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We study the radio continuum emission of four galaxies experiencing ram-pressure stripping in four clusters of the Shapley Supercluster at redshift |$z\sim 0.05$|⁠. Multiband (235–1367 MHz) radio data, complemented by integral-field spectroscopy, allow us to detect and analyse in detail the non-thermal component both in the galaxy discs and the radio continuum tails. Three galaxies present radio continuum tails that are tens of kiloparsecs long. By deriving the radio spectral index in the inner and outer tails and comparing our findings with the distribution of the extraplanar ionized gas and the results of N -body/hydrodynamical simulations, we demonstrate that these tails are caused by the ram pressure, which, together with the ionized gas, sweeps the magnetic field from the galaxy discs. We suggest that the radio continuum emission in these tails can be differently powered by (i) in situ star formation, (ii) relativistic electrons stripped from the disc, and (iii) shock excitation or a combination of them. All the ram-pressure-stripped galaxies are found in environments where cluster–cluster interactions occurred and/or are ongoing, thus strongly supporting the thesis that cluster and group collisions and mergers may locally increase the ram pressure and trigger hydrodynamical interactions between the intracluster medium and the interstellar medium of galaxies. [ABSTRACT FROM AUTHOR]

The Main galaxy cluster in the Abell 781 system is undergoing a significant merger and accretion process with peripheral emission to the north and south-eastern flanks of the merging structure. Here we present a full polarimetric study of this field, using radio interferometric data taken at 21 and 92 cm with the Westerbork Synthesis Radio Telescope (WSRT), to a sensitivity better than any 21 cm (L band) observation to date. We detect evidence of extended low-level emission of 1.9 mJy associated with the Main cluster at 21 cm, although this detection necessitates further follow-up by modern instruments due to the limited resolution of the WSRT. Our polarimetric study indicates that, most likely, the peripheral emission associated with this cluster is not a radio relic. [ABSTRACT FROM AUTHOR]

Galaxy clusters are assembled via merging of smaller structures, in a process that generates shocks and turbulence in the intra cluster medium and produces radio diffuse emission in the form of halos and relics. The cluster pair A 399–A 401 represents a special case: both clusters host a radio halo. Recent Low Frequency Array (LOFAR) observations at 140 MHz revealed the presence of a radio bridge connecting the two clusters along with two relic candidates. These relics include one South of A 399 and the other in between the two clusters, in proximity of a shock front detected in X-ray observations. In this paper we present observations of the A 399–A 401 cluster pair at 1.7, 1.4, 1.2 GHz, and 346 MHz from the Westerbork Synthesis Radio Telescope (WSRT). We detect the radio halo in the A 399 cluster at 346 MHz, extending up to ∼650 kpc and with a 125 ± 6 mJy flux density. Its spectral index between 140 MHz and 346 MHz is α = 1.75 ± 0.14. The two candidate relics are also seen at 346 MHz and we determine their spectral indices to be α = 1.10 ± 0.14 and α = 1.46 ± 0.14. The low surface brightness bridge connecting the two clusters is below the noise level at 346 MHz therefore we constrain the bridge average spectral index to be steep, i.e. α > 1.5 at 95 per cent confidence level. This result favours the scenario where dynamically-induced turbulence is a viable mechanism to reaccelerate a population of mildly relativistic particles and amplify magnetic fields on scales of a few Mpcs. [ABSTRACT FROM AUTHOR]

We aim to carry out a radio study of the SoUthern Cluster sCale Extended Source Survey (SUCCESS) sample consisting of 20 massive (M500 > 5 × 1014 M⊙), nearby (redshift <0.3) and southern (−50° < δ < −30°) galaxy clusters detected by the Planck satellite and the South Pole Telescope. Here, we report targeted GMRT observations (325/610 MHz) for a subsample of nine clusters. We also use the first data release of MeerKAT Galaxy Cluster Legacy Survey (1283 MHz) for five of these nine clusters. The properties of the mini-halo in RXC J0528.9-3927, a candidate mini-halo in A3322, the radio halo and candidate double relics in A3399, and the radio halo in RXC J0232.2-4420 are presented. We also report a detection of candidate radio relics at distances 1 and 1.9 Mpc from the centre of RXC J0232.2-4420. The southeast relic of A3399 is consistent with the radio power–mass scaling relation for radio relics, while the candidate relics around RXC J0232.2-4420 are outliers. This indicates an origin of the candidate relics near RXC J0232.2-4420 to be independent of this cluster and a cluster merger-shock origin for the relic in A3399. In this subsample of clusters, 1/9 hosts a radio halo and double relics, 1/9 hosts a radio halo and 2/9 host mini-haloes. The dynamical states based on X-ray morphology show that A3399 is a disturbed cluster; however, the radio halo cluster RXC J0232.2-4420 is relaxed, and the mini-halo clusters have intermediate morphologies, adding to the cases of the less commonly found associations. [ABSTRACT FROM AUTHOR]

In this paper, we present results from a radio–optical study of the galaxy populations of the galaxy clusters Abell 1300 and MACS J1931.8−2634, a merger and a relaxed system respectively both located at z ∼ 0.3, aimed at finding evidence of merger-induced radio emission. Radio observations are taken at 1.28 GHz with the MeerKAT interferometer during its early-stage commissioning phase, and combined with archive optical data. We generated catalogues containing 107 and 162 radio sources in the A 1300 and MACS J1931.8−2634 cluster fields, respectively, above a 0.2 mJy threshold and within a 30 arcmin radius from the cluster centre (corresponding to 8.1 and 8.8 Mpc, respectively). By cross-correlating the radio and optical catalogues, and including spectroscopic information, nine and six sources were found to be cluster members and used to construct the radio luminosity functions, respectively, for both clusters. The comparison of the radio source catalogues between the two cluster fields leads to a marginal difference, with a 2σ statistical significance. We derived the radio luminosity function at 1.28 GHz in both clusters, in the power range of |$22.81 \lt \rm {log~\mathit{ P}_{1.28~GHz}~(W\,Hz^{-1})} \lt 25.95$|⁠ , and obtained that in A 1300 the radio luminosity function averaged over the full radio power interval is only 3.3 ± 1.9 times higher than the MACS J1931.8−2634 one, suggesting no statistical difference in their probability to host nuclear radio emission. We conclude that, at least for the two clusters studied here, the role of cluster mergers in affecting the statistical properties of the radio galaxy population is negligible. [ABSTRACT FROM AUTHOR]

Giant radio galaxies (GRGs), with extended structures reaching hundreds of kpc, are among the most spectacular examples of ejection of relativistic plasma from supermassive black holes. In this work, third of a series, we present Low-Frequency Array (LOFAR) images at 144 MHz, collected in the framework of the LOFAR Two-metre Sky Survey Data Release 2 (LoTSS DR2), for nine sources extracted from our sample of hard X-ray-selected GRGs (HXGRGs, i.e. from INTEGRAL /Imager on-Board the INTEGRAL Satellite and Swift /Burst Alert Telescope catalogues at >20 keV). Thanks to the resolution and sensitivity of LoTSS, we could probe the complex morphology of these GRGs, unveiling cases with diffuse (Mpc-scale) remnant emission, presence of faint off-axis wings, or a misaligned inner jet. In particular, for one source (B2 1144+35B), we could clearly detect a ∼300 kpc wide off-axis emission, in addition to an inner jet whose orientation is not aligned with the lobes axis. For another source (J1153.9+5848), a structure consistent with jet precession was revealed, appearing as an X-shaped morphology with relic lobes having an extension larger than the present ones, and with a different axis orientation. From an environment analysis, we found two sources showing an overdensity of cosmological neighbours, and a correspondent association with a galaxy cluster from catalogues. Finally, a comparison with radio-selected GRGs from LoTSS DR1 suggested that, on average, HXGRGs can grow to larger extents. These results highlight the importance of deep low-frequency observations to probe the evolution of radio galaxies, and ultimately estimate the duty cycle of their jets. [ABSTRACT FROM AUTHOR]

We present an update on the sample of soft gamma-ray selected giant radio galaxies (GRGs) extracted from INTEGRAL/IBIS and Swift /BAT surveys; it includes eight new sources and one candidate object. In the new sample, all but one source display FR II radio morphologies; the only exception is B21144+35B, which is an FR I. The objects belong to both type 1 and type 2 active galactic nucleus (AGN) optical classes and have redshifts in the range 0.06–0.35, while the radio sizes span from 0.7 to 1 Mpc. In this study, we present for the first time two objects that were never discussed as GRGs before and propose a new candidate GRG. We confirm the correlation between the X-ray luminosity and the radio core luminosity found for other soft gamma-ray selected GRGs and expected for AGNs powered by efficient accretion. We also corroborate previous results that indicate that the luminosity of the radio lobes is relatively low compared with the nuclear X-ray emission. This supports the idea that the nucleus of these GRGs is now more powerful than in the past, consistent with a restarting activity scenario. [ABSTRACT FROM AUTHOR]

About 6 per cent of radio galaxies (RGs) can reach linear sizes larger than 0.7 Mpc, and are then classified as giant radio galaxies (GRGs). The conditions that make possible the formation of such big structures are still not clear – either core accretion properties or environmental factors. Recent studies have shown that GRGs can be up to four times more abundant in hard X-ray selected (i.e. from INTEGRAL /IBIS and Swift /BAT at >20 keV) RG samples. Moreover, a high fraction of young radio sources found in their cores suggest a recently restarted activity, as suggested from the discrepancy between the measured jet and lobes power, with respect to the one expected from core X-ray luminosity. Here, we present a radio morphological study of a sample of 15 hard X-ray selected GRGs, discussing low-frequency images from our GMRT campaign complemented with others from the literature: among them, 7/15 show evidence of restarted radio activity either in the form of double–double/X-shaped morphology, or as a cocoon emission embedding more recent jets. This, together with the objects from this sample already found hosting a young radio source in their core, suggests that at least 13 over 15 of these hard X-ray selected GRGs show features that are consistent with the possibility of restarted radio activity. [ABSTRACT FROM AUTHOR]

Radio haloes and radio relics are diffuse synchrotron sources that extend over Mpc-scales and are found in a number of merger galaxy clusters. They are believed to form as a consequence of the energy that is dissipated by turbulence and shocks in the intracluster medium (ICM). However, the precise physical processes that generate these steep synchrotron spectrum sources are still poorly constrained. We present a new LOFAR observation of the double galaxy cluster Abell 1758. This system is composed of A1758N, a massive cluster hosting a known giant radio halo, and A1758S, which is a less massive cluster whose diffuse radio emission is confirmed here for the first time. Our observations have revealed a radio halo and a candidate radio relic in A1758S, and a suggestion of emission along the bridge connecting the two systems which deserves confirmation. We combined the LOFAR data with archival VLA and GMRT observations to constrain the spectral properties of the diffuse emission. We also analysed a deep archival Chandra observation and used this to provide evidence that A1758N and A1758S are in a pre-merger phase. The ICM temperature across the bridge that connects the two systems shows a jump which might indicate the presence of a transversal shock generated in the initial stage of the merger. [ABSTRACT FROM AUTHOR]

Pairs of radio relics are believed to form during cluster mergers and are best observed when the merger occurs in the plane of the sky. Mergers can also produce radio haloes, through complex processes likely linked to turbulent re-acceleration of cosmic ray electrons. However, only some clusters with double relics also show a radio halo. Here, we present a novel method to derive upper limits on the radio halo emission and analyse archival X-ray Chandra data, as well as galaxy velocity dispersions and lensing data, in order to understand the key parameter that switches on radio halo emission. We place upper limits on the halo power below the P1.4 GHz-M500 correlation for some clusters, confirming that clusters with double relics have different radio properties. Computing X-ray morphological indicators, we find that clusters with double relics are associated with the most disturbed clusters.We also investigate the role of different mass-ratios and time-since-merger. Data do not indicate that the merger mass-ratio has an impact on the presence or absence of radio haloes (the null hypothesis that the clusters belong to the same group cannot be rejected). However, the data suggest that the absence of radio haloes could be associated with early and late mergers, but the sample is too small to perform a statistical test. Our study is limited by the small number of clusters with double relics. Future surveys with LOFAR, ASKAP, MeerKat and SKA will provide larger samples to better address this issue. [ABSTRACT FROM AUTHOR]

Using the recent INTEGRAL/IBIS and Swift/BAT surveys we have extracted a sample of 64 confirmed plus three candidate radio galaxies selected in the soft gamma-ray band. The sample covers all optical classes and is dominated by objects showing a Fanaroff-Riley type II radio morphology; a large fraction (70 per cent) of the sample is made of 'radiative mode' or highexcitation radio galaxies. We measured the source size on images from the NRAO VLA Sky Survey, the Faint Images of the Radio Sky at twenty-cm and the Sydney University Molonglo Sky Survey images and have compared our findings with data in the literature obtaining a good match. We surprisingly found that the soft gamma-ray selection favours the detection of large size radio galaxies: 60 per cent of objects in the sample have size greater than 0.4 Mpc while around 22 per cent reach dimension above 0.7 Mpc at which point they are classified as giant radio galaxies (GRGs), the largest and most energetic single entities in the Universe. Their fraction among soft gamma-ray selected radio galaxies is significantly larger than typically found in radio surveys, where only a few per cent of objects (1-6 per cent) are GRGs. This may partly be due to observational biases affecting radio surveys more than soft gamma-ray surveys, thus disfavouring the detection of GRGs at lower frequencies. The main reasons and/or conditions leading to the formation of these large radio structures are still unclear with many parameters such as high jet power, long activity time and surrounding environment all playing a role; the first two may be linked to the type of active galactic nucleus discussed in this work and partly explain the high fraction of GRGs found in the present sample. Our result suggests that high energy surveys may be a more efficient way than radio surveys to find these peculiar objects. [ABSTRACT FROM AUTHOR]

In order to investigate the role of absorption in active galactic nuclei (AGN) with jets, we have studied the column density distribution of a hard X-ray selected sample of radio galaxies, derived from the INTEGRAL/Imager on Board the Integral Satellite (IBIS) and Swift/The Burst Alert Telescope (BAT) AGN catalogues (~7-10 per cent of the total AGN population). The 64 radio galaxies have a typical FR II radio morphology and are characterized by high 20-100 keV luminosities (from 1042 to 1046 erg s-1) and high Eddington ratios (log LBol/LEdd typically larger than ~0.01). The observed fraction of absorbed AGN (NH > 1022 cm-2) is around 40 per cent among the total sample, and ~75 per cent among type 2 AGN. The majority of obscuredAGNare narrow-line objects, while unobscuredAGNare broad-line objects, obeying to the zeroth-order predictions of unified models. A significant anti-correlation between the radio core dominance parameter and the X-ray column density is found. The observed fraction of Compton thick AGN is ~2-3 per cent, in comparison with the 5-7 per cent found in radio-quiet hard X-ray selected AGN. We have estimated the absorption and Compton thick fractions in a hard X-ray sample containing both radio galaxies and non-radio galaxies and therefore affected by the same selection biases. No statistical significant difference was found in the absorption properties of radio galaxies and non-radio galaxies sample. In particular, the Compton thick objects are likely missing in both samples and the fraction of obscured radio galaxies appears to decrease with luminosity as observed in hard X-ray non-radio galaxies. [ABSTRACT FROM AUTHOR]

The presence of megaparsec-scale radio haloes in galaxy clusters has already been established by many observations over the last two decades. The emerging explanation for the formation of these giant sources of diffuse synchrotron radio emission is that they trace turbulent regions in the intracluster medium, where particles are trapped and accelerated during cluster mergers. Our current observational knowledge is, however, mainly limited to massive systems. Here we present observations of a sample of 14 mass-selected galaxy clusters, i.e. M500 > 4×1014 M⊙, in the Southern hemisphere, aimed to study the occurrence of radio haloes in low-mass clusters and test the correlation between the radio halo power at 1.4 GHz P1.4 and the cluster mass M500. Our observations were performed with the 7-element Karoo Array Telescope at 1.86 GHz. We found three candidates to host diffuse cluster-scale emission and derived upper limits at the level of 0.6-1.9×1024 Watt Hz-1 for ~ 50 per cent of the clusters in the sample, significantly increasing the number of clusters with radio halo information in the considered mass range. Our results confirm that bright radio haloes in less massive galaxy clusters are statistically rare. [ABSTRACT FROM AUTHOR]

We report results of a multiband monitoring campaign of the flat spectrum radio quasar TXS 0536+145 at redshift 2.69. This source was detected during a very high γ-ray activity state in 2012 March by the Large Area Telescope on board Fermi, becoming the γ-ray flaring blazar at the highest redshift detected so far. At the peak of the flare the source reached an apparent isotropic γ-ray luminosity of 6.6 × 1049 erg s−1 which is comparable to the values achieved by the most luminous blazars. This activity triggered radio-to-X-rays monitoring observations by Swift, Very Long Baseline Array, European VLBI Network, and Medicina single-dish telescope. Significant variability was observed from radio to X-rays supporting the identification of the γ-ray source with TXS 0536+145. Both the radio and γ-ray light curves show a similar behaviour, with the γ-rays leading the radio variability with a time lag of about 4-6 months. The luminosity increase is associated with a flattening of the radio spectrum. No new superluminal component associated with the flare was detected in high-resolution parsec-scale radio images. During the flare the γ-ray spectrum seems to deviate from a power law, showing a curvature that was not present during the average activity state. The γ-ray properties of TXS 0536+145 are consistent with those shown by the high-redshift γ-ray blazar population. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the flux density variability and changes in the pc-scale radio structure of the flat spectrum radio quasar PKS 1510−089. This source was the target of multi-epoch very long baseline interferometry (VLBI) and space-VLBI observations at 4.8, 8.4 and 22 GHz carried out between 1999 and 2001. Comparison of the pc-scale structure observed at different epochs shows the presence of a non-stationary jet feature moving with a superluminal apparent velocity of 16.2 c± 0.7 c. Over three epochs at 8.4 GHz, during this period, the core flux density varied by about 50 per cent, while the scatter in the jet flux density was within 10 per cent. The polarization percentage of both the core and the jet components significantly changed from 2 to 9 per cent, while the polarization angle of the core showed an abrupt change of about 90°, becoming roughly perpendicular to the jet direction, consistent with a change in the opacity. To complete the picture of the physical processes at work, we complemented our observations with multi-epoch Very Long Baseline Array data at 15 GHz from the MOJAVE programme spanning a time-baseline from 1995 to 2010. Since 1995 jet components were ejected roughly once per year with the same position angle and an apparent speed between 15 c and 20 c, indicating that no jet precession has taken place on a time-scale longer than a decade in our frame. The variability of the total intensity flux density together with variations in the polarization properties may be explained assuming either a change between the optically-thick and optically-thin regimes produced by a shock that varies the opacity, or a highly ordered magnetic field produced by the compression of the relativistic plasma by a shock propagating along the jet. Taking into account the high γ-ray emission from this source observed by the AGILE and Fermi satellites, we investigated the connection between the radio and γ-ray activity during 2007-10. Multiwavelength flux and polarization observations suggest that during some γ-ray-flaring episodes the emission at high (γ-ray) and low (radio) energies has origin in the same region. [ABSTRACT FROM AUTHOR]

Deep radio observations of the galaxy cluster Abell 781 have been carried out using the Giant Metrewave Radio Telescope at 325 MHz and have been compared to previous 610-MHz observations and to archival Very Large Array (VLA) 1.4-GHz data. The radio emission from the cluster is dominated by a diffuse source located at the outskirts of the X-ray emission, which we tentatively classify as a radio relic. We detected residual diffuse emission at the cluster centre at the level of S∼ 15-20 mJy. Our analysis disagrees with Govoni et al., and on the basis of simple spectral considerations, we do not support their claim of a radio halo with the flux density of 20-30 mJy at 1.4 GHz. Abell 781, a massive and merging cluster, is an intriguing case. Assuming that the residual emission is indicative of the presence of a radio halo barely detectable at our sensitivity level, it could be a very steep spectrum source. [ABSTRACT FROM AUTHOR]

We present results on the compact steep-spectrum quasar 3C 48 from observations with the Very Long Baseline Array (VLBA), the Multi-Element Radio Linked Interferometer Network (MERLIN) and the European Very long baseline interferometry (VLBI) Network (EVN) at multiple radio frequencies. In the 1.5-GHz VLBI images, the radio jet is characterized by a series of bright knots. The active nucleus is embedded in the southernmost VLBI component A, which is further resolved into two sub-components A1 and A2 at 4.8 and 8.3 GHz, respectively. A1 shows a flat spectrum and A2 shows a steep spectrum. The most strongly polarized VLBI components are located at component C ∼ 0.25 arcsec north of the core, where the jet starts to bend to the north-east. The polarization angles at C show gradual changes across the jet width at all observed frequencies, indicative of a gradient in the emission-weighted intrinsic polarization angle across the jet and possibly a systematic gradient in the rotation measure; moreover, the percentage of polarization increases near the curvature at C, likely consistent with the presence of a local jet–interstellar-medium interaction and/or changing magnetic-field directions. The hot spot B shows a higher rotation measure, and has no detected proper motion. These facts provide some evidence for a stationary shock in the vicinity of B. Comparison of the present VLBI observations with those made 8.43 yr ago suggests a significant northward motion for A2 with an apparent transverse velocity . The apparent superluminal motion suggests that the relativistic jet plasma moves at a velocity of if the jet is viewed at an inclination angle less than . A simple precessing jet model and a hydrodynamical isothermal jet model with helical-mode Kelvin–Helmholtz instabilities are used to fit the oscillatory jet trajectory of 3C 48 defined by the bright knots. [ABSTRACT FROM AUTHOR]

We report and discuss the results of a 22-cm radio survey carried out with the Australia Telescope Compact Array (ATCA) covering the A3528 complex, a chain formed by the merging ACO clusters A3528–A3530–A3532, located in the central region of the Shapley Concentration. Simultaneous 13-cm observations are also presented. Our final catalogue includes a total of 106 radio sources above the flux density limit of 0.8 mJy. By cross-correlation with optical and spectroscopic catalogues we found 32 optical counterparts, nine of them belonging to the A3528 complex. We explored the effects of cluster mergers on the radio emission properties of radio galaxies by means of the radio luminosity function (RLF) and radio source counts. We found that the radio source counts are consistent with the background counts, as already found for the A3558 complex. The RLF for this complex is consistent, in both shape and normalization, with the general cluster luminosity function for early-type galaxies derived by Ledlow & Owen. This result is different from the one we obtained for the A3558 merging complex, the RLF of which is significantly lower than that derived by Ledlow & Owen. We propose that the different stage of the merger is responsible for the different RLFs in the two cluster complexes in the core of the Shapley Concentration. The early stage of merger for the A3528 complex, proposed by many authors, may not yet have affected the radio properties of cluster galaxies, while in the more much advanced merger in the A3558 region we actually see the effects of this process on the radio emission. [ABSTRACT FROM AUTHOR]

We present the results of a 22-cm radio survey carried out with the Australia Telescope Compact Array (ATCA) covering the A3558 complex, a chain formed by the merging ACO clusters A3556–A3558–A3562 and the two groups SC 1327-312 and SC 1323-313, located in the central region of the Shapley Concentration. The purpose of our survey is to study the effects of cluster mergers on the statistical properties of radio galaxies and to investigate the connection between mergers and the presence of radio haloes and relic sources. We found that the radio source counts in the A3558 complex are consistent with the background source counts. The much higher optical density compared with the background is not reflected as a higher density of radio sources. Furthermore, we found that no correlation exists between the local density and the radio source power, and that steep-spectrum radio galaxies are not segregated in denser optical regions. The radio luminosity function for elliptical and S0 galaxies is significantly lower than for cluster early-type galaxies and for those not selected to be in clusters at radio powers log P[sub 1.4]&gsim;22.5, implying that the probability of a galaxy becoming a radio source above this power limit is lower in the Shapley Concentration compared with any other environment. Possible explanations will be presented. The detection of a head–tail source in the centre of A3562, coupled with careful inspection of the 20-cm NRAO VLA Sky Survey (NVSS) and of 36-cm MOST observations, allowed us to spot two extended sources in the region between A3562 and SC 1329-313, i.e. a candidate radio halo at the centre of A3562 and low brightness extended emission around a 14.96-mag Shapley galaxy. The relation between these two extended galaxies and the ongoing group merger in this region of the Shapley Concentration are discussed. [ABSTRACT FROM AUTHOR]