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et al., [Context] The Shapley Supercluster (⟨z⟩≈0.048) contains several tens of gravitationally bound clusters and groups, making it an ideal subject for radio studies of cluster mergers., [Aims] We used new high sensitivity radio observations to investigate the less energetic events of mass assembly in the Shapley Supercluster from supercluster down to galactic scales., [Methods] We created total intensity images of the full region between A3558 and A3562, from ∼230 to ∼1650 MHz, using ASKAP, MeerKAT and the GMRT, with sensitivities ranging from ∼6 to ∼100 μJy beam−1. We performed a detailed morphological and spectral study of the extended emission features, complemented with ESO-VST optical imaging and X-ray data from XMM-Newton., [Results] We report the first GHz frequency detection of extremely low brightness intercluster diffuse emission on a ∼1 Mpc scale connecting a cluster and a group, namely: A3562 and the group SC 1329–313. It is morphologically similar to the X-ray emission in the region. We also found (1) a radio tail generated by ram pressure stripping in the galaxy SOS 61086 in SC 1329–313; (2) a head-tail radio galaxy, whose tail is broken and culminates in a misaligned bar; (3) ultrasteep diffuse emission at the centre of A3558. Finally (4), we confirm the ultra-steep spectrum nature of the radio halo in A3562., [Conclusions] Our study strongly supports the scenario of a flyby of SC 1329–313 north of A3562 into the supercluster core. This event perturbed the centre of A3562, leaving traces of this interaction in the form of turbulence between A3562 and SC 1329–313, at the origin of the radio bridge and eventually affecting the evolution of individual supercluster galaxies by triggering ram pressure stripping. Our work shows that minor mergers can be spectacular and have the potential to generate diffuse radio emission that carries important information on the formation of large-scale structures in the Universe., T. Venturi and G. Bernardi acknowledge the support from the Ministero degli Affari Esteri e della Cooperazione Internazionale, Direzione Generale per la Promozione del Sistema Paese, Progetto di Grande Rilevanza ZA18GR02. R. Kale acknowledges the support of the Department of Atomic Energy, Government of India under project no. 12-R&D-TFR-5.02-0700. S. P. Sikhosana acknowledges funding support from the South African Radio Astronomy Observatory (SARAO) and the National Research Foundation (NRF). O. Smirnov’s research is supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation.

Galaxy clusters are massive dark matter-dominated systems filled with X-ray emitting, optically thin plasma. Their large size and relative simplicity (at least as astrophysical objects go) make them a unique laboratory to measure some of the interesting plasma properties that are inaccessible by other means but fundamentally important for understanding and modeling many astrophysical phenomena -- from solar flares to black hole accretion to galaxy formation and the emergence of the cosmological Large Scale Structure. While every cluster astrophysicist is eagerly anticipating the direct gas velocity measurements from the forthcoming microcalorimeters onboard XRISM, Athena and future missions such as Lynx, a number of those plasma properties can best be probed by high-resolution X-ray imaging of galaxy clusters. Chandra has obtained some trailblazing results, but only grazed the surface of such studies. In this white paper, we discuss why we need arcsecond-resolution, high collecting area, low relative background X-ray imagers (with modest spectral resolution), such as the proposed AXIS and the imaging detector of Lynx., Comment: Science white paper submitted for Astro2020 Decadal survey. 9 pages, 4 figures

Extended cluster radio galaxies show different morphologies compared to those found isolated in the field. Indeed, symmetric double radio galaxies are only a small percentage of the total content of radio loud cluster galaxies, which show mainly tailed morphologies. Moreover, cluster mergers can deeply affect the statistical properties of their radio activity. In order to better understand the morphological and radio activity differences of the brightest central galaxies (BCGs) in major merging and non/tidal-merging clusters, we performed a multifrequency study of extended radio galaxies inside two cluster complexes, A3528 and A3558, belonging to the Shapley Concentration Core.

It has been recently found that a few galaxy clusters host diffuse radio halo emis- sion with very steep synchrotron spectra (α > 1.6), which may be classified as Ultra Steep Spectrum Radio Halos (USSRHs). USSRHs are expected in the turbulence re-acceleration model for the origin of cluster radio halos, and are best discovered and studied at low fre- quencies. We performed GMRT follow up observations of three galaxy clusters at 150 MHz, selected from the GMRT radio halo survey, which are known to host an USSRH or candi- date very steep spectrum diffuse emission. This project is aimed to characterize the low frequency spectrum of USSRHs for a detailed study of their origin and connection with cluster mergers. We present preliminary results at 150 MHz of the cluster A 697.

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 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(325 MHz)~15-20 mJy. Our analysis disagrees with Govoni et al. (2011), and on the basis of simple spectral considerations we do not support their claim of a radio halo with 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., 5 pages, 4 figures, 1 table - Accepted for publication on Monthly Notices of the Royal Astronomical Society Letters

We present results from Chandra observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. We focus on three main results. 1) The diffuse gas shows clear signatures from three distinct outbursts of the central AGN, with three pairs of roughly collinear cavities. The inner two cavity pairs are associated with unambiguous elliptical shock fronts, with Mach numbers M~1.7 and M~1.5 for the inner and outer shocks, respectively. 2) The mean power of the two most recent outbursts differs by a factor of six, indicating that the mean jet power varies over long (~10^7 yr) time scales. 3) The heating from the shocks alone is sufficient to balance radiative cooling of the gas within at least the central 10 kpc, allowing feedback to operate isotropically at small radii., 2 pages, 2 figures. To appear the proceedings of the conference "Non-thermal Phenomena in Colliding Galaxy Clusters" (Nice, 15-18 November, 2010), Memorie della Societa Astronomica Italiana, vol 82, n 3

High sensitivity observations of radio halos in galaxy clusters at frequencies lower than 330 MHz are still relatively rare, and very little is known compared to the classical 1.4 GHz images. The few radio halos imaged down to 150-240 MHz show a considerable spread in size, morphology and spectral properties. All clusters belonging to the GMRT Radio Halo Survey with detected or candidate cluster-scale diffuse emission have been imaged at 325 MHz with the GMRT. Few of them were also observed with the GMRT at 240 MHz and 150 MHz. For A1682, imaging is particularly challenging due to the presence of strong and extended radio galaxies at the center. Our data analysis suggests that the radio galaxies are superposed to very low surface brightness radio emission extended on the cluster scale, which we present here., Comment: 4 pages, 1 figure, to be published on the Journal of Astrophysics and Astronomy

We present results from new Chandra, GMRT, and SOAR observations of NGC 5813, the dominant central galaxy in a nearby galaxy group. The system shows three pairs of collinear cavities at 1 kpc, 8 kpc, and 20 kpc from the central source, from three distinct outbursts of the central AGN, which occurred 3x10^6, 2x10^7, and 9x10^7 yr ago. The H-alpha and X-ray observations reveal filaments of cool gas that has been uplifted by the X-ray cavities. The inner two cavity pairs are filled with radio emitting plasma, and each pair is associated with an elliptical surface brightness edge, which we unambiguously identify as shocks (with measured temperature jumps) with Mach numbers of M~1.7 and M~1.5 for the inner and outer shocks, respectively. Such clear signatures from three distinct AGN outbursts in an otherwise dynamically relaxed system provide a unique opportunity to study AGN feedback and outburst history. The mean power of the two most recent outbursts differs by a factor of six, from 1.5--10x10^42 erg/s, indicating that the mean jet power changes significantly over long (~10^7 yr) timescales. The total energy output of the most recent outburst is also more than an order of magnitude less than the total energy of the previous outburst (1.5x10^56 erg versus 4x10^57 erg), which may be a result of the lower mean power, or may indicate that the most recent outburst is ongoing. The outburst interval implied by both the shock and cavity ages (~10^7 yr) indicates that, in this system, shock heating alone is sufficient to balance radiative cooling close to the central AGN, which is the relevant region for regulating feedback between the ICM and the central SMBH., Replaced with version submitted to ApJ. Some reorganization of the text, and typos corrected in Tables 1 & 2

We present results from combined low-frequency radio and X-ray studies of nearby galaxy groups. We consider two main areas: firstly, the evolutionary process from spiral-dominated, HI-rich groups to elliptical-dominated systems with hot, X-ray emitting gas halos; secondly, the mechanism of AGN feedback which appears to balance radiative cooling of the hot halos of evolved groups. The combination of radio and X-ray observations provides a powerful tool for these studies, allowing examination of gas in both hot and cool phases, and of the effects of shock heating and AGN outbursts. Low-frequency radio data are effective in detecting older and less energetic electron populations and are therefore vital for the determination of the energetics and history of such events. We present results from our ongoing study of Stephan's Quintet, a spiral-rich group in which tidal interactions and shock heating appear to be transforming HI in the galaxies into a diffuse X-ray emitting halo, and show examples of AGN feedback from our sample of elliptical-dominated groups, where multi-band low-frequency radio data have proved particularly useful., Comment: 5 pages, 3 figures, Oral contribution to the Conference "The Low Frequency Radio Universe", ASP Conference Series, Eds. D.J. Saikia, D. Green, Y. Gupta and T. Venturi

(abridged) A Chandra observation of the X-ray bright group NGC 5044 shows that the X-ray emitting gas has been strongly perturbed by recent outbursts from the central AGN and also by motion of the central dominant galaxy relative to the group gas. The NGC 5044 group hosts many small radio quiet cavities with a nearly isotropic distribution, cool filaments, a semi-circular cold front and a two-armed spiral shaped feature of cool gas. A GMRT observation of NGC 5044 at 610 MHz shows the presence of extended radio emission with a "torus-shaped" morphology. The largest X-ray filament appears to thread the radio torus, suggesting that the lower entropy gas within the filament is material being uplifted from the center of the group. The radio emission at 235 MHz is much more extended than the emission at 610 MHz, with little overlap between the two frequencies. One component of the 235 MHz emission passes through the largest X-ray cavity and is then deflected just behind the cold front. A second detached radio lobe is also detected at 235 MHz beyond the cold front. All of the smaller X-ray cavities in the center of NGC 5044 are undetected in the GMRT observations. Since the smaller bubbles are probably no longer momentum driven by the central AGN, their motion will be affected by the group "weather" as they buoyantly rise outward. Hence, most of the enthalpy within the smaller bubbles will likely be deposited near the group center and isotropized by the group weather. The total mechanical power of the smaller radio quiet cavities is $P_c = 9.2 \times 10^{41}$erg s$^{-1}$ which is sufficient to suppress about one-half of the total radiative cooling within the central 10 kpc. This is consistent with the presence of H$\alpha$ emission within this region which shows that at least some of the gas is able to cool.

We present a detailed radio morphological study and spectral analysis of the wide-angle-tail radio source 4C +24.36 associated with the dominant galaxy in the relaxed galaxy cluster AWM 4. Our study is based on new high sensitivity GMRT observations at 235 MHz, 327 MHz and 610 MHz, and on literature and archival data at other frequencies. We find that the source major axis is likely oriented at a small angle with respect to the plane of the sky. The wide-angle-tail morphology can be reasonably explained by adopting a simple hydrodynamical model in which both ram pressure (driven by the motion of the host galaxy) and buoyancy forces contribute to bend the radio structure. The spectral index progressively steepens along the source major axis from $\alpha \sim$0.3 in the region close to the radio nucleus to beyond 1.5 in the lobes. The results of the analysis of the spectral index image allow us to derive an estimate of the radiative age of the source of $\sim$ 160 Myr. The cluster X-ray emitting gas has a relaxed morphology and short cooling time, but its temperature profile is isothermal out to at least 160 kpc from the centre. Therefore we seek evidence of energy ejection from the central AGN to prevent catastrophic cooling. We find that the energy injected by 4C +24.36 in the form of synchrotron luminosity during its lifetime is far less than the energy required to maintain the high gas temperature in the core. We also find that it is not possible for the central source to eject the requisite energy in the intracluster gas in terms of the enthalpy of buoyant bubbles of relativistic fluid, without creating discernible large cavities in the existing X-ray XMM-Newton observations., Comment: 15 pages, 12 figures, accepted for publication in ApJ

We present the GMRT 235 MHz images of three radio galaxies and 610 MHz images of two sources belonging to a complete sample of cD galaxies in rich and poor galaxy clusters. The analysis of the spectral properties confirms the presence of aged radio emission in two of the presented sources., Comment: 3 pages, 2 figures, To appear in the Proceedings of "Heating vs. Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching (Germany)

The lack of very cool gas at the cores of groups and clusters of galaxies, even where the cooling time is significantly shorter than the Hubble time, has been interpreted as evidence of sources that re-heat the intergalactic medium. Most studies of rich clusters adopt AGN feedback to be this source of heating. From ongoing GMRT projects involving clusters and groups, we demonstrate how low-frequency GMRT radio observations, together with Chandra/XMM-Newton X-ray data, present a unique insight into the nature of feedback, and of the energy transfer between the AGN and the IGM., Comment: 5 pages, 3 figures, To appear in ASP Conference Series, Vol. 407, The Low-Frequency Radio Universe, Eds: D. J. Saikia, D. A. Green, Y. Gupta and T. Venturi (Invited talk, conference held at NCRA-TIFR, Pune, INDIA, 8-12 December, 2008)

It has been recently found that a few galaxy clusters host diffuse radio halo emission with very steep synchrotron spectra ($\alpha$ > 1.6), which may be classified as Ultra Steep Spectrum Radio Halos (USSRHs). USSRHs are expected in the turbulence re-acceleration model for the origin of cluster radio halos, and are best discovered and studied at low frequencies. We performed GMRT follow up observations of three galaxy clusters at 150MHz, selected from the GMRT radio halo survey, which are known to host an USSRH or candidate very steep spectrum diffuse emission. This project is aimed to characterize the low frequency spectrum of USSRHs for a detailed study of their origin and connection with cluster mergers. We present preliminary results at 150 MHz of the cluster A697., Comment: 4 pages, 2 figures, Proceedings of the conference "Non-thermal phenomena in colliding galaxy clusters" (Nice, 15-18 November 2010)

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 $z \sim 0.3$, 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, 9 and 6 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 $2��$ statistical significance. We derived the radio luminosity function at 1.28 GHz in both clusters, in the power range $22.81 < \rm {log~P_{1.28~GHz}~(W/Hz)} < 25.95$, and obtained that in A 1300 the radio luminosity function averaged over the full radio power interval is only $3.3 \pm 1.9$ 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., 18 pages, 8 figures, MNRAS accepted

Many galaxy clusters host Mpc scale diffuse radio sources called radio halos. Their origin is connected to the processes that lead to the formation of clusters themselves. In order to unveil this connection, statistical studies of radio halos are necessary. We selected a sample of galaxy clusters with M500>6e14Msun and z=0.08-0.33 from the Planck SZ catalogue. In paper I, we presented the radio and X-ray data analysis that we carried out on these clusters. Here, we study the radio properties of the sample, in connection to the mass and dynamical state of clusters. We used the dynamical information derived from the X-ray data to assess the role of mergers in the origin of radio halos. We studied the distribution of clusters in the radio power-mass diagram and the role of dynamics on the radio luminosity and emissivity of radio halos. We measured the occurrence of radio halos as a function of the cluster mass and we compared it with the expectations of turbulent acceleration models. We found that more than the 90% of radio halos are in merging clusters and that their radio power correlates with the mass of the host clusters. The correlation shows a large dispersion. Interestingly, we showed that cluster dynamics contributes significantly to this dispersion with more disturbed clusters being more radio luminous. Clusters without radio halos are generally relaxed and the upper limits to their diffuse emission lie below the correlation. We showed that the radio emissivity of clusters exhibits an apparent bimodality, with the emissivity of radio halos being at least 5 times larger than the non-emission associated with more relaxed clusters. We found that the fraction of radio halos drops from ~70% in high mass clusters to ~35% in the lower mass systems of the sample and we showed that this result is in good agreement with the expectations from turbulent re-acceleration models., 15 pages, 10 figures, accepted for publication in A&A

Diffuse radio emission at the centre of galaxy clusters has been observed both in merging clusters on scales of Mpc, called giant radio haloes, and in relaxed systems with a cool-core on smaller scales, named mini haloes. Giant radio haloes and mini haloes are thought to be distinct classes of sources. However, recent observations have revealed the presence of diffuse radio emission on Mpc scales in clusters that do not show strong dynamical activity. RX J1720.1+2638 is a cool-core cluster, presenting both a bright central mini halo and a fainter diffuse, steep-spectrum emission extending beyond the cluster core that resembles giant radio halo emission. In this paper, we present new observations performed with the LOFAR Low Band Antennas (LBA) at 54 MHz. These observations, combined with data at higher frequencies, allow us to constrain the spectral properties of the radio emission. The large-scale emission presents an ultra-steep spectrum with $\alpha_{54}^{144}\sim3.2$. The radio emission inside and outside the cluster core have strictly different properties, as there is a net change in spectral index and they follow different radio-X-ray surface brightness correlations. We argue that the large-scale diffuse emission is generated by particles re-acceleration after a minor merger. While for the central mini halo we suggest that it could be generated by secondary electrons and positrons from hadronic interactions of relativistic nuclei with the dense cool-core gas, as an alternative to re-acceleration models., Comment: 13 pages, 10 figures; accepted for publication in MNRAS

Several cool-core clusters are known to host a radio mini-halo, a diffuse, steep-spectrum radio source located in their cores, thus probing the presence of non-thermal components as magnetic field and relativistic particles on scales not directly influenced by the central AGN. The nature of the mechanism that produces a population of radio-emitting relativistic particles on the scale of hundreds of kiloparsecs is still unclear. At the same time, it is still debated if the central AGN may play a role in the formation of mini-halos by providing the seed of the relativistic particles. We aim to investigate these open issues by studying the connection between thermal and non-thermal components of the intra-cluster medium. We performed a point-to-point analysis of the radio and the X-ray surface brightness of a compilation of mini-halos. We find that mini-halos have super-linear scalings between radio and X-rays, with radio brightness declining more steeply than the X-ray brightness. This trend is opposite to that generally observed in giant radio halos, thus marking a possible difference in the physics of the two radio sources. Finally, using the scalings between radio and X-rays and assuming a hadronic origin of mini-halos we derive constraints on the magnetic field in the core of the hosting clusters., Comment: Proceedings of the IAU Symposium IAUS342, "Perseus in Sicily: from black hole to cluster outskirts". 4 pages, 2 figures

The fate of cooling gas in the centers of galaxy clusters and groups is still not well understood, as is also the case for the complex processes of triggering star formation in central dominant galaxies (CDGs), re-heating of cooled gas by AGN, and the triggering/feeding of supermassive black hole outbursts. We present CO observations of the early type galaxy NGC 5044, which resides at the center of an X-ray bright group with a moderate cooling flow. For our analysis we combine CO(2-1) data from the 7m antennae of the Atacama Compact Array (ACA), and the ACA total power array (TP). We demonstrate, using the 7m array data, that we can recover the total flux inferred from IRAM 30m single dish observations, which corresponds to a total molecular mass of about 4x10^7 Msun. Most of the recovered flux is blueshifted with respect to the galaxy rest frame and is extended on kpc-scales, suggesting low filling factor dispersed clouds. We find 8 concentrations of molecular gas out to a radius of 10 arcsec (1.5 kpc), which we identify with giant molecular clouds. The total molecular gas mass is more centrally concentrated than the X-ray emitting gas, but extended in the north-east/south-west direction beyond the IRAM 30m beam. We also compare the spatial extent of the molecular gas to the Halpha emission: The CO emission coincides with the very bright Halpha region in the center. We do not detect CO emission in the fainter Halpha regions. Furthermore, we find two CO absorption features spatially located at the center of the galaxy, within 5 pc projected distance of the AGN, infalling at 255 and 265 km/s relative to the AGN. This indicates that the two giant molecular clouds seen in absorption are most likely within the sphere of influence of the supermassive black hole., 23 pages, 16 figures

Diffuse synchrotron radio emission from cosmic ray electrons is observed at the centre of a number of galaxy clusters. These sources can be classified either as giant radio haloes, which occur in merging clusters, or as mini haloes, which are found only in cool-core clusters. In this paper, we present the first discovery of a cool-core cluster with an associated mini halo that also shows ultra-steep-spectrum emission extending well beyond the core that resembles radio halo emission. The large-scale component is discovered thanks to LOFAR observations at 144 MHz. We also analyse GMRT observations at 610 MHz to characterize the spectrum of the radio emission. An X-ray analysis reveals that the cluster is slightly disturbed, and we suggest that the steep-spectrum radio emission outside the core could be produced by a minor merger that powers electron re-acceleration without disrupting the cool core. This discovery suggests that, under particular circumstances, both a mini halo and giant halo could co-exist in a single cluster, opening new perspectives for particle acceleration mechanisms in galaxy clusters.

Using new 610 MHz and 235 MHz observations from the Giant Metrewave Radio Telescope (GMRT) in combination with archival GMRT and Very Large Array (VLA) survey data we present the radio properties of the dominant early-type galaxies in the low$-$richness sub-sample of the Complete Local-volume Groups Sample (CLoGS; 27 galaxy groups) and provide results for the radio properties of the full CLoGS sample for the first time. We find a high radio detection rate in the dominant galaxies of the low-richness sub-sample of 82% (22/27); for the full CLoGS sample, the detection rate is 87% (46/53). The group-dominant galaxies exhibit a wide range of radio power, 10$^{20}$ $-$ 10$^{25}$ W Hz$^{-1}$ in the 235 and 610 MHz bands, with the majority (53%) presenting point-like radio emission, 19% hosting currently active radio jets, 6% having remnant jets, 9% being diffuse and 13% having no detected radio emission. The mean spectral index of the detected radio sources in the 235$-$610 MHz frequency range is found to be $\alpha_{235}^{610}\sim$0.68, and $\alpha_{235}^{1400}\sim$0.59 in the 235$-$1400 MHz one. In agreement with earlier studies, we find that the fraction of ultra-steep spectrum sources ($\alpha>$1.3) is $\sim$4%, mostly dependent on the detection limit at 235 MHz. The majority of point-like systems are found to reside in dynamically young groups, whereas jet systems show no preference between spiral-rich and spiral-poor group environments. The mechanical power of the jet sources in the low$-$richness sample groups is estimated to be $\sim$10$^{42}$ $-$ 10$^{44}$ erg s$^{-1}$ with their black hole masses ranging between 2$\times$10$^{8}$ $-$ 5$\times$10$^{9}$ M$_{\odot}$. We confirm previous findings that, while radio jet sources tend to be associated with more massive black holes, black hole mass is not the decisive factor in determining jet activity or power., Comment: Accepted for publication in MNRAS, 48 pages of which 18 Manuscript pages with 10 tables and 8 figures, plus 30 pages of appendices with 25 figures and 1 table

Radio halos are synchrotron diffuse sources at the centre of a fraction of galaxy clusters. The study of large samples of clusters with adequate radio and X-ray data is necessary to investigate the origin of radio halos and their connection with the cluster dynamics and formation history. The aim of this paper is to compile a well-selected sample of galaxy clusters with deep radio observations to perform an unbiased statistical study of the properties of radio halos. We selected 75 clusters with M > = 6e14 Msun at z=0.08-0.33 from the Planck Sunyaev-Zel'dovich catalogue. Clusters without suitable radio data were observed with the Giant Metrewave Radio Telescope (GMRT) and/or the Jansky Very Large Array (JVLA) to complete the information about the possible presence of diffuse emission. We used archival Chandra X-ray data to derive information on the clusters' dynamical states. This observational campaign led to the detection of several cluster-scale diffuse radio sources and candidates that deserve future follow-up observations. Here we summarise their properties and add information resulting from our new observations. For the clusters where we did not detect any hint of diffuse emission, we derived new upper limits to their diffuse flux. We have built the largest mass-selected (> 80 per cent complete in mass) sample of galaxy clusters with deep radio observations available to date. The statistical analysis of the sample, which includes the connection between radio halos and cluster mergers, the radio power-mass correlation, and the occurrence of radio halos as a function of the cluster mass, will be presented in paper II., 35 pages, 42 figures, accepted for publication in A&A

The radio mini halos (MH) observed in relaxed clusters probe the presence of relativistic particles on scales of hundreds of kpc, beyond the scales directly influenced by the central AGN, but the nature of the mechanism that produces the relativistic electrons is still debated. In this work we explore the connection between thermal and non-thermal components of the ICM in a sample of MH and we study its implications for hadronic models for the origin of the relativistic electrons. We studied the thermal and non-thermal connection by carrying out a point-to-point comparison of the radio and the X-ray surface brightness. We extended the method generally applied to giant radio halos by considering the effects of a grid randomly generated through a Monte Carlo chain. Contrary to what is generally observed for giant radio halos, we find that the mini-halos in our sample have super-linear scaling between radio and X-rays, which suggests a peaked distribution of relativistic electrons and magnetic field. We used the radio and X-ray correlation to constrain the physical parameters of a hadronic model and we compared the model predictions with current observations. Specifically, we focus on a model where cosmic rays are injected by the central AGN and they generate secondaries in the ICM, and we assume that the role of turbulent re-acceleration is negligible. This model allows us to constrain the AGN cosmic ray luminosity in the range $\sim10^{44-46}$ erg s$^{-1}$ and the central magnetic field in the range 10-40 $\mu$G. The resulting $\gamma$-ray fluxes calculated assuming these model parameters do not violate the upper limits on $\gamma$-ray diffuse emission set by the Fermi-LAT telescope. Further studies are now required to explore the consistency of these large magnetic fields with Faraday rotation studies and to study the interplay between the secondary electrons and the ICM turbulence., Comment: 17 pages, 13 figures. Accepted for publication on A&A

A fraction of galaxy clusters host diffuse radio sources whose origins are investigated through multi-wavelength studies of cluster samples. We investigate the presence of diffuse radio emission in a sample of seven galaxy clusters in the largely unexplored intermediate redshift range (0.3 < z < 0.44). In search of diffuse emission, deep radio imaging of the clusters are presented from wide band (1.1-3.1 GHz), full resolution ($\sim$ 5 arcsec) observations with the Australia Telescope Compact Array (ATCA). The visibilities were also imaged at lower resolution after point source modelling and subtraction and after a taper was applied to achieve better sensitivity to low surface brightness diffuse radio emission. In case of non-detection of diffuse sources, we set upper limits for the radio power of injected diffuse radio sources in the field of our observations. Furthermore, we discuss the dynamical state of the observed clusters based on an X-ray morphological analysis with XMM-Newton. We detect a giant radio halo in PSZ2 G284.97-23.69 (z=0.39) and a possible diffuse source in the nearly relaxed cluster PSZ2 G262.73-40.92 (z=0.421). Our sample contains three highly disturbed massive clusters without clear traces of diffuse emission at the observed frequencies. We were able to inject modelled radio halos with low values of total flux density to set upper detection limits; however, with our high-frequency observations we cannot exclude the presence of RH in these systems because of the sensitivity of our observations in combination with the high z of the observed clusters., 12 pages, 7 figures, accepted for publication in A&A

High sensitivity observations of radio halos in galaxy clusters at frequencies ν ≤ 330 MHz are still relatively rare, and very little is known compared to the classical 1.4 GHz images. The few radio halos imaged down to 150-240 MHz show a considerable spread in size, morphology and spectral properties. All clusters belonging to the GMRT Radio Halo Survey with detected or candidate cluster-scale diffuse emission have been imaged at 325 MHz with the GMRT. Few of them were also observed with the GMRT at 240 MHz and 150 MHz. For A 1682, imaging is particularly challenging due to the presence of strong and extended radio galaxies at the center. Our data analysis suggests that thew radio galaxies are superposed to very low surface brightness radio emission extended on the cluster scale, which we present here. High sensitivity observations of radio halos in galaxy clusters at frequencies ν ≤ 330 MHz are still relatively rare, and very little is known compared to the classical 1. 4 GHz images. The few radio halos imaged down to 150-240 MHz show a considerable spread in size, morphology and spectral properties. All clusters belonging to the GMRT Radio Halo Survey with detected or candidate cluster-scale diffuse emission have been imaged at 325 MHz with the GMRT. Few of them were also observed with the GMRT at 240 MHz and 150 MHz. For A 1682, imaging is particularly challenging due to the presence of strong and extended radio galaxies at the center. Our data analysis suggests that thew radio galaxies are superposed to very low surface brightness radio emission extended on the cluster scale, which we present here. © 2011 Indian Academy of Sciences.

The Shapley Concentration ($z\approx0.048$) covers several degrees in the Southern Hemisphere, and includes galaxy clusters in advanced evolutionary stage, groups of clusters in the early stages of merger, fairly massive clusters with ongoing accretion activity, and smaller groups located in filaments in the regions between the main clusters. With the goal to investigate the role of cluster mergers and accretion on the radio galaxy population, we performed a multi-wavelength study of the BCGs and of the galaxies showing extended radio emission in the cluster complexes of Abell 3528 and Abell 3558. Our study is based on a sample of 12 galaxies. We observed the clusters with the GMRT at 235, 325 and 610 MHz, and with the VLA at 8.46 GHz. We complemented our study with the TGSS at 150 MHz, the SUMSS at 843 MHz and ATCA at 1380, 1400, 2380, and 4790 MHz data. Optical imaging with ESO-VST and mid-IR coverage with WISE are also available for the host galaxies. We found deep differences in the properties of the radio emission of the BCGs in the two cluster complexes. The BCGs in the A3528 complex and in A3556, which are relaxed cool-core objects, are powerful active radio galaxies. They also present hints of restarted activity. On the contrary, the BCGs in A3558 and A3562, which are well known merging systems, are very faint, or quiet, in the radio band. The optical and IR properties of the galaxies are fairly similar in the two complexes, showing all passive red galaxies. Our study shows remarkable differences in the radio properties of the BGCs, which we relate to the different dynamical state of the host cluster. On the contrary, the lack of changes between such different environments in the optical band suggests that the dynamical state of galaxy clusters does not affect the optical counterparts of the radio galaxies, at least over the life-time of the radio emission., 24 pages, 11 figures, accepted for publication in Astronomy & Astrophysics

Clusters of galaxies are sites of acceleration of charged particles and sources of non-thermal radiation. We report on new constraints on the population of cosmic rays in the Intra Cluster Medium (ICM) obtained via radio observations of a fairly large sample of massive, X-ray luminous, galaxy clusters in the redshift interval 0.2--0.4. The bulk of the observed galaxy clusters does not show any hint of Mpc scale synchrotron radio emission at the cluster center (Radio Halo). We obtained solid upper limits to the diffuse radio emission and discuss their implications for the models for the origin of Radio Halos. Our measurements allow us to derive also a limit to the content of cosmic ray protons in the ICM. Assuming spectral indices of these protons delta =2.1-2.4 and microG level magnetic fields, as from Rotation Measures, these limits are one order of magnitude deeper than present EGRET upper limits, while they are less stringent for steeper spectra and lower magnetic fields., 14 pages, 5 figures, ApJ Letter, accepted

The intra-cluster medium contains cosmic rays and magnetic fields that are manifested through the large scale synchrotron sources, termed as radio halos, relics and mini-halos. The Extended Giant Metrewave Radio Telescope (GMRT) Radio Halo Survey (EGRHS) is an extension of the GMRT Radio Halo Survey (GRHS) designed to search for radio halos using GMRT 610/235 MHz observations. The GRHS+EGRHS consists of 64 clusters in the redshift range 0.2 -- 0.4 that have an X-ray luminosity larger than 5x10^44 erg/s in the 0.1 -- 2.4 keV band and with declinations > -31 deg in the REFLEX and eBCS X-ray cluster catalogues. In this second paper in the series, GMRT 610/235 MHz data on the last batch of 11 galaxy clusters and the statistical analysis of the full sample are presented. A new mini-halo in RXJ2129.6+0005 and candidate diffuse sources in Z5247, A2552 and Z1953 are discovered. A unique feature of this survey are the upper limits on the detections of 1 Mpc sized radio halos; 4 new are presented here making a total of 31 in the survey. Of the sample, 58 clusters that have adequately sensitive radio information were used to obtain the most accurate occurrence fractions so far. The occurrence of radio halos in our X-ray selected sample is ~22%, that of mini-halos is 13% and that of relics is ~5%. The radio power - X-ray luminosity diagrams for the radio halos and mini-halos with the detections and upper limits are presented. The morphological estimators namely, centroid shift (w), concentration parameter (c) and power ratios (P_3/P_0) derived from the Chandra X-ray images are used as proxies for the dynamical states of the GRHS+EGRHS clusters. The clusters with radio halos and mini-halos occupy distinct quadrants in the c-w, c-P_3/P_0 and w - P_3/P_0 planes, corresponding to the more and less morphological disturbance, respectively. The non-detections span both the quadrants., 24 pages, 5 tables, 25 figures, accepted for publication in A&A

Brightest Cluster Galaxies (BCGs) show exceptional properties over the whole electromagnetic spectrum. Their special location at the centres of galaxy clusters raises the question of the role of the environment on their radio properties. To decouple the effect of the galaxy mass and of the environment in their statistical radio properties, we investigate the possible dependence of the occurrence of radio loudness and of the fractional radio luminosity function on the dynamical state of the hosting cluster. We studied the radio properties of the BCGs in the Extended GMRT Radio Halo Survey (EGRHS). We obtained a statistical sample of 59 BCGs, which was divided into two classes, depending on the dynamical state of the host cluster, i.e. merging (M) and relaxed (R). Among the 59 BCGs, 28 are radio-loud, and 31 are radio--quiet. The radio-loud sources are located favourably located in relaxed clusters (71\%), while the reverse is true for the radio-quiet BCGs, mostly located in merging systems (81\%). The fractional radio luminosity function (RLF) for the BCGs is considerably higher for BCGs in relaxed clusters, where the total fraction of radio loudness reaches almost 90\%, to be compared to the $\sim$30\% in merging clusters. For relaxed clusters, we found a positive correlation between the radio power of the BCGs and the strength of the cool core, consistent with previous studies on local samples. Our study suggests that the radio loudness of the BCGs strongly depends on the cluster dynamics, their fraction being considerably higher in relaxed clusters. We compared our results with similar investigations, and briefly discussed them in the framework of AGN feedback., Comment: 12 pages, 6 figures, 3 tables, A&A accepted

We report the discovery of a giant radio halo in the galaxy cluster RXC J1514.9-1523 at z=0.22 with a relatively low X-ray luminosity, $L_{X \, [0.1-2.4 \rm \, kev]} \sim 7 \times 10^{44}$ erg s$^{-1}$. This faint, diffuse radio source is detected with the Giant Metrewave Radio Telescope at 327 MHz. The source is barely detected at 1.4 GHz in a NVSS pointing that we have reanalyzed. The integrated radio spectrum of the halo is quite steep, with a slope \alpha = 1.6 between 327 MHz and 1.4 GHz. While giant radio halos are common in more X-ray luminous cluster mergers, there is a less than 10% probability to detect a halo in systems with $L_X \ltsim 8 \times 10^{44}$ erg s$^{-1}$. The detection of a new giant halo in this borderline luminosity regime can be particularly useful for discriminating between the competing theories for the origin of ultrarelativistic electrons in clusters. Furthermore, if our steep radio spectral index is confirmed by future deeper radio observations, this cluster would provide another example of the recently discovered population of ultra-steep spectrum radio halos, predicted by the model in which the cluster cosmic ray electrons are produced by turbulent reacceleration., Comment: 4 pages, 2 figures - Accepted for publication on A&A Research Notes

Aims. We report the discovery of a radio minihalo in RXCJ1504.1-0248, a massive galaxy cluster that has an extremely luminous cool core. To date, only 9 radio minihalos are known, thus the discovery of a new one, in one of the most luminous cool-core clusters, provides important information on this peculiar class of sources and sheds light on their origin. Methods. The diffuse radio source is detected using GMRT at 327 MHz and confirmed by pointed VLA data at 1.46 GHz. The minihalo has a radius of $\sim$140 kpc. A Chandra gas temperature map shows that the minihalo emission fills the cluster cool core and has some morphological similarities to it, as has been previously observed for other minihalos. Results. The Chandra data reveal two subtle cold fronts in the cool core, likely created by sloshing of the core gas, as observed in most cool-core clusters. Following previous work, we speculate that the origin of the minihalo is related to sloshing. Sloshing may result in particle acceleration by generating turbulence and/or amplifying the magnetic field in the cool core, leading to the formation of a minihalo., 4 pages, 1 table, 3 color figures. Accepted for publication in A&A Letters

The frequently observed association between giant radio halos and merging galaxy clusters has driven present theoretical models of non-thermal emission from galaxy clusters, which are based on the idea that the energy dissipated during cluster-cluster mergers could power the formation of radio halos. To quantitatively test the merger-halo connection we present the first statistical study based on deep radio data and X-ray observations of a complete X-ray selected sample of galaxy clusters with X-ray luminosity > 5x 10^44 erg/s and redshift 0.2, Comment: 5 pages, 1 figure, ApJ Letter in press

We present a radio morphological study and spectral analysis for a sample of 13 cD galaxies in rich and poor clusters of galaxies.} Our study is based on new high sensitivity Giant Metrewave Radio Telescope (GMRT) observations at 1.28 GHz, 610 MHz and 235 MHz, and on archival data. From a statistical sample of cluster cD galaxies we selected those sources with little information available in the literature and promising for the detection of aged radio emission. Beyond the high sensitivity images for all 13 radio galaxies, we present also a detailed spectral analysis for 7 of them. We found a variety of morphologies and linear sizes, as typical for radio galaxies in the radio power range sampled here (low to intermediate power radio galaxies). The spectral analysis shows that 10/13 radio galaxies have steep radio spectrum, with spectral index $\alpha \ge 1$. In general, the radiative ages and growth velocities are consistent with previous findings that the evolution of radio galaxies at the cluster centres is affected by the dense external medium (i.e. low growth velocities and old ages. We suggest that the dominant galaxies in A 2622 and MKW 03s are dying radio sources, which at present are not fed by nuclear activity. On the other hand, the spectacular source at the centre of A 2372 might be a very interesting example of restarted radio galaxy. For this source we estimated a life cycle of the order of 10$^6$ yr., Comment: Accepted by A&A, 25 pages, 28 figures, 6 tables and appendix Full version including high quality images available at http://www.ira.inaf.it/~tventuri/pap/Venturi.pdf

Aims. We present the first results of an ongoing project devoted to the search of giant radio halos in galaxy clusters located in the redshift range z=0.2--0.4. One of the main goals of our study is to measure the fraction of massive galaxy clusters in this redshift interval hosting a radio halo, and to constrain the expectations of the particle re--acceleration model for the origin of non--thermal radio emission in galaxy clusters. Methods. We selected 27 REFLEX clusters and here we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for 11 of them. The sensitivity (1$\sigma$) in our images is in the range 35--100~$\mu$Jy beam$^{-1}$ for all clusters. Results. We found three new radio halos, doubling the number of halos known in the selected sample. In particular, giant radio halos were found in A 209 and RXCJ 2003.5--2323, and one halo (of smaller size) was found in RXCJ 1314.4--2515. Candidate extended emission on smaller scale was found around the central galaxy in A 3444 which deserves further investigation. Furthermore, a radio relic was found in A 521, and two relics were found in RXCJ 1314.5--2515. The remaining six clusters observed do not host extended emission of any kind., Comment: 18 pages, 10 figures & appendix. Accepted by A&A

We present a new detailed multiband study of the merging cluster A3562, in the core of the Shapley Concentration Supercluster. We analyzed new, low frequency radio data performed at 240 MHz, 332 MHz and 610 MHz with the Giant Metrewave Radio Telescope (GMRT). The new GMRT data allowed us to carry out a detailed study of the radio halo at the centre of A3562, as well as of the head--tail radio galaxy J1333--3141 embedded in it, and of the extended emission around the peripheral cluster galaxy J1332--3146a. Thanks to the present observations we could derive the integrated spectrum of the radio halo with five data points in the frequency range 240 MHz -- 1.4 GHz. Our data show a clear steepening of the total spectrum in this frequency range. Furthermore, by comparing the GMRT 332 MHz image with a previously published VLA 1.4 GHz image, we produced an image of the halo spectral index distribution. The image shows a very complex structure, with an average value of $\alpha^{1.4GHz}_{332MHz} \sim 1.5$ and a number of knots steepening up to $\sim 2$. We performed a combined morphological and statistical analysis using the radio images and the quantities derived from XMM--Newton and {\it Chandra} observations. We discuss our results in the light of particle re--acceleration processes in galaxy clusters. In particular, we outline an overall picture, consistent with the available radio and X-ray data, in which the cluster merger kinematics, the injection of turbulence and B--amplification induced by the merger between A 3562 and SC 1329--313 are jointly taken into account., Comment: 14 pages, 13 figures, A&A in press Paper with high quality figures can be downloaded from http://www.ira.cnr.it/~tventuri/pap

We present high sensitivity radio observations of the merging cluster A521, at a mean redsfhit z=0.247. The observations were carried out with the GMRT at 610 MHz and cover a region of $\sim$1 square degree, with a sensitivity limit of $1\sigma$ = 35 $\mu$Jy b$^{-1}$. The most relevant result of these observations is the presence of a radio relic at the cluster periphery, at the edge of a region where group infalling into the main cluster is taking place. Thanks to the wealth of information available in the literature in the optical and X-ray bands, a multi--band study of the relic and its surroundings was performed. Our analysis is suggestive of a connection between this source and the complex ongoing merger in the A521 region. The relic might be ``revived' fossil radio plasma through adiabatic compression of the magnetic field or shock re--acceleration due to the merger events. We also briefly discussed the possibility that this source is the result of induced ram pressure stripping of radio lobes associated with the nearby cluster radio galaxy J0454--1016a. Allowing for the large uncertainties due to the small statistics, the number of radio emitting early--type galaxies found in A521 is consistent with the expectations from the standard radio luminosity function for local (z$\le$0.09) cluster ellipticals., Comment: 30 pages 8 figures, 5 tables, accepted by New Astronomy

In this paper we present Very Large Array (VLA) 1.4 GHz (21 cm) observations of the region between the centres of A3558 and A3562, in the majorcluster merger complex of the Shapley Concentration. Our final catalogue includes a total of 174 radio sources above the flux density limit of 0.25 mJy b$^{-1}$. By cross-correlation with optical and spectroscopic catalogues we found 33 optical counterparts belonging to the Shapley Concentration. We investigated the effects of cluster merger on the radio emission properties of the galaxy population by means of the radio source counts and the radio luminosity functions (RLF). We found that the radio source counts are consistent with the field source counts.The RLF of elliptical and S0 galaxies in the region surveyed here, is consistent with the ``universal'' RLF for early--type galaxies.This result suggests that the deficit in radio galaxies found in ourprevious works over the whole A3558 chain, is entirely due to the cluster A3558. A population of faint radio galaxies (logP$_{1.4 GHz}$(W Hz$^{-1}$) \ltsim 22) is also found.Half of these objects are also blue, suggesting that starburst is the main mechanism driving the radio emission. Finally, we detected 14 spiral radio galaxies, whose ratio between radio and optical emission is similar to those found in galaxies located in rich and dynamically evolved clusters. Our results are briefly discussed in the light of the age and stage of the merger in the A3558 cluster complex., Comment: 23 pages, 8 figures, Submitted to A&A