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5. Radio Halos and Relics in Galaxy Groups and Clusters

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5. Radio Halos and Relics in Galaxy Groups and Clusters 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.
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