil plasma sources constitute the primary population of steep-spectrum sources at these frequencies, emphasising the large discovery potential of ground-based decameter observations., Comment: 19 pages, 9 figures, 1 table. Submitted version, full version is published by Nature Astronomy

In this study, we compare the radio, optical and environmental properties of GRGs with those of a control sample of smaller RGs we found in the three LOw-Frequency ARray (LOFAR) deep fields, namely the Bootes, ELAIS-N1, Lockman Hole, for a total area of about 95 deg^2. We inspected the LOFAR deep fields and created a catalogue of 1609 extended radio galaxies (ERGs). By visual inspection, we identified their host galaxies and spectroscopically or photometrically classified 280 of these as GRGs. We studied their properties, such as their accretion state, stellar mass and star formation rate (SFR) using deep optical and infrared survey data. Moreover, we explored the environment in terms of the surface number density of neighbouring galaxies within these surveys. Integrated flux densities and radio luminosities were also determined for a subset of ERGs through available survey images at 50, 150, 610, and 1400 MHz to compute integrated spectral indices. Considering the fraction of GRGs displaying an FRII morphology alongside the host galaxy properties, we suggest that GRGs consistently possess sufficient power to overcome jet frustration caused by the interstellar medium. Moreover, clear differences emerge in the environmental densities between GRGs and smaller RGs, using the number of neighbouring galaxies within 10 Mpc from the host galaxy as a proxy. GRGs preferentially reside in sparser environments compared to their smaller counterparts. In particular, only 3.6% of the GRGs reside within a 3D comoving distance of 5 Mpc from a previously reported galaxy cluster. We found that larger sources exhibit steeper integrated spectral indices, suggesting that GRGs are late-stage versions of RGs. These results suggest that GRGs are amongst the oldest radio sources with the most stable nuclear activity that reside in sparse environments., Comment: 21 pages, 11 figures. Accepted for publication with A&A. The full table will be available in electronic form at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (130.79.128.5) or via https://cdsarc.cds.unistra.fr/cgi-bin/qcat?J/A+A/

We present a comprehensive study of the physical origin of radio emission in optical quasars at redshifts z < 2.5. We focus particularly on the associations between compact radio emission, dust reddening, and outflows identified in our earlier work. Leveraging the deepest low-frequency radio data available to date (LoTSS Deep DR1), we achieve radio detection fractions of up to 94%, demonstrating the virtual ubiquity of radio emission in quasars, and a continuous distribution in radio loudness. Through our analysis of radio properties, combined with spectral energy distribution modeling of multiwavelength photometry, we establish that the primary source of radio emission in quasars is the AGN, rather than star formation. Modeling the dust reddening of the accretion disk emission shows a continuous increase in radio detection in quasars as a function of the reddening parameter E(B-V), suggesting a causal link between radio emission and dust reddening. Confirming previous findings, we observe that the radio excess in red quasars is most pronounced for sources with compact radio morphologies and intermediate radio loudness. We find a significant increase in [Oiii] and Civ outflow velocities for red quasars not seen in our control sample, with particularly powerful [Oiii] winds in those around the radio-quiet/radio-loud threshold. Based on the combined characterisation of radio, reddening, and wind properties in our sample, we favor a model in which the compact radio emission observed in quasars originates in compact radio jets and their interaction with a dusty, circumnuclear environment. Our results align with the theory that jet-induced winds and shocks resulting from this interaction are the origin of the enhanced radio emission in red quasars. Further investigation of this model is crucial for advancing our understanding of quasar feedback mechanisms and their role in galaxy evolution., Comment: 11 pages, 8 Figures, submitted to A&A

We present the detailed analysis of the thermal, diffuse emission of the proto-intracluster medium (ICM) detected in the halo of the Spiderweb Galaxy at z=2.16, within a radius of $\sim$ 150 kpc. We combined deep X-ray data from Chandra and millimeter observations of the Sunyaev-Zeldovich (SZ) effect obtained by ALMA. Thanks to independent measurements of the pressure profile from ALMA SZ observation and the electron density profile from the available X-ray data, we derived, for the first time, the temperature profile in the ICM of a z>2 protocluster. It reveals the presence of a strong cool core (comparable to the local ones) that may host a significant mass deposition flow, consistent with measured local star formation values. We also find mild evidence of an asymmetry in the X-ray surface brightness distribution, which may be tentatively associated with a cavity carved into the proto-ICM by the radio jets or, alternatively, may be due to the young dynamical status of the halo. The cooling time of baryons in the core of the Spiderweb Protocluster is estimated to be $\sim$ 0.1 Gyr, implying that the baryon cycle in the first stages of the protocluster formation is characterised by a high-duty cycle and a very active environment. In the case of the Spiderweb protocluster, we are witnessing the presence of a strongly peaked core that is possibily hosting a cooling flow with a mass deposition rate up to 250-1000 $M_{\odot}$/yr, responsible for feeding both the central supermassive black hole and the high star formation rate observed in the Spiderweb Galaxy. This phase is expected to be rapidly followed by active galactic nucleus feedback events, whose onset may have already left an imprint in the radio and X-ray appearance of the Spiderweb protocluster, eventually driving the ICM into a self-regulated, long-term evolution in less than one Gyr.

Radio galaxies dominate the radio sky and are essential to the galaxy evolution puzzle. High-resolution studies of statistical samples of radio galaxies are expected to shed light on the triggering mechanisms of the AGN, alternating between the phases of activity and quiescence. In this work, we focus on the sub-arcsec radio structures in the central regions of the 35 radio galaxies over 6.6 $deg^2$ of the Lockman Hole region. These sources were previously classified as active, remnant, and candidate restarted radio galaxies using 150 MHz LOFAR observations. We examine the morphologies and study the spectral properties of their central regions to explore their evolutionary stages and revise the criteria used to select the initial sample. We use the newly available LOFAR 150 MHz image obtained using international baselines, achieving 0.38'' x 0.30'' resolution, making this the first systematic study of the nuclear regions at high resolution and low frequency. We use publicly available images from the FIRST survey at 1.4 GHz and the Karl G. Jansky VLA Sky Survey at 3 GHz to achieve our goals. In addition, for one restarted candidate we present new dedicated observations with the VLA at 3 GHz. We have found various morphologies of the central regions of the radio galaxies in our sample, some resembling miniature double-double radio galaxies. We also see the beginnings of active jets or distinct detections unrelated to the large-scale structure. Furthermore, we have found diverse radio spectra in our sample - flat, steep, or peaked between 150 MHz and 3 GHz, indicative of the different life-cycle phases. Based on these analyses, we confirm five of six previously considered restarted candidates and identify three more from the active sample, supporting previous results suggesting that the restarted phase can occur after a relatively short remnant phase (i.e. a few tens of millions of years)., Comment: Accepted for publication in A&A, 22 pages, 5 tables, 8 figures

It has long been known that luminous, ultra-steep spectrum radio sources are preferentially associated with massive galaxies at high redshifts. Here we describe a pilot project directed at such objects, to demonstrate the feasibility and importance of using LOFAR to study the most distant forming massive galaxies and protoclusters. We have successfully imaged four high-redshift ($z>2$) high-luminosity radio galaxies with sub-arcsecond resolution, at 144 MHz, using the International LOFAR Telescope (ILT). Our targets were 4C 41.17 ($z=3.8$), the "Anthill", B2 0902+34 ($z=3.4$), 4C 34.34 ($z=2.4$) and 4C 43.15 ($z=2.5$). Their low-frequency morphologies and the spatial distributions of their low-frequency spectral indices have been mapped, and compared with available optical, infrared, and X-ray images. Both for the Anthill at $z = 3.8$ and B2 0902+34 at $z=3.4$, the location of the steepest radio emission coincides with the Ly$\alpha$ emitting ionized gas halo. Our pilot project demonstrates that, because of its outstanding sensitivity and high angular resolution at low frequencies, the ILT is a unique facility for studying the co-evolution and interaction of massive galaxies, galaxy clusters, and supermassive black holes in the early Universe., Comment: 10 pages, 10 figures, 2 tables

We present a detailed study of the cosmic star formation history over $90$ per cent of cosmic time ($0\lesssim z\lesssim4$), using deep, radio continuum observations that probe star formation activity independent of dust. The Low Frequency Array Two Metre Sky Survey has imaged three well-studied extragalactic fields, Elais-N1, Bo\"otes and the Lockman Hole, reaching $\sim20\,\mu\rm{Jy/beam}$ rms sensitivity at $150\,\rm{MHz}$. The availability of high-quality ancillary data from ultraviolet to far-infrared wavelengths has enabled accurate photometric redshifts and the robust separation of radio-bright AGN from their star-forming counterparts. We capitalise on this unique combination of deep, wide fields and robustly-selected star-forming galaxies to construct radio luminosity functions and derive the cosmic star formation rate density. We carefully constrain and correct for scatter in the $L_{150\,\rm{MHz}}-\rm{SFR}$ relation, which we find to be $\sim0.3\,\rm{dex}$. Our derived star formation rate density lies between previous measurements at all redshifts studied. We derive higher star formation rate densities between $z\sim0$ and $z\sim3$ than are typically inferred from short wavelength emission; at earlier times, this discrepancy is reduced. Our measurements are generally in good agreement with far-infrared and radio-based studies, with small offsets resulting from differing star formation rate calibrations., Comment: 22 pages, 14 figures; accepted for publication in MNRAS

In the nearby universe jets from AGN are observed to have a dramatic impact on their surrounding extragalactic environment. Their effect at the `cosmic noon' (z>1.5), the epoch when star formation and AGN activity peak, is instead much less constrained. Here we present a study of the giant (750 kpc) radio galaxy 103025+052430 located at the centre of a protocluster at redshift z=1.7, with a focus on its interaction with the external medium. We present new LOFAR observations at 144 MHz, which we combine with VLA 1.4 GHz and 0.5-7 keV Chandra archival data. The new map at 144 MHz confirms that the source has a complex morphology, possibly consistent with the `hybrid morphology' classification. The large size of the source gave us the possibility to perform a resolved radio spectral index analysis, a very unique opportunity for a source at such high redshift. This reveals a tentative flattening of the radio spectral index at the edge of the backflow in the Western lobe, which might be indicating plasma compression. The spatial coincidence between this region and the thermal X-ray bubble C suggests a causal connection between the two. Contrary to previous estimates for the bright X-ray component A, we find that inverse Compton scattering between the radio-emitting plasma of the Eastern lobe and the CMB photons can account for a large fraction (~45%-80%) of its total 0.5-7 keV measured flux. Finally, the X-ray bubble C, which is consistent with a thermal origin, is found to be significantly overpressurised with respect to the ambient medium. This suggests that it will tend to expand and release its energy in the surroundings, contributing to the overall intracluster medium heating. Overall, 103025+052430 gives us the chance to investigate the interaction between AGN jets and the surrounding gas in a system that is likely the predecessor of the rich galaxy clusters we all well know at z=0., Comment: 12 pages, 5 figures, 2 tables, accepted for publication in A&A

The Low Frequency Array (LOFAR) is the only existing radio interferometer able to observe at ultra-low frequencies (<100 MHz) with high resolution (<15") and high sensitivity (<1 mJy/beam). To exploit these capabilities, the LOFAR Surveys Key Science Project is using the LOFAR Low Band Antenna (LBA) to carry out a sensitive wide-area survey at 41-66 MHz named the LOFAR LBA Sky Survey (LoLSS). LoLSS is covering the whole northern sky above declination 24 deg with a resolution of 15" and a sensitivity of 1-2 mJy/beam (1 sigma) depending on declination, field properties, and observing conditions. Here we present the first data release. An automated pipeline was used to reduce the 95 fields included in this data release. The data reduction procedures developed for this project have general application and are currently being used to process LOFAR LBA interferometric observations. Compared to the preliminary release, direction-dependent errors have been corrected for during the calibration process. This results in a typical sensitivity of 1.55 mJy/beam at the target resolution of 15". The first data release of the LOFAR LBA Sky Survey covers 650 sqdeg in the HETDEX spring field. The resultant data products released to the community include mosaic images (I and V Stokes) of the region, and a catalogue of 42463 detected sources and related Gaussian components used to describe sources' morphologies. Separate catalogues for 6 in-band frequencies are also released. The first data release of LoLSS shows that, despite the influences of the ionosphere, LOFAR can conduct large-scale surveys in the frequency window 42-66 MHz with unprecedentedly high sensitivity and resolution. The data can be used to derive unique information on the low-frequency spectral properties of many thousands of sources with a wide range of applications in extragalactic and galactic astronomy., Comment: 20 pages, 22 figures, images and catalogues available at https://www.lofar-surveys.org/lolss.html

We present a sample of 373 peaked-spectrum (PS) sources with spectral peaks around 150MHz, selected using a subset of two LOFAR all-sky surveys, the LOFAR Two Meter Sky Survey and the LOFAR LBA Sky Survey. These surveys are the most sensitive low-frequency widefield surveys to date, allowing us to select low-luminosity PS sources. Our sample increases the number of known PS sources in our survey area by a factor 50. The 5GHz luminosity distribution of our PS sample shows we sample the lowest luminosity PS sources to-date by nearly an order of magnitude. Since high-frequency PS sources and compact steep-spectrum sources are hypothesised to be the precursors to large radio galaxies, we investigate whether this is also the case for our sample of low-frequency PS sources. Using optical line emission criteria, we find that our PS sources are predominately high-excitation radio galaxies instead of low-excitation radio galaxies, corresponding to a quickly evolving population. We compute the radio source counts of our PS sample, and find they are scaled down by a factor of $\sim$40 compared to a general sample of radio-loud active galactic nuclei (AGN). This implies that the lifetimes of PS sources are 40 times shorter than large scale radio galaxies, if their luminosity functions are identical. To investigate this, we compute the first radio luminosity function for a homogeneously-selected PS sample. We find that for 144MHz luminosities $\gtrsim 10^{25}$W Hz$^{-1}$, the PS luminosity function has the same shape as an unresolved radio-loud AGN population but shifted down by a factor of $\sim$10. We interpret this as strong evidence that these high-luminosity PS sources evolve into large-scale radio-loud AGN. For local, low-luminosity PS sources, there is a surplus of PS sources, which we hypothesise to be the addition of frustrated PS sources that do not evolve into large-scale AGN., Comment: 21 pages, accepted in Astronomy & Astrophysics

High redshift quasars ($z>5$) that also shine brightly at radio wavelengths are unique signposts of supermassive black hole activity in the early universe. However, bright radio sources at $z\ge5$ are extremely rare and therefore we have started a campaign to search for new high-$z$ quasars by combining an optical dropout selection driven by the $g$, $r$, and $z$ bands from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys with low-frequency radio observations from the LOFAR Two-metre Sky Survey (LoTSS). Currently, LoTSS covers a large fraction of the northern sky (5720 deg$^2$) to such a depth (median noise level of 83 $\mu$Jy beam$^{-1}$) that about 30% of the general quasar population is detected $-$ which is a factor of 5-10 more than previous large sky radio surveys such as NVSS and FIRST, respectively. In this paper, we present the discovery of 20 new quasars (and the independent confirmation of 4) between $4.9\leq z\leq 6.6$. Out of the 24 quasars, 21 satisfy the traditional radio-loudness criterion of $R=f_{5\text{GHz}}/f_{4400A} > 10$, with the full sample spanning $R\sim$6-1000, thereby more than doubling the sample of known radio-loud quasars at $z \ge 5$. Our radio detection requirement strongly decreases the contamination of stellar sources and allows one to select these quasars in a broad redshift range. Despite selecting our quasar candidates using fewer and less conservative colour restrictions, both the optical and near-infrared colours, Ly$\alpha$ emission line properties, and dust reddening, $E(B-V)$, measurements of our quasar sample do not deviate from the known radio-quiet quasar population, suggesting similar optical quasar properties of the radio-loud and radio-quiet quasar population at high-$z$. Our campaign demonstrates the potential for discovering new high-$z$ quasar populations through next generation radio continuum surveys., Comment: 19 pages, 11 figures, Tab A1 corrected, accepted for publication in A&A

We present the X-ray imaging and spectral analysis of the diffuse emission around the Spiderweb galaxy at z=2.16 and of its nuclear emission, based on a deep (700 ks) Chandra observation. We characterize the nuclear emission and computed the contamination in the surrounding regions due to the wings of the instrument PSF. Then, we quantified the extended emission within 12". We find that the Spiderweb galaxy hosts a mildly absorbed quasar, with modest yet significant variability on a timescale of ~1 year. We find that the emission in the jet regions is well described by a power law with Gamma~2-2.5, and it is consistent with IC upscattering of the CMB photons by the relativistic electrons. We also find a roughly symmetric, diffuse emission within a radius of ~100 kpc. This emission is consistent with thermal bremsstrahlung from a hot ICM with a temperature of kT=2.0_{-0.4}^{+0.7} keV, and a metallicity of Z<1.6Z_sun. The average electron density within 100 kpc is n_e=(1.51+-0.24+-0.14)E-2 cm^{-3}, corresponding to an upper limit for the total ICM mass of <=(1.76+-0.30+-0.17)E+12 M_sun (where error bars are 1 sigma statistical and systematic, respectively). If we apply hydrostatic equilibrium to the ICM, we measure a total gravitational mass M(<100 kpc)=(1.5^{+0.5}_{-0.3})E+13 M_sun and, extrapolating at larger radii, we estimate a total mass M_{500}=(3.2^{+1.1}_{-0.6})E+13 M_sun within a radius of r_{500}=(220+-30) kpc. We conclude that the Spiderweb protocluster shows significant diffuse emission within a radius of 12 arcsec, whose major contribution is provided by IC scattering associated with the radio jets. Outside the jet regions, we also identified thermal emission within a radius of ~100 kpc, revealing the presence of hot, diffuse baryons that may represent the embryonic virialized halo of the forming cluster., Comment: 19 pages, 16 figures, accepted in Astronomy & Astrophysics

We present the first detailed analysis of the radio halo in the merging galaxy cluster Abell 2256 using the LOFAR, uGMRT, and VLA. These observations combined with archival X-ray data allowed us to study the halo emission with unprecedented detail. The integrated radio emission from the entire halo is characterized by an ultra-steep spectrum, which can be described by a power law with $\alpha_{144 \rm MHz}^{1.5 \rm GHz}=-1.63\pm0.03$, and a radial steepening in the outer regions. The halo is significantly underluminous according to the scaling relations between radio power and mass at 1.4 GHz but not at 150 MHz; ultra-steep spectrum halos are predicted to be statistically underluminous. Despite the complex structure of this system, the radio halo morphology is remarkably similar to that of the X-ray emission. The radio surface brightness distribution across the halo is strongly correlated with the X-ray brightness of the intracluster medium. The derived correlations show sublinear slopes and there are distinct structures: the core is $\rm I_{R}\propto I_{X}^{1.51}$, the outermost region $\rm I_{R}\propto I_{X}^{0.41}$, and we find radio morphological connections with X-ray discontinuities. We also find a strong anti-correlation between the radio spectral index and the X-ray surface brightness, implying radial steepening. We suggests that the halo core is either related to old plasma from previous AGN activity, being advected, compressed and re-accelerated by mechanisms activated by the cold front or less turbulent with strong magnetic field in the core. The change in the radio vs X-ray correlation slopes in the outer regions of the halo could be due to a radial decline of magnetic field, increase in the number density of seed particles or increasing turbulence. Our findings suggest that that the emitting volume is not homogenous according to turbulence re-acceleration models., Comment: 19 pages, 15 figures, accepted for publication in A&A

We present deep broadband radio polarization observations of the Spiderweb radio galaxy (J1140-2629) in a galaxy proto-cluster at $z=2.16$. These yield the most detailed polarimetric maps yet made of a high redshift radio galaxy. The intrinsic polarization angles and Faraday Rotation Measures (RMs) reveal coherent magnetic fields spanning the $\sim60$ kpc length of the jets, while $\sim50$% fractional polarizations indicate these fields are well-ordered. Source-frame absolute RM values of $\sim1,000$ rad/m/m are typical, and values up to $\sim11,100$ rad/m/m are observed. The Faraday-rotating gas cannot be well-mixed with the synchrotron-emitting gas, or stronger-than-observed depolarization would occur. Nevertheless, an observed spatial coincidence between a localized absolute RM enhancement of $\sim1,100$ rad/m/m, a bright knot of Ly$\alpha$ emission, and a deviation of the radio jet provide direct evidence for vigorous jet-gas interaction. We detect a large-scale RM gradient totaling $\sim1,000$s rad/m/m across the width of the jet, suggesting a net clockwise (as viewed from the AGN) toroidal magnetic field component exists at 10s-of-kpc-scales, which we speculate may be associated with the operation of a Poynting-Robertson cosmic battery. We conclude the RMs are mainly generated in a sheath of hot gas around the radio jet, rather than the ambient foreground proto-cluster gas. The estimated magnetic field strength decreases by successive orders-of-magnitude going from the jet hotspots ($\sim90$ $\mu$G) to the jet sheath ($\sim10$ $\mu$G) to the ambient intracluster medium ($\sim1$ $\mu$G). Synthesizing our results, we propose that the Spiderweb radio galaxy is actively magnetizing its surrounding proto-cluster environment, with possible implications for theories of the origin and evolution of cosmic magnetic fields., Comment: 12 pages, 7 figures, 2 tables. Accepted for publication in ApJ

3C186, a radio-loud quasar at $z=1.0685$, was previously reported to have both velocity and spatial offsets from its host galaxy, and has been considered as a promising candidate for a gravitational wave recoiling black hole triggered by a black hole merger. Another possible scenario is that 3C186 is in an on-going galaxy merger, exhibiting a temporary displacement. In this study, we present analyses of new deep HST/WFC3-IR and ACS images, aiming to characterize the host galaxy and test this alternative scenario. We carefully measure the light-weighted center of the host and reveal a significant spatial offset from the quasar core ($11.1\pm0.1$kpc). The direction of the confirmed offset aligns almost perpendicularly to the radio jet. We do not find evidence of a recent merger, such as a young starburst in disturbed outskirts, but only marginal light concentration in F160W at $\sim30$kpc. The host consists of matured ($>200$Myr) stellar populations and one compact star-forming region. We compare with hydro-dynamical simulations and find that those observed features are consistently seen in late-stage merger remnants. Taken together, those pieces of evidence indicate that the system is not an on-going/young merger remnant, suggesting that the recoiling black hole scenario is still a plausible explanation for the puzzling nature of 3C186., Comment: Accepted for publication in ApJ. 14 pages, 8 figures, 2 tables

Feedback from low-excitation radio galaxies (LERGs) plays a key role in the lifecycle of massive galaxies in the local Universe; their evolution, and the impact of these active galactic nuclei on early galaxy evolution, however, remain poorly understood. We use a sample of 10481 LERGs from the first data release of the LOFAR Two-meter Sky Survey Deep Fields, covering $\sim$ 25 deg$^2$, to present the first measurement of the evolution of the radio luminosity function (LF) of LERGs out to $z\sim2.5$; this shows relatively mild evolution. We split the LERGs into those hosted by quiescent and star-forming galaxies, finding a new dominant population of LERGs hosted by star-forming galaxies at high redshifts. The incidence of LERGs in quiescent galaxies shows a steep dependence on stellar-mass out to $z \sim1.5$, consistent with local Universe measurements of accretion occurring from cooling of hot gas haloes. The quiescent-LERGs dominate the LFs at $z<1$, showing a strong decline in space density with redshift, tracing that of the available host galaxies, while there is an increase in the characteristic luminosity. The star-forming LERG LF increases with redshift, such that this population dominates the space densities at most radio-luminosities by $z \sim 1$. The incidence of LERGs in star-forming galaxies shows a much weaker stellar-mass dependence, and increases with redshift, suggesting a different fuelling mechanism compared to their quiescent counterparts, potentially associated with the cold gas supply present in the star-forming galaxies., Comment: 26 pages, 16 figures, accepted for publication in MNRAS. LF data available at: https://github.com/rohitk-10/AGN_LF_Kondapally22 . Author list error corrected in updated version

Deep Chandra and VLA imaging reveals a clear correlation between X-ray and radio emission on scales $\sim 100$~kpc in the Spiderweb radio galaxy at z=2.16. The X-ray emission associated with the extended radio source is likely dominated by inverse Compton up-scattering of cosmic microwave background photons by the radio emitting relativistic electrons. For regions dominated by high surface brightness emission, such as hot spots and jet knots, the implied magnetic fields are $\sim 50~\mu$G to $70~\mu$G. The non-thermal pressure is these brighter regions is then $\sim 9\times 10^{-10}$ dyne cm$^{-2}$, or three times larger than the non-thermal pressure derived assuming minimum energy conditions, and an order of magnitude larger than the thermal pressure in the ambient cluster medium. Assuming ram pressure confinement implies an average advance speed for the radio source of $\sim 2400$ km s$^{-1}$, and a source age of $\sim 3\times 10^7$ years. Considering the lower surface brightness, diffuse radio emitting regions, we identify an evacuated cavity in the Ly$\alpha$ emission coincident with the tail of the eastern radio lobe. Making reasonable assumptions for the radio spectrum, we find that the relativistic electrons and fields in the lobe are plausibly in pressure equilibrium with the thermal gas, and close to a minimum energy configuration. The radio morphology suggests that the Spiderweb is a high-$z$ example of the rare class of hybrid morphology radio sources (or HyMoRS), which we attribute to interaction with the asymmetric gaseous environment indicated by the Ly$\alpha$ emission., Comment: 8 figures, 25 pages, to appear in the ApJ

(Abridged) We present an analysis of the 700 ks Chandra ACIS-S observation of the field around the Spiderweb Galaxy at z=2.156, focusing on the nuclear activity in the associated large-scale environment. We identify unresolved X-ray sources down to flux limits of 1.3X10^{-16} and 3.9X10^{-16} erg/s/cm^2 in the soft and hard band, respectively. We search for counterparts in the optical, NIR and submm bands to identify X-ray sources belonging to the protocluster. We detect 107 X-ray unresolved sources within 5 arcmin (corresponding to 2.5 Mpc) of J1140-2629, among which 13 have optical counterparts with spectroscopic redshift 2.11~1.84+-0.04. The best-fit intrinsic absorption for 5 protocluster X-ray members is N_H>10^{23} cm^{-2}, while other 6 have upper limits of the order of fewX10^{22} cm^{-2}. Two sources can only be fitted with very flat \Gamma<=1, and are therefore considered Compton-thick candidates. Their 0.5-10 keV rest frame luminosities are larger than 2X10^{43} erg/s, significantly greater than X-ray luminosities expected from star formation activity. The X-ray luminosity function of AGN in the volume associated to the Spiderweb protocluster in the range 10^{43}10.5, corresponding to an enhancement of 6.0^{+9.0}_{-3.0} with respect to the COSMOS field at comparable redshifts and stellar mass range. We conclude that the galaxy population in the Spiderweb Protocluster is characterized by enhanced X-ray nuclear activity triggered by environmental effects on Mpc scales., Comment: Astronomy & Astrophysics. Minor changes in this version. Version accepted in A&A

In this data release from the LOFAR Two-metre Sky Survey (LoTSS) we present 120-168MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44$^\circ$30' and 1h00m +28$^\circ$00' and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451hrs (7.6PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4,396,228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6" resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144MHz have: a median rms sensitivity of 83$\mu$Jy/beam; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2"; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8mJy/beam. By creating three 16MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of +/-0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20" resolution 120-168MHz continuum images have a median rms sensitivity of 95$\mu$Jy/beam, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 x 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8mJy/beam at 4' and 2.2mJy/beam at 20"; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data., Comment: 23 figures, 1 table and 29 pages. The catalogues, images and uv-data associated with this data release are publicly available via https://lofar-surveys.org/

Radio-loud active galaxies have two accretion modes [radiatively inefficient (RI) and radiatively efficient (RE)], with distinct optical and infrared signatures, and two jet dynamical behaviours, which in arcsec- to arcmin-resolution radio surveys manifest primarily as centre- or edge-brightened structures [Fanaroff-Riley (FR) class I and II]. The nature of the relationship between accretion mode and radio morphology (FR class) has been the subject of long debate. We present a comprehensive investigation of this relationship for a sample of 286 well-resolved radio galaxies in the LOFAR Two-metre Sky Survey Deep Fields (LoTSS-Deep) first data release, for which robust morphological and accretion mode classifications have been made. We find that two-thirds of luminous FRII radio galaxies are RI, and identify no significant differences in the visual appearance or source dynamic range (peak/mean surface brightness) of the RI and RE FRIIs, demonstrating that both RI and RE systems can produce FRII structures. We also find a significant population of low-luminosity FRIIs (predominantly RI), supporting our earlier conclusion that FRII radio structures can be produced at all radio luminosities. We demonstrate that in the luminosity range where both morphologies are present, the probability of producing FRI or FRII radio morphology is directly linked to stellar mass, while across all morphologies and luminosities, RE accretion occurs in systems with high specific star formation rate, presumably because this traces fuel availability. In summary, the relationship between accretion mode and radio morphology is very indirect, with host-galaxy environment controlling these two key parameters in different ways., Comment: 22 pages, 17 figures, 7 tables. Accepted for publication in MNRAS. Update with minor changes to match published version

We present deep and high fidelity images of the merging galaxy cluster Abell 2256 at low frequencies, using the upgraded Giant Metrewave Radio Telescope (uGMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most prominent known relics, with a remarkably spectacular network of filamentary substructures. The new uGMRT (300-850 MHz) and LOFAR (120-169 MHz) observations, combined with the archival Karl G. Jansky Very Large Array (VLA; 1-4 GHz) data, allowed us to carry out the first spatially resolved spectral analysis of the exceptional relic emission down to 6 arcsec resolution over a broad range of frequencies. Our new sensitive radio images confirm the presence of complex filaments of magnetized relativistic plasma also at low frequencies. We find that the integrated spectrum of the relic is consistent with a single power law, without any sign of spectral steepening, at least below 3 GHz. Unlike previous claims, the relic shows an integrated spectral index of $-1.07\pm0.02$ between 144 MHz and 3 GHz, which is consistent with the (quasi)stationary shock approximation. The spatially resolved spectral analysis suggests that the relic surface very likely traces the complex shock front, with a broad distribution of Mach numbers propagating through a turbulent and dynamically active intracluster medium. Our results show that the northern part of the relic is seen edge-on and the southern part close to face-on. We suggest that the complex filaments are regions where higher Mach numbers dominate the (re-)acceleration of electrons that are responsible for the observed radio emission., Comment: 27 pages, 14 Figures, accepted for publication in ApJ

We present observations obtained with the VLT/MUSE optical integral field spectrograph of the radio source 3C277.3, located at a redshift of 0.085 and associated with the galaxy Coma A. An emission line region fully enshrouds the double-lobed radio source, which is ~60 kpc x 90 kpc in size. Based on the emission line ratios, we identified five compact knots in which the gas ionization is powered by young stars located as far as ~60 kpc from the host. The emission line filaments surrounding the radio emission are compatible with ionization from fast shocks (with a velocity of 350-500 km/s), but a contribution from star formation occurring at the edges of the radio source is likely. Coma A might be a unique example in the local Universe in which the expanding outflow triggers star formation throughout the whole radio source., Comment: Pre-proofs version - Accepted for publication in A&A

Optically luminous quasars at $z > 5$ are important probes of super-massive black hole (SMBH) formation. With new and future radio facilities, the discovery of the brightest low-frequency radio sources in this epoch would be an important new probe of cosmic reionization through 21-cm absorption experiments. In this work, we systematically study the low-frequency radio properties of a sample of 115 known spectroscopically confirmed $z>5$ quasars using the second data release of the Low Frequency Array (LOFAR) Two Metre Sky survey (LoTSS-DR2), reaching noise levels of $\sim$80 $\mu$Jy beam$^{-1}$ (at 144 MHz) over an area of $\sim5720$ deg$^2$. We find that 41 sources (36%) are detected in LoTSS-DR2 at $>2 \sigma$ significance and we explore the evolution of their radio properties (power, spectral index, and radio loudness) as a function of redshift and rest-frame ultra-violet properties. We obtain a median spectral index of $-0.29^{+0.10}_{-0.09}$ by stacking 93 quasars using LoTSS-DR2 and Faint Images of the Radio Sky at Twenty Centimetres (FIRST) data at 1.4 GHz, in line with observations of quasars at $z<3$. We compare the radio loudness of the high-$z$ quasar sample to a lower-$z$ quasar sample at $z\sim2$ and find that the two radio loudness distributions are consistent with no evolution, although the low number of high-z quasars means that we cannot rule out weak evolution. Furthermore, we make a first order empirical estimate of the $z=6$ quasar radio luminosity function, which is used to derive the expected number of high-$z$ sources that will be detected in the completed LoTSS survey. This work highlights the fact that new deep radio observations can be a valuable tool in selecting high-$z$ quasar candidates for follow-up spectroscopic observations by decreasing contamination of stellar dwarfs and reducing possible selection biases introduced by strict colour cuts., Comment: 11 pages, 10 figures, accepted for publication in A&A

[abridged] The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3" x 0.2" although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $\mu$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $\mu$Jy/bm for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image ~900 sources per LoTSS pointing. This equates to ~3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit., Comment: Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR. 24 pages, 16 figures

Relatively little information is available about the Universe at ultra-low radio frequencies, i.e. below 50 MHz (ULF), although the ULF spectral window contains a wealth of unique diagnostics for studying galactic and extragalactic phenomena. Sub-arcsecond resolution imaging at these frequencies is extremely difficult, due to the long baselines (>1000 km) required and large ionospheric perturbations. We have conducted a pilot project to investigate the ULF performance and potential of the International LOFAR Telescope (ILT), a trans-European interferometric array with baselines up to ~2000 km and observing frequencies down to 10 MHz. We have successfully produced images with sub-arcsecond resolution for 6 radio sources at frequencies down to 30 MHz. This is more than an order of magnitude better resolution than pre-ILT observations at similar frequencies. The six targets that we have imaged (3C 196, 3C 225, 3C 273, 3C 295, 3C 298 and 3C 380) are bright radio sources with compact structures. By comparing our data of 3C 196 and 3C 273 with observations at higher frequencies, we investigate their spatially resolved radio spectral properties. Our success shows that at frequencies down to 30 MHz, sub-arcsecond imaging with the ILT is possible. Further analysis is needed to determine the feasibility of observations of fainter sources or sources with less compact emission., Comment: Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR. 11 pages, 8 figures, 3 tables

The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30006 observations of 24713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of $\geq$ 200 km baselines is identified by the detection of compact flux density, for declinations north of 30 degrees and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2 arcsec, for sources observed in the survey. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2 arcsec and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the Murchison Widefield Array (MWA) shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS., Comment: Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR

We report a detailed CO(1-0) survey of a galaxy protocluster field at $z=2.16$, based on 475 hours of observations with the Australia Telescope Compact Array. We constructed a large mosaic of 13 individual pointings, covering an area of 21 arcmin$^2$ and $\pm6500$ km/s range in velocity. We obtain a robust sample of 46 CO(1-0) detections spanning $z=2.09-2.22$, constituting the largest sample of molecular gas measurements in protoclusters to date. The CO emitters show an overdensity at $z=2.12-2.21$, suggesting a galaxy super-protocluster or a protocluster connected to large-scale filaments with ~120 cMpc size. We find that 90% CO emitters have distances $>0'.5-4'$ to the center galaxy, indicating that small area surveys would miss the majority of gas reservoirs in similar structures. Half of the CO emitters have velocities larger than escape velocities, which appears gravitationally unbound to the cluster core. These unbound sources are barely found within the $R_{200}$ radius around the center, which is consistent with a picture in which the cluster core is collapsed while outer regions are still in formation. Compared to other protoclusters, this structure contains relatively more CO emitters with relatively narrow line width and high luminosity, indicating galaxy mergers. We use these CO emitters to place the first constraint on the CO luminosity function and molecular gas density in an overdense environment. The amplitude of the CO luminosity function is 1.6$\pm$0.5 orders of magnitudes higher than observed for field galaxy samples at $z\sim2$, and one order of magnitude higher than predictions for galaxy protoclusters from semi-analytical SHARK models. We derive a high molecular gas density of $0.6-1.3\times10^{9}$ $M_\odot$ cMpc$^{-3}$ for this structure, consistent with predictions for cold gas density of massive structures from hydro-dynamical DIANOGA simulations., Comment: 21 pages + Appendices, 9 figures, 6 tables. Accepted for publication in A&A

LOFAR is the only radio telescope that is presently capable of high-sensitivity, high-resolution (<1 mJy/b and <15") observations at ultra-low frequencies (<100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range 42-66 MHz, at a resolution of 15 arcsec and at a sensitivity of 1 mJy/b. Here we outline the survey strategy, the observational status, the current calibration techniques, and briefly describe several scientific motivations. We also describe the preliminary public data release. The preliminary images were produced using a fully automated pipeline that aims to correct all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, are not yet corrected, the images presented in this work are still 10 times more sensitive than previous surveys available at these low frequencies. The preliminary data release covers 740 sqdeg around the HETDEX spring field region at a resolution of 47" with a median noise level of 5 mJy/b. The images and the catalogue with 25,247 sources are publicly released. We demonstrate that the system is capable of reaching an rms noise of 1 mJy/b and the resolution of 15" once direction-dependent effects are corrected for. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique dataset that can be used for a wide range of science topics such as: the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, and from AGN activity., Comment: 19 pages, 14 figures, Accepted A&A, catalogue and images on www.lofar-surveys.org

We present the final observations of a complete sample of 37 radio galaxies from the Third Cambridge Catalog (3C) with redshift <0.3 and declination <20 degrees obtained with the VLT/MUSE optical integral field spectrograph. These data were obtained as part of the MURALES survey (a MUse RAdio Loud Emission line Snapshot survey) with the main goal of exploring the AGN feedback process in the most powerful radio sources. We present the data analysis and, for each source, the resulting emission line images and the 2D gas velocity field. Thanks to such an unprecedented depth these observations reveal emission line regions (ELRs) extending several tens of kiloparsec in most objects. The gas velocity shows ordered rotation in 25 galaxies, but in several sources it is highly complex. We find that the 3C sources show a connection between radio morphology and emission line properties. In the ten FRI sources the line emission region is generally compact, only a few kpc in size, and only in one case it exceeds the size of the host. Conversely, all but two of the FRII galaxies show large-scale structures of ionized gas. The median extent is 16 kpc with the maximum reaching a size of ~80 kpc. There are no apparent differences in extent or strength between the ELR properties of the FRII sources of high and low gas excitation. We confirm that the previous optical identification of 3C258 is incorrect: this radio source is likely associated with a QSO at z~ 1.54., Comment: 29 pages; accepted for publication on A&A

Various studies have laid claim to finding an alignment of the polarization vectors or radio jets of active galactic nuclei (AGN) over large distances, but these results have proven controversial and so far, there is no clear explanation for this observed alignment. To investigate this case further, we tested the hypothesis that the position angles of radio galaxies are randomly oriented in the sky by using data from the Low-Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS). A sample of 7,555 double-lobed radio galaxies was extracted from the list of 318,520 radio sources in the first data release of LoTSS at 150 MHz. We performed statistical tests for uniformity of the two-dimensional (2D) orientations for the complete 7,555 source sample. We also tested the orientation uniformity in three dimensions (3D) for the 4,212 source sub-sample with photometric or spectroscopic redshifts. Our sample shows a significant deviation from uniformity (p-value < $10^{-5}$) in the 2D analysis at angular scales of about four degrees, mainly caused by sources with the largest flux densities. No significant alignment was found in the 3D analysis. Although the 3D analysis has access to fewer sources and suffers from uncertainties in the photometric redshift, the lack of alignment in 3D points towards the cause of the observed effect being unknown systematics or biases that predominantly affect the brightest sources, although this has yet to be demonstrated irrefutably and should be the subject of subsequent studies., Comment: 10 pages, 13 figures. Accepted for publication in Astronomy & Astrophysics

We explore the low-frequency radio properties of the sources in the Fanaroff-Riley class 0 catalog (FR0CAT) as seen by the LOw Frequency ARray (LOFAR) observations at 150 MHz. This sample includes 104 compact radio active galactic nuclei (AGN) associated with nearby (z<0.05) massive early-type galaxies. Sixty-six FR0CAT sources are in the sky regions observed by LOFAR and all of them are detected, usually showing point-like structures with sizes smaller than 3-6 kpc. However, 12 FR0s present resolved emission of low surface brightness which contributes between 5% and 40% of the total radio power at 150 MHz, usually with a jetted morphology extending between 15 and 50 kpc. No extended emission is detected around the other FR0s, with a typical luminosity limit of 5 x 10$^{22}$ W/Hz over an area of 100 kpc x 100 kpc. The spectral slopes of FR0s between 150 MHz and 1.4 GHz span a broad range (-0.7 < $\alpha$ < 0.8) with a median value of $\overline\alpha \sim 0.1$; 20% of them have a steep spectrum ($\alpha$ > 0.5), an indication of the presence of substantial extended emission confined within the spatial resolution limit. The fraction of FR0s showing evidence for the presence of jets, by including both spectral and morphological information, is at least ~40%. This study confirms that FR0s and FRIs can be interpreted as two extremes of a continuous population of jetted sources, with the FR0s representing the low end in size and radio power., Comment: Pre-proofs version - Accepted for publication in A&A

The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (<100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15 arcsec with a high dynamic range and good image fidelity. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna (LBA) system. We reduced the datasets and obtained an image for each A-team source. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further., Comment: 7 pages, 2 figures, accepted A&A, online data on A&A website

We present observations of a complete sub-sample of 20 radio galaxies from the Third Cambridge Catalog (3C) with redshift <0.3 obtained from VLT/MUSE optical integral field spectrograph. These data have been obtained as part of the survey MURALES (a MUse RAdio Loud Emission line Snapshot survey) with the main goal of exploring the Active Galactic Nuclei (AGN) feedback process in a sizeable sample of the most powerful radio sources at low redshift. We present the data analysis and, for each source, the resulting emission line images and the 2D gas velocity field. Thanks to their unprecedented depth (the median 3 sigma surface brightness limit in the emission line maps is 6X10^-18 erg s-1 cm-2 arcsec-2, these observations reveal emission line structures extending to several tens of kiloparsec in most objects. In nine sources the gas velocity shows ordered rotation, but in the other cases it is highly complex. 3C sources show a connection between radio morphology and emission line properties. Whereas, in three of the four Fanaroff and Riley Class I radio galaxies (FRIs), the line emission regions are compact, ~1 kpc in size; in all but one of the Class II radiogalaxies FRIIs, we detected large scale structures of ionized gas with a median extent of 17 kpc. Among the FRIIs, those of high and low excitation show extended gas structures with similar morphological properties, suggesting that they both inhabit regions characterized by a rich gaseous environment on kpc scale., Comment: Accepted for publication in A&A

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45$^\circ$00$'$00$''$ to 57$^\circ$00$'$00$''$) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325,694 sources are detected with a signal of at least five times the noise, and the source density is a factor of $\sim 10$ higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S$_{\rm 144 MHz} = 71\,\mu$Jy beam$^{-1}$ and the point-source completeness is 90% at an integrated flux density of 0.45mJy. The resolution of the images is 6$''$ and the positional accuracy is within 0.2$''$. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further $\sim$20 articles that highlight the scientific potential of LoTSS., Comment: 16 figures, 1 table and 22 pages. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available on lofar-surveys.org upon publication of the journal

The LOFAR Two-metre Sky Survey (LoTSS) is a sensitive, high-resolution 120-168 MHz survey of the Northern sky. The LoTSS First Data Release (DR1) presents 424 square degrees of radio continuum observations over the HETDEX Spring Field (10h45m00s $<$ right ascension $<$ 15h30m00s and 45$^\circ$00$'$00$'$ $<$ declination $<$ 57$^\circ$00$'$00$''$) with a median sensitivity of 71$\mu$Jy/beam and a resolution of 6$''$. In this paper we present photometric redshifts (photo-$z$) for 94.4% of optical sources over this region that are detected in the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) 3$\pi$ steradian survey. Combining the Pan-STARRS optical data with mid-infrared photometry from the Wide-field Infrared Survey Explorer, we estimate photo-$z$s using a novel hybrid photometric redshift methodology optimised to produce the best possible performance for the diverse sample of radio continuum selected sources. For the radio-continuum detected population, we find an overall scatter in the photo-$z$ of 3.9% and an outlier fraction ($\left | z_{\rm{phot}} - z_{\rm{spec}} \right | / (1+z_{\rm{spec}}) > 0.15$) of 7.9%. We also find that, at a given redshift, there is no strong trend in photo-$z$ quality as a function of radio luminosity. However there are strong trends as a function of redshift for a given radio luminosity, a result of selection effects in the spectroscopic sample and/or intrinsic evolution within the radio source population. Additionally, for the sample of sources in the LoTSS First Data Release with optical counterparts, we present rest-frame optical and mid-infrared magnitudes based on template fits to the consensus photometric (or spectroscopic when available) redshift., Comment: 16 figures, 1 table and 18 pages. This paper is part of the LOFAR surveys Data Release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, Volume 622. The catalogues and images from the data release will be publicly available at http://www.lofar-surveys.org upon publication of the volume

We constructed a sample of 23,344 radio-loud active galactic nuclei (RLAGN) from the catalogue derived from the LOFAR Two-Metre Sky Survey (LoTSS) survey of the HETDEX Spring field. Although separating AGN from star-forming galaxies remains challenging, the combination of spectroscopic and photometric techniques we used gives us one of the largest available samples of candidate RLAGN. We used the sample, combined with recently developed analytical models, to investigate the lifetime distribution of RLAGN. We show that large or giant powerful RLAGN are probably the old tail of the general RLAGN population, but that the low-luminosity RLAGN candidates in our sample, many of which have sizes $<100$ kpc, either require a very different lifetime distribution or have different jet physics from the more powerful objects. We then used analytical models to develop a method of estimating jet kinetic powers for our candidate objects and constructed a jet kinetic luminosity function based on these estimates. These values can be compared to observational quantities, such as the integrated radiative luminosity of groups and clusters, and to the predictions from models of RLAGN feedback in galaxy formation and evolution. In particular, we show that RLAGN in the local Universe are able to supply all the energy required per comoving unit volume to counterbalance X-ray radiative losses from groups and clusters and thus prevent the hot gas from cooling. Our computation of the kinetic luminosity density of local RLAGN is in good agreement with other recent observational estimates and with models of galaxy formation., Comment: 21 pages, 11 colour figures. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available at https://lofar-surveys.org/ upon publication of the journal

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120-168 MHz survey of the Northern sky with diverse and ambitious science goals. Many of the scientific objectives of LoTSS rely upon, or are enhanced by, the association or separation of the sometimes incorrectly catalogued radio components into distinct radio sources, and the identification and characterisation of the optical counterparts to these sources. Here we present the source associations and optical and/or IR identifications for sources in the first data release, which are made using a combination of statistical techniques and visual association and identification. We document in detail the colour- and magnitude-dependent likelihood ratio method used for statistical identification as well as the Zooniverse project, called LOFAR Galaxy Zoo, used for the visual classification. We describe the process used to select which of these two different methods is most appropriate for each LoTSS source. The final LoTSS-DR1-IDs value-added catalogue presented contains 318,520 radio sources, of which 231,716 (73%) have optical and/or IR identifications in Pan-STARRS and WISE. The value-added catalogue is available online at https://lofar-surveys.org/, as part of this data release., Comment: 8 figures, 5 tables and 23 pages. This paper is part of the LOFAR surveys data release 1 and has been accepted for publication in a special edition of A&A that will appear in Feb 2019, volume 622. The catalogues and images from the data release will be publicly available at https://lofar-surveys.org/ upon publication in the journal

An understanding of the relationship between radio-loud active galaxies and their large-scale environments is essential for realistic modelling of radio-galaxy evolution and environmental impact, for understanding AGN triggering and life cycles, and for calibrating galaxy feedback in cosmological models. We use the LOFAR Two-Metre Sky Survey (LoTSS) Data Release 1 catalogues to investigate this relationship. We cross-matched 8,745 radio-loud AGN with 0.08

We observed the FRII radio galaxy 3C459 (z=0.22) with the MUSE spectrograph at the Very Large Telescope (VLT) as part of the MURALES project (a MUse RAdio Loud Emission line Snapshot survey). We detected diffuse nuclear emission and a filamentary ionized gas structure forming a one-sided, triangular-shaped region extending out to $\sim$80 kpc. The central emission line region is dominated by two compact knots of similar flux: the first (N1) cospatial with the radio core and the (N2) second located 1.2" (5.3 kpc) to the SE. The two regions differ dramatically from the point of view of velocity (with an offset of ~400 km/s), line widths, and line ratios. This suggests that we are observing a dual AGN system formed by a radio loud AGN and type 2 QSO companion, which is the result of the recent merger that also produced its disturbed host morphology. The alternative possibility that N2 is just a bright emission line knot resulting from, for example, a jet-cloud interaction, is disfavored because of 1) the presence of a high ionization bicone whose apex is located at N2; 2) the observed narrow line widths; 3) its line luminosity (~10^42 erg s-1) typical of luminous QSOs; and 4) its location, which is offset from the jet path. The putative secondary AGN must be highly obscured, since we do not detect any emission in the Chandra and infrared Hubble Space Telescope images., Comment: 6 pages, 6 figures, A&A in press

We report the discovery of the most distant radio galaxy to date, TGSS1530 at a redshift of $z=5.72$ close to the presumed end of the Epoch of Reionisation. The radio galaxy was selected from the TGSS ADR1 survey at 150 MHz for having to an ultra-steep spectral index, $\alpha^{\textrm{150 MHz}}_{\textrm{1.4 GHz}} = -1.4$ and a compact morphology obtained using VLA imaging at 1.4 GHz. No optical or infrared counterparts for it were found in publicly available sky surveys. Follow-up optical spectroscopy at the radio position using GMOS on Gemini North revealed the presence of a single emission line. We identify this line as Lyman alpha at $z=5.72$, because of its asymmetric line profile, the absence of other optical/UV lines in the spectrum and a high equivalent width. With a Ly$\alpha$ luminosity of $5.7 \times 10^{42}$ erg s$^{-1}$ and a FWHM of $370$ km s$^{-1}$, TGSS1530 is comparable to `non-radio' Lyman alpha emitters (LAEs) at a similar redshift. However, with a radio luminosity of $\log L_{\textrm{150 MHz}} = 29.1$ W Hz$^{-1}$ and a deconvolved physical size $3.5$ kpc, its radio properties are similar to other known radio galaxies at $z>4$. Subsequent $J$ and $K$ band imaging using LUCI on the Large Binocular Telescope resulted in non-detection of the host galaxy down to $3\sigma$ limits of $J>24.4$ and $K>22.4$ (Vega). The $K$ band limit is consistent with $z>5$ from the $K-z$ relation for radio galaxies, suggesting stellar mass limits using simple stellar population models of $M_{\textrm{stars}}< 10^{10.5}$ $M_\odot$. Its high redshift coupled with relatively small radio and Ly$\alpha$ sizes suggest that TGSS1530 may be a radio galaxy in an early phase of its evolution., Comment: 11 pages, 7 figures. Latest published version 10 October 2018

The circumgalactic medium (CGM) of the massive Spiderweb Galaxy, a conglomerate of merging proto-cluster galaxies at z=2.2, forms an enriched interface where feedback and recycling act on accreted gas. This is shown by observations of [CI], CO(1-0) and CO(4-3) performed with the Atacama Large Millimeter Array (ALMA) and Australia Telescope Compact Array (ATCA). [CI] and CO(4-3) are detected across ~50 kpc, following the distribution of previously detected low-surface-brightness CO(1-0) across the CGM. This confirms our previous results on the presence of a cold molecular halo. The central radio galaxy MRC1138-262 shows a very high global $L'_{\rm CO(4-3)}$/$L'_{\rm CO(1-0)}$ ~ 1, suggesting that mechanisms other than FUV-heating by star formation prevail at the heart of the Spiderweb Galaxy. Contrary, the CGM has $L'_{\rm CO(4-3)}$/$L'_{\rm CO(1-0)}$ and $L'_{\rm [CI]}$/$L'_{\rm CO(1-0)}$ similar to the ISM of five galaxies in the wider proto-cluster, and its carbon abundance, $X_{\rm [CI]}$/$X_{\rm H2}$, resembles that of the Milky Way and starforming galaxies. The molecular CGM is thus metal-rich and not diffuse, confirming a link between the cold gas and in-situ star formation. Thus, the Spiderweb Galaxy grows not directly through accretion of gas from the cosmic web, but from recycled gas in the GCM., Comment: Accepted for publication as a Letter in MNRAS (5 pages, 4 figures)

Ultra-steep spectrum (USS) radio sources are good tracers of powerful radio galaxies at $z > 2$. Identification of even a single bright radio galaxy at $z > 6$ can be used to detect redshifted 21cm absorption due to neutral hydrogen in the intervening IGM. Here we describe a new sample of high-redshift radio galaxy (HzRG) candidates constructed from the TGSS ADR1 survey at 150 MHz. We employ USS selection ($\alpha \le -1.3$) in $\sim10000$ square degrees, in combination with strict size selection and non-detections in all-sky optical and infrared surveys. We apply flux density cuts that probe a unique parameter space in flux density ($50 < S_{\textrm{150}} < 200$ mJy) to build a sample of 32 HzRG candidates. Follow-up Karl G. Jansky Very Large Array (VLA) observations at 1.4 GHz with an average beam size of $1.3$ arcseconds ($"$) revealed $\sim 48\%$ of sources to have a single radio component. P-band (370 MHz) imaging of 17 of these sources revealed a flattening radio SED for ten sources at low frequencies, which is expected from compact HzRGs. Two of our sources lie in fields where deeper multi-wavelength photometry and ancillary radio data are available and for one of these we find a best-fit photo-z of $4.8 \pm 2.0$. The other source has $z_{\textrm{phot}}=1.4 \pm 0.1$ and a small angular size ($3.7"$), which could be associated with an obscured star forming galaxy or with a "dead" elliptical. One USS radio source not part of the HzRG sample but observed with the VLA nonetheless is revealed to be a candidate giant radio galaxy with a host galaxy photo-z of $1.8\pm0.5$, indicating a size of 875 kpc., Comment: 17 pages, 13 figures, accepted for publication in MNRAS

This paper presents a study of the redshift evolution of radio-loud active galactic nuclei (AGN) as a function of the properties of their galaxy hosts in the Bo\"otes field. To achieve this we match low-frequency radio sources from deep $150$-MHz LOFAR observations to an $I$-band-selected catalogue of galaxies, for which we have derived photometric redshifts, stellar masses and rest-frame colours. We present spectral energy distribution (SED) fitting to determine the mid-infrared AGN contribution for the radio sources and use this information to classify them as High- versus Low-Excitation Radio Galaxies (HERGs and LERGs) or Star-Forming galaxies. Based on these classifications we construct luminosity functions for the separate redshift ranges going out to $z = 2$. From the matched radio-optical catalogues, we select a sub-sample of $624$ high power ($P_{150\mathrm{\,MHz}}>10^{25}$ W Hz$^{-1}$) radio sources between $0.5 \leq z < 2$. For this sample, we study the fraction of galaxies hosting HERGs and LERGs as a function of stellar mass and host galaxy colour. The fraction of HERGs increases with redshift, as does the fraction of sources in galaxies with lower stellar masses. We find that the fraction of galaxies that host LERGs is a strong function of stellar mass as it is in the local Universe. This, combined with the strong negative evolution of the LERG luminosity functions over this redshift range, is consistent with LERGs being fuelled by hot gas in quiescent galaxies., Comment: 25 pages, 21 figures, accepted for publication in MNRAS

We study the mutual alignment of radio sources within two surveys, FIRST and TGSS. This is done by producing two position angle catalogues containing the preferential directions of respectively $30\,059$ and $11\,674$ extended sources distributed over more than $7\,000$ and $17\,000$ square degrees. The identification of the sources in the FIRST sample was performed in advance by volunteers of the Radio Galaxy Zoo project, while for the TGSS sample it is the result of an automated process presented here. After taking into account systematic effects, marginal evidence of a local alignment on scales smaller than $2.5\deg$ is found in the FIRST sample. The probability of this happening by chance is found to be less than $2$ per cent. Further study suggests that on scales up to $1.5\deg$ the alignment is maximal. For one third of the sources, the Radio Galaxy Zoo volunteers identified an optical counterpart. Assuming a flat $\Lambda$CDM cosmology with $\Omega_m = 0.31, \Omega_\Lambda = 0.69$, we convert the maximum angular scale on which alignment is seen into a physical scale in the range $[19, 38]$ Mpc $h_{70}^{-1}$. This result supports recent evidence reported by Taylor and Jagannathan of radio jet alignment in the $1.4$ deg$^2$ ELAIS N1 field observed with the Giant Metrewave Radio Telescope. The TGSS sample is found to be too sparsely populated to manifest a similar signal.

Previous studies have shown that CIZA J2242.8+5301 (the 'Sausage' cluster, $z=0.192$) is a massive merging galaxy cluster that hosts a radio halo and multiple relics. In this paper we present deep, high fidelity, low-frequency images made with the LOw-Frequency Array (LOFAR) between 115.5 and 179 MHz. These images, with a noise of 140 mJy/beam and a resolution of $\theta_{\text{beam}}=7.3"\times5.3"$, are an order of magnitude more sensitive and five times higher resolution than previous low-frequency images of this cluster. We combined the LOFAR data with the existing GMRT (153, 323, 608 MHz) and WSRT (1.2, 1.4, 1.7, 2.3 GHz) data to study the spectral properties of the radio emission from the cluster. Assuming diffusive shock acceleration (DSA), we found Mach numbers of $\mathcal{M}_{n}=2.7{}_{-0.3}^{+0.6}$ and $\mathcal{M}_{s}=1.9_{-0.2}^{+0.3}$ for the northern and southern shocks. The derived Mach number for the northern shock requires an acceleration efficiency of several percent to accelerate electrons from the thermal pool, which is challenging for DSA. Using the radio data, we characterised the eastern relic as a shock wave propagating outwards with a Mach number of $\mathcal{M}_{e}=2.4_{-0.3}^{+0.5}$, which is in agreement with $\mathcal{M}_{e}^{X}=2.5{}_{-0.2}^{+0.6}$ that we derived from Suzaku data. The eastern shock is likely to be associated with the major cluster merger. The radio halo was measured with a flux of $346\pm64\,\text{mJy}$ at $145\,\text{MHz}$. Across the halo, we observed a spectral index that remains approximately constant ($\alpha^{\text{145 MHz-2.3 GHz}}_{\text{across \(\sim\)1 Mpc}^2}=-1.01\pm0.10$) after the steepening in the post-shock region of the northern relic. This suggests a generation of post-shock turbulence that re-energies aged electrons.

We present a study of the low-frequency radio properties of star forming (SF) galaxies and active galactic nuclei (AGN) up to redshift $z=2.5$. The new spectral window probed by the Low Frequency Array (LOFAR) allows us to reconstruct the radio continuum emission from 150 MHz to 1.4 GHz to an unprecedented depth for a radio-selected sample of $1542$ galaxies in $\sim 7~ \rm{deg}^2$ of the LOFAR Bo\"otes field. Using the extensive multi-wavelength dataset available in Bo\"otes and detailed modelling of the FIR to UV spectral energy distribution (SED), we are able to separate the star-formation (N=758) and the AGN (N=784) dominated populations. We study the shape of the radio SEDs and their evolution across cosmic time and find significant differences in the spectral curvature between the SF galaxy and AGN populations. While the radio spectra of SF galaxies exhibit a weak but statistically significant flattening, AGN SEDs show a clear trend to become steeper towards lower frequencies. No evolution of the spectral curvature as a function of redshift is found for SF galaxies or AGN. We investigate the redshift evolution of the infrared-radio correlation (IRC) for SF galaxies and find that the ratio of total infrared to 1.4 GHz radio luminosities decreases with increasing redshift: $ q_{\rm 1.4GHz} = (2.45 \pm 0.04) \times (1+z)^{-0.15 \pm 0.03} $. Similarly, $q_{\rm 150MHz}$ shows a redshift evolution following $ q_{\rm 150GHz} = (1.72 \pm 0.04) \times (1+z)^{-0.22 \pm 0.05}$. Calibration of the 150 MHz radio luminosity as a star formation rate tracer suggests that a single power-law extrapolation from $q_{\rm 1.4GHz}$ is not an accurate approximation at all redshifts., Comment: Accepted for publication in MNRAS, 21 pages, 16 figures

Extreme laser pulses driving non-equilibrium processes in high density plasmas permit an increase of the fusion of hydrogen with the boron isotope 11 by nine orders of magnitude of the energy gains above the classical values. This is the result of initiating the reaction by non-thermal ultrahigh acceleration of plasma blocks by the nonlinear (ponderomotive) force of the laser field, in addition to the avalanche reaction that has now been experimentally and theoretically manifested. The design of a very compact fusion power reactor is scheduled to produce then environmentally fully clean and inexhaustible generation of energy at profitably low costs. The reaction within a volume of cubic millimetres during a nanosecond can only be used for controlled power generation., Comment: 10 pages, 5 fugures