5. Radio halos and Relics in galaxy groups and clusters

5.1 (Oral) Particle acceleration in galaxy clusters : from radio halos to gamma rays Gianfranco Brunetti (IRA‐INAF) ([email protected]) In this talk I will start with a brief (and critical) review of the most important observational aspects that guide our modeling of particle acceleration and diffuse non‐thermal emission in galaxy clusters. Then I will discuss in detail relevant theoretical aspects. In particular, one of the most popular models for the origin of radio halos assumes that relativistic particles are reaccelerated by turbulence generated during cluster‐cluster mergers. I will focus on the physics and on the most important expectations of this scenario, from radio to gamma‐rays, discussing the most relevant tests of this model with present and future telescopes.

5.2 (Oral) The connection between radio halos and cluster mergers and the statistical properties of the radio halo population Rossella Cassano (IRA‐ INAF, Bologna, ITALY) ([email protected]) I will discuss the statistical properties of the radio halo population in galaxy clusters. Radio bimodality is observed in galaxy clusters: a fraction of clusters host giant radio halos while the majority of clusters do not show evidence of diffuse cluster‐scale radio emission. Present data sclearly suggest that the observed radio bimodality has a correspondence in terms of dynamical state of the hosting clusters. I will report on these evidences in some details and discuss the role of cluster mergers in the generation of giant radio halos and their evolution. Finally I will show some expectations on the halo statistical properties in the case where the emitting electrons are re‐accelerated by merger‐ turbulence, and discuss the role of incoming LOFAR surveys.

5.3 (Oral) The GMRT survey of MACS clusters : Discovery of a giant radio halo at z=0.443 K. S. Dwarakanath (RRI)([email protected]), Siddharth Malu (RRI), Ruta Kale (RRI) We have imaged six of the hottest (~ 8 keV) and most luminous (L_x > 10**45 erg/s) clusters selected from the MAssive Cluster Survey (MACS) at 230 and 610 MHz using the GMRT. All the clusters (median z=0.37) chosen display disturbed X‐ray morphologies, suggesting that they may have undergone mergers. The images of these six cluster fields at 610 MHz have rms values in the range 40‐100 microJy/beam, with a beam of ~ 10" X 10". Four out of the six cluster fields exhibit diffuse and filamentary radio emissions which are likely to be of cluster origin. The surface brightnesses of these diffuse emissions are in the range 0.1 ‐ 3 mJy/beam. The most spectacular radio halo was discovered in the second brightest cluster in the MACS sample (MACSJ0417.5‐1154, z=0.443) which has an L_x of 2.9*10**45 erg/s. This radio halo is ~ 1.2 Mpc in extent and is extended along the North‐West, similar to the morphology of the X‐ray emission that is confining the halo emission. The 1400 MHz radio luminosity (L_r) of the halo is ~ 2*10**25 W/Hz, in good agreement with the value expected from the L_x‐L_r correlation for cluster halos. The implications of these detections are being explored.

5.4 (Oral) Simulations of halos and relics in galaxy clusters Matthias Hoeft (Thueringer Landessternwarte) (hoeft@tls‐tautenburg.de) Clusters of galaxies are the largest and most massive gravitationally bound structures in the Universe. During mergers of clusters large amounts of kinetic energy are dissipated into the intra‐ cluster medium via shock fronts and turbulence. We will report on current efforts to simulate by high resolution cosmological simulations shock fronts and turbulence induced by cluster mergers. The simulations reproduce the properties of large‐scale relics found in the periphery of massive galaxy clusters. This clearly supports that relics trace structure formation shocks. Upcoming low frequency radio telescopes ‐as LOFAR and LWA‐are expected to detect many relics. We will show that future observations may allow to put constraints on the evolution of magnetic fields in the universe.

5.5 (Oral) Detection of Diffuse Radio Emission in Southern REXCESS Clusters Melanie Johnston‐Hollitt (Victoria University of Wellington)(Melanie.Johnston‐[email protected]) Sara Shakouri (Victoria University of Wellington) Gabriel Pratt (Centre d'Etudes d'Astrophysique) Saclay, Hans Bohringer (Max Planck Inst. fuer Extraterrestrische Physik) Judith Croston (University of outhampton) Alexis Finoguenov (Max Planck Inst. fuer Extraterrestrische Physik) We present new 1.4 GHz ATCA imaging of 18 clusters South of ‐20 degrees in the Representative XMM‐Newton Cluster Structure Survey (REXCESS). REXCESS is a luminosity selected, representative sample of nearby X‐ray clusters and thus provides an ideal sample with which to study statistically both the occurrence of diffuse radio emission (halos and relics) in galaxy clusters. We present new relics and halos found in the sample and discuss their statistics in the context of the recent progress on characterizing the prevalence of diffuse radio emission with cluster dynamical state.

5.6 (Oral) Double relics in the outskirts of A3376: accretion flows meet merger shocks ? Ruta Kale (RRI) ([email protected]), K. S. Dwarakanath (RRI), Joydeep Bagchi (IUCAA), Surajit Paul (IUCAA) The case of spectacular ring‐like double radio relics in the merging, rich galaxy cluster A3376 is of great interest to study non‐thermal phenomena at cluster outskirts. We present the first low frequency (330 and 150 MHz) images of the double relics using the GMRT. With our GMRT 330 MHz map and the VLA 1400 MHz map (Bagchi et al 2006), we have constructed and analyzed the distribution of spectral indices over the radio relics. We find flat spectral indices at the outer edges of both the relics and a gradual steepening of spectral indices toward the inner regions. This supports the model of outgoing merger shock waves. The eastern relic has a complex morphology and spectral index distribution toward the inner region. This will be discussed in the context of the effect of large scale accretion flows on the outgoing merger shocks as reported in the recent simulations (Paul et al 2011).

5.7 (Poster) A zoo of radio relics: from cluster cores to filaments Ruta Kale (RRI) ([email protected]), K. S. Dwarakanath (RRI) Radio relics in galaxy clusters can be electrons accelerated at cluster merger shocks or adiabatically compressed fossil radio cocoons or dying radio galaxies. The spectral evolution of radio relics is affected by the pressure of the surrounding thermal plasma. We present a low frequency study of three radio relics representing environments of dense cluster core (A4038), cluster outskirts (A1664) and filaments (A786). Multi‐frequency (three or more frequencies in the range 150‐ 1400 MHz) imaging of these relics led to the following results: 1. The spectral indices (sp. ind) of the relics progressively steepen as the environment becomes denser: relics in sparse environments, A786 (spectral index ~ ‐1.0) and A1664 (sp. ind. ~ ‐1.1) have flat spectra as compared to the relic at the centre of A4038 (sp. ind ~ ‐1.8). Similarly the sizes of the relics range from ~ 200 kpc to ~ 1.6 Mpc as one goes from dense (electron densities ~ 0.01‐0.001 /cc) to sparse (<0.0001 /cc) environments. These trends are consistent with the expectation of steepening of spectral index due to confinement of the relativistic plasma by the intra‐cluster medium. 2. GMRT low frequency images (150, 240 and 606 MHz) of the relic in A4038 led to discovery of steep spectrum emission (sp. ind. ~ ‐1.8 to ‐2.8) much larger in extent (200 kpc) than was known from 1400 MHz images (56 kpc). 3. Best fits to the integrated spectra of these relics in the framework of the adiabatic compression model were obtained. The spectrum of A4038 relic can be best fit by a model of adiabatically compressed plasma. The spectra of relics in A1664 and A786 are best fit by a model of a fossil radio galaxy. 4. Spectral index maps of all the three relics do not show any obvious gradient indicative of shock.

5.8 (Oral) Disentangling the diffuse radio emission from galaxy clusters Uri Keshet (Harvard University) ([email protected]) Some galaxy clusters show radio emission that is not associated with any particular galaxy. These radio sources, classified for example as peripheral relics and central halos, were so far believed to arise from different electron injection mechanisms. I will show that diffuse radio emission from the intracluster medium, in its various forms, can be explained as arising from hadronic collisions involving the same population of cosmic ray ions, distributed homogeneously across the cluster. This resolves several previous discrepancies, gauges additional hadronic signals, and allows one to directly measure the magnetic field, as well as its recent evolution.

5.9 (Poster) Interaction of the WAT source in Abell 3395 with the Intracluster medium Kiran Lakhchaura(TIFR, Mumbai)([email protected]), K. P. Singh (TIFR, Mumbai), D. J. Saikia, (NCRA) R. W. Hunstead ( SIfA, School of Physics, Univ. of Sydney, Australia) Using X‐ray observations from Chandra and XMM‐Newton and radio observations from the Australia Telescope Large Array (ATCA), we have examined the merging environment of the bimodal cluster Abell 3395. The radio images of the cluster show the presence of a Wide Angle Tail (WAT) galaxy slightly offset from the X‐ray emission peak of the southern part of the cluster, which is a clear signature of an ongoing merger in the cluster. The WAT galaxy is possibly responsible for the absence of cooling flows in the southern part of the cluster through feedback processes. The unsharp masked Chandra image of the cluster does not show any deficit in the X‐ray flux near the location of the source. This may be either due to a) as the source is very extended, the cavities form at large distances from the centre and hence, the detector is not sensitive enough to detect the cavities. or b) the radio plasma of the lobes is in pressure equilibrium with the thermal gas. Though, the synchrotron ! pressure of the radio lobes is atleast an order of magnitude lesser than the thermal pressure of the gas but pressure equilibrium can still be achieved by considering the entrainment of thermal gas by the radio jets.

5.10 (Oral) The Radio Halo and SZE effect in the Bullet Cluster at 18 and 22 GHz Siddharth S. Malu (RRI)([email protected]), Ravi Subrahmanyan (RRI), Mark Wieringa (ATCA, CSIRO, Australia), D. Narasimha(TIFR) We present 18~GHz observations of the Bullet cluster from the Australia Telescope Compact Array; in particular, a high angular resolution measurement of the substructure in Sunyaev‐Zel'dovich Effect (SZE). We report observations of the radio halo as well as the discovery of a compact SZE `hole' in the galaxy cluster. The SZE hole does not correspond to any bright feature in X‐ray, optical or lensing maps; in general, the relatively deeper SZE features appear to avoid the regions with the most intense X‐ray emission. These imply that the gas pressure distribution differs significantly from the distributions in gas emission measure, galaxy and dark matter distributions. This has implications for the gas physics and evolution in the cluster merger event. The SZE distributions in the western parts of the cluster are cospatial with the radio halo indicative of a common origin for the hot and relativistic electrons in the turbulent wake of the Bullet.

5.11 (Oral) ATLAS, and WATs in ATLAS Minnie Y. Mao (University of Tasmania)([email protected]),Rob Sharp (ANU), Dhruba J. Saikia(N.C.R.A‐ T.I.F.R), Ray P. Norris (A.T.N.F., CSIRO Astronomy and Space Science) Melanie Johnston‐Hollitt (Victoria University of Wellington), Enno Middelberg (Astronomisches Institut, Ruhr‐ University Bochum) ATLAS, the Australia Telescope Large Area Survey, is the widest deep radio survey attempted to date, and aims to understand the formation and evolution of galaxies through cosmic time. Using the Australia Telescope Compact Array, ATLAS is imaging two fields totaling 7 square degrees down to 10 uJy at 1.4 GHz. We have found 6 wide‐angle tail galaxies (WATs), 4 of which are associated with galaxy over densities. The largest WAT, at a redshift of 0.22, appears to be associated with an over density of galaxies that is spread over an unusually large area of 12Mpc, with a velocity range of 4500 km/s. In this talk, I will give discuss some results concerning the WATs, as well as the implications of these observations for future large‐scale radio surveys such as ASKAP‐EMU.

5.12 (Oral) Entropy of the intracluster medium and AGN feedback Biman Nath (RRI)([email protected]) X‐ray observations of the intracluster medium suggest that it has higher entropy than expected from purely gravitational interactions. It is believed that active galaxies provide the additional entropy although the physical mechanisms remain uncertain. Radio halos and relics may be indicative of such a feedback mechanism. We review what is known about these processes, as well as some phenomenological studies of the entropy of the intracluster gas in order to determine the redshift evolution of AGN feedback.

5.13 (Oral) Large scale structure emission: the polarization aspects Roberto Pizzo (ASTRON) ([email protected]) Since a few decades, the existence of magnetic fields associated with the intra‐cluster medium in galaxy clusters has been well established through different methods of analysis. Furthermore, recent low‐frequency radio observations allowed the detection of weak magnetic fields in between galaxy clusters. Such results provide the convincing evidence of the role that the new generation of low‐ frequency radio interferometers will play in the detection and study of polarized emission associated with the cosmic web. In this talk I will review the recent efforts made through radio observations to measure the strengths and power spectra of cluster magnetic fields and I will discuss the main techniques that have led to our knowledge on magnetic fields in clusters of galaxies and in their outskirts. In particular, I will focus on RM‐synthesis, a powerful tool for the detection of weak magnetic fields and for the study of the 3‐dimensional geometry of galaxy clusters. I will refer to the my recent work on Abell 2255, for which I performed a deep RM tomography between 1.4 GHz and 350 MHz that helped to understand the origin of its polarized radio halo.

5.14 (Oral) The effect of AGN Feedback on the intergalactic medium of galaxy groups and clusters Somak Raychaudhury (University of Birmingham, UK)([email protected]), Ewan O'Sullivan (University of Birmingham, UK), Jan Vrtilek (Harvard‐Smithsonian Center for Astrophysics, USA), Simona Giacintucci (Harvard‐Smithsonian Center for Astrophysics, USA), Larry David (Harvard‐ Smithsonian Center for Astrophysics, USA), C. H. Ishwara‐Chandra (NCRA‐TIFR). The lack of very cool gas at the cores of galaxy groups and clusters, even where the cooling time is significantly shorter than the Hubble time, has been interpreted as evidence of re‐heating of the diffuse intergalactic medium. Gravitational heating, mergers and conduction are among the candidates, but most studies of rich clusters adopt active galactic nuclei (AGN) to be the predominant source. Here we assess the roles of various sources of feedback in clusters and groups of galaxies. Putting together X‐ray observations from the Chandra satellite and the low‐frequency radio observations from the GMRT yields spectacular evidence of the energy transfer between the supermassive black holes and the intracluster gas, in systems spanning a wide range of mass, and provides a unique insight into the nature of AGN feedback.

5.15 (Oral) Diffuse emission in clusters: tracers of cluster mergers. Recent results and prospects for LOFAR. Huub Rottgering (Sterrewacht Leiden) ([email protected]) Radio observations of clusters are important to understand the impact of shocks and mergers on the general evolution of clusters. In the last two years we have embarked on a large project to elucidate the relation of the diffuse radio relics and properties of the ICM. In this talk we will first discuss results on studies of individual clusters including ZwCl 2341.1+0000, MACS J0715.5+3745 and Abell 2256 and the spectacularly long and narrow relic CIZA CIZAJ2242.8+5301 The radio observations presented all show how that shocks produced in cluster mergers are clearly related to the presence of diffuse radio emission. We then present results from a study of a sample of relics. Using this sample, we found clear relations between the various properties of the relics (the Mach number of merger shocks as traced by the radio spectral index, projected distance from the cluster center and the radio power). In the second part of the talk I will briefly review the status of LOFAR, a new radio telescope that is currently being commissioned. First results on nearby clusters will be shown.

5.16 (Oral) Manifestations of Magnetic Field Inhomogeneities Lawrence Rudnick (University of Minnesota) ([email protected]) Both observations and simulations reveal large inhomogeneities in magnetic field distributions in diffuse plasmas. I will look at how these may be distinguished observationally from relativistic electron changes and then re‐examine the old issue of how inhomogeneous fields affect particle lifetimes. I will also consider the consequences of these inhomogeneities on observed radio spectra. Finally, I will preview some new results on EVLA observations of the tantalizing rotation measure structure in the complex relic in Abell 2256.

5.17 (Oral) Diffuse radio emission in galaxy clusters: current knowledge and future prospects Tiziana Venturi (INAF, Istituto di Radioastronomia) ([email protected]) The GMRT Radio Halo Survey, its extension and low frequency follow up; have considerably helped our understanding of the radio halo phenomenon and its connection to cluster mergers. In this presentation I will focus on the most recent progress in our knowledge of the properties of radio halos and relics and will present some of the open questions and future observational prospects in this field.

5.18 (Oral) Radio relics as probes of galaxy cluster mergers R J van Weeren (Leiden Observatory)([email protected]), H J A Rottgering (Leiden Observatory), M Bruggen (Jacobs University Bremen), M Hoeft (Thuringer Landessternwarte Tautenburg) Radio relics are diffuse elongated radio sources found in disturbed galaxy clusters, indicative of an ongoing merger event. It is proposed that these relics trace shock waves, in which particles are (re)accelerated by the diffusive shock acceleration mechanism. We will present GMRT, WSRT and VLA radio observations of newly discovered relics. By compiling a sample of more than 30 relics, we find correlations between the position and orientation of relics with respect to the X‐ray elongation measured from ROSAT. The correlations are consistent with the scenario that relics trace merger shock waves. In addition, we will present initial results from simulations of idealized binary cluster merger events. These simulations can be used to constrain the timescales, mass ratios, and impact parameters of merger events by making use of the position and sizes of double radio relics.