Giant Radio Halos Research Articles

THE SCALING RELATIONS AND THE FUNDAMENTAL PLANE FOR RADIO HALOS AND RELICS OF GALAXY CLUSTERS

Diffuse radio emission in galaxy clusters is known to be related to cluster mass and cluster dynamical state. We collect the observed fluxes of radio halos, relics, and mini-halos for a sample of galaxy clusters from the literature, and calculate their radio powers. We then obtain the values of cluster mass or mass proxies from previous observations, and also obtain the various dynamical parameters of these galaxy clusters from optical and X-ray data. The radio powers of relics, halos, and mini-halos are correlated with the cluster masses or mass proxies, as found by previous authors, while the correlations concerning giant radio halos are in general the strongest. We found that the inclusion of dynamical parameters as the third dimension can significantly reduce the data scatter for the scaling relations, especially for radio halos. We therefore conclude that the substructures in X-ray images of galaxy clusters and the irregular distributions of optical brightness of member galaxies can be used to quantitatively characterize the shock waves and turbulence in the intracluster medium responsible for re-accelerating particles to generate the observed diffuse radio emission. The power of radio halos and relics is correlated with cluster mass proxies and dynamical parameters in the form of a fundamental plane.

The challenge of turbulent acceleration of relativistic particles in the intra-cluster medium

Acceleration of cosmic-ray electrons (CRe) in the intra-cluster medium (ICM) is probed by radio observations that detect diffuse, megaparsec-scale, synchrotron sources in a fraction of galaxy clusters. Giant radio halos are the most spectacular manifestations of non-thermal activity in the ICM and are currently explained assuming that turbulence, driven during massive cluster–cluster mergers, reaccelerates CRe at several giga-electron volts. This scenario implies a hierarchy of complex mechanisms in the ICM that drain energy from large scales into electromagnetic fluctuations in the plasma and collisionless mechanisms of particle acceleration at much smaller scales. In this paper we focus on the physics of acceleration by compressible turbulence. The spectrum and damping mechanisms of the electromagnetic fluctuations, and the mean free path (mfp) of CRe, are the most relevant ingredients that determine the efficiency of acceleration. These ingredients in the ICM are, however, poorly known, and we show that calculations of turbulent acceleration are also sensitive to these uncertainties. On the other hand this fact implies that the non-thermal properties of galaxy clusters probe the complex microphysics and the weakly collisional nature of the ICM.

Occurrence of radio halos in galaxy clusters

Giant radio halos (RH) are diffuse Mpc-scale synchrotron sources detected in a fraction of massive and merging galaxy clusters. An unbiased study of the statistical properties of RHs is crucial to constrain their origin and evolution. We aim at investigating the occurrence of RHs and its dependence on the cluster mass in a SZ-selected sample of galaxy clusters, which is as close as possible to be a mass-selected sample. Moreover, we analyse the connection between RHs and merging clusters. We select from the Planck SZ catalogue (Planck Collaboration XXIX 2014) clusters with $M\geq 6\times10^{14} M_\odot$ at z=0.08-0.33 and we search for the presence of RHs using the NVSS for z<0.2 and the GMRT RH survey (GRHS, Venturi et al. 2007, 2008) and its extension (EGRHS, Kale et al. 2013, 2015) for 0.2<z<0.33. We use archival Chandra X-ray data to derive information on the clusters dynamical status. We confirm that RH clusters are merging systems while the majority of clusters without RH are relaxed, thus supporting the idea that mergers play a fundamental role in the generation of RHs. We find evidence for an increase of the fraction of clusters with RHs with the cluster mass and this is in line with expectations derived on the basis of the turbulence re-acceleration scenario. Finally, we discuss the effect of the incompleteness of our sample on this result.

A giant radio halo in the cool core cluster CL1821+643

Abstract Giant radio haloes are Mpc-size sources found in some merging galaxy clusters. The synchrotron emitting electrons are thought to be (re)accelerated by plasma turbulence induced by the merging of two massive clusters. Cool core galaxy clusters have a low-temperature core, likely an indication that a major merger has not recently occurred. CL1821+643 is one of the strongest cool core clusters known so far. Surprisingly, we detect a giant radio halo with a largest linear size of ∼1.1 Mpc. We discuss the radio and X-ray properties of the cluster in the framework of the proposed models for giant radio haloes. If a merger is causing the radio emission, despite the presence of a cool core, we suggest that it should be off-axis, or in an early phase, or a minor one.

THE DISCOVERY OF A RADIO HALO IN PLCK G147.3–16.6 AT Z = 0.65

Recent X-ray and Sunyaev-Zel'dovich (SZ) observations have dramatically increased the number of known distant galaxy clusters. In some merging, low-redshift (z<0.4) clusters, centrally-located, diffuse, extended radio emission (called radio halos) has been found. Using the Giant Metrewave Radio Telescope (GMRT), we report the detection of diffuse radio emission in the binary-merging cluster PLCK G147.3-16.6 located at z = 0.65. We classify the emission as a giant radio halo due to the large physical extent of about 0.9 Mpc and low-surface brightness. We measure an integrated flux density of $7.3\pm1.1$ mJy at 610 MHz for the radio halo, resulting in a 1.4 GHz radio power of $5.1 \times 10^{24}$ W/Hz. The radio halo power is consistent with that expected from the known correlation between X-ray luminosity or the cluster integrated SZ signal and radio power. Our observations also suggest that more of these distant radio halos could be discovered with the GMRT.

NEW DETECTIONS OF RADIO MINIHALOS IN COOL CORES OF GALAXY CLUSTERS

Cool cores of some galaxy clusters exhibit faint radio "minihalos." Their origin is unclear, and their study has been limited by their small number. We undertook a systematic search for minihalos in a large sample of X-ray luminous clusters with high-quality radio data. In this article, we report four new minihalos (A 478, ZwCl 3146, RXJ 1532.9+3021, and A 2204) and five candidates found in the reanalyzed archival Very Large Array observations. The radio luminosities of our minihalos and candidates are in the range of 1023–25 W Hz−1 at 1.4 GHz, which is consistent with these types of radio sources. Their sizes (40–160 kpc in radius) are somewhat smaller than those of previously known minihalos. We combine our new detections with previously known minihalos, obtaining a total sample of 21 objects, and briefly compare the cluster radio properties to the average X-ray temperature and the total masses estimated from Planck. We find that nearly all clusters hosting minihalos are hot and massive. Beyond that, there is no clear correlation between the minihalo radio power and cluster temperature or mass (in contrast with the giant radio halos found in cluster mergers, whose radio luminosity correlates with the cluster mass). Chandra X-ray images indicate gas sloshing in the cool cores of most of our clusters, with minihalos contained within the sloshing regions in many of them. This supports the hypothesis that radio-emitting electrons are reaccelerated by sloshing. Advection of relativistic electrons by the sloshing gas may also play a role in the formation of the less extended minihalos.

On the physics of radio haloes in galaxy clusters: scaling relations and luminosity functions

The underlying physics of giant and mini radio haloes in galaxy clusters is still an open question. We find that mini haloes (such as in Perseus and Ophiuchus) can be explained by radio-emitting electrons that are generated in hadronic cosmic ray (CR) interactions with protons of the intracluster medium. By contrast, the hadronic model either fails to explain the extended emission of giant radio haloes (as in Coma at low frequencies) or would require a flat CR profile, which can be realized through outward streaming and diffusion of CRs (in Coma and A2163 at 1.4 GHz). We suggest that a second leptonic component could be responsible for the missing flux in the outer parts of giant haloes within a new hybrid scenario and we describe its possible observational consequences. To study the hadronic emission component of the radio-halo population statistically, we use a cosmological mock galaxy cluster catalogue built from the MultiDark simulation. Because of the properties of CR streaming and the different scalings of the X-ray luminosity (LX) and the Sunyaev–Zel'dovich flux (Y) with gas density, our model can simultaneously reproduce the observed bimodality of radio-loud and radio-quiet clusters at the same LX as well as the unimodal distribution of radio-halo luminosity versus Y; thereby suggesting a physical solution to this apparent contradiction. We predict radio-halo emission down to the mass scale of galaxy groups, which highlights the unique prospects for low-frequency radio surveys (such as the Low Frequency Array Tier 1 survey) to increase the number of detected radio haloes by at least an order of magnitude.

REVISITING SCALING RELATIONS FOR GIANT RADIO HALOS IN GALAXY CLUSTERS

Many galaxy clusters host Megaparsec-scale radio halos, generated by ultrarelativistic electrons in the magnetized intracluster medium. Correlations between the power of radio halos and the thermal properties of the hosting clusters were established in the last decade, including the connection between the presence of a halo and cluster mergers. The X-ray luminosity and redshift limited Extended GMRT Radio Halo Survey provides a rich and unique dataset for statistical studies of the halos. We uniformly analyze the radio and X-ray data for the GMRT cluster sample, and use the new Planck SZ catalog, to revisit the correlations between the power of halos and the thermal properties of galaxy clusters. We find that the radio power of halos at 1.4 GHz scales with the cluster X-ray (0.1--2.4 keV) luminosity computed within R_500 as P_1.4 L_500^2.0. Our bigger and more homogenous sample confirms that the X-ray luminous (L_500 > 5x10^44 erg/s) clusters branch into two populations --- radio halos lie on the correlation, while clusters with upper limits to radio-halo emission are well below that correlation. This bimodality remains if we excise cool cores from the X-ray luminosities. Correlating with Planck data, we find that P_1.4 scales with the cluster integrated SZ signal within R_500 as P_1.4 Y_500^2.1, in line with previous findings. However, contrary to previous studies that were limited by incompleteness and small sample size, we find that the "SZ-luminous" Y_500 > 6x10^-5 Mpc^2 clusters show a bimodal behavior similar to that in the radio-X-ray diagram. Bimodality of both correlations can be traced to clusters dynamics, with radio halos found exclusively in merging clusters. These results confirm the key role of mergers for the origin of giant radio halos, suggesting that they trigger the relativistic particle acceleration.

Low-frequency radio emission in the massive galaxy cluster MACSJ0717.5 + 3745

To investigate the nonthermal emission mechanism and their interaction during cluster mergers, we analyze multiple low-frequency radio data for the X-ray luminous massive galaxy cluster MACS J0717.5 + 3745, located at z = 0.5548. Large-scale structure-formation models in the Universe suggest that galaxy clusters grow via constant accretion of gas and the merger of galaxy groups and smaller clusters. Low-frequency radio observations trace these mergers in the form of relics and halos. The dual frequency observations were performed on MACS J0717.5 + 3745 to investigate the spectral index pattern of the nonthermal emission and its interaction within the intracluster medium (ICM), during merger process. Methods: Continuum observations were carried out using GMRT at 0.235 and 0.61 GHz on MACS J0717.5 + 3745 and archival data from the VLA (0.074 and 1.42 GHz) and WSRT (0.325 GHz) was used to complement the results. Furthermore, to explore the thermal and nonthermal interactions within the ICM and the morphological distribution, Chandra X-ray and HST data were used. Results: A highly complex nonthermal radio emission distribution is seen in the cluster at very low frequencies, with a global spectral index α0.2350.61˜-1.17±0.37. We have detected a giant radio halo within the cluster system with a linear size of 1.58 Mpc and a Chair-shaped filament structure between the merging subclusters of linear size 853 kpc at 0.235 GHz. This is the most powerful halo ever observed with P1.4 = 9.88 × 1025 WHz-1 and an equipartition magnetic field estimate of ~6.49 μG. The bright filament structure is well located in the central merging region of subclusters with enhanced temperature, as shown by Chandra and HST data analysis, further indicating the formation of this structure due to shock waves encountered within the ICM during the merger events.

THERMODYNAMICS OF THE COMA CLUSTER OUTSKIRTS

We present results from a large mosaic of Suzaku observations of the Coma Cluster, the nearest and X-ray brightest hot, dynamically active, non-cool core system, focusing on the thermodynamic properties of the ICM on large scales. For azimuths not aligned with an infalling subcluster towards the southwest, our measured temperature and X-ray brightness profiles exhibit broadly consistent radial trends, with the temperature decreasing from about 8.5 keV at the cluster center to about 2 keV at a radius of 2 Mpc, which is the edge of our detection limit. The SW merger significantly boosts the surface brightness, allowing us to detect X-ray emission out to ~2.2 Mpc along this direction. Apart from the southwestern infalling subcluster, the surface brightness profiles show multiple edges around radii of 30-40 arcmin. The azimuthally averaged temperature profile, as well as the deprojected density and pressure profiles, all show a sharp drop consistent with an outward propagating shock front located at 40 arcmin, corresponding to the outermost edge of the giant radio halo observed at 352 MHz with the WSRT. The shock front may be powering this radio emission. A clear entropy excess inside of r_500 reflects the violent merging events linked with these morphological features. Beyond r_500, the entropy profiles of the Coma Cluster along the relatively relaxed directions are consistent with the power-law behavior expected from simple models of gravitational large-scale structure formation. The pressure is also in agreement at these radii with the expected values measured from SZ data from the Planck satellite. However, due to the large uncertainties associated with the Coma Cluster measurements, we cannot yet exclude an entropy flattening in this system consistent with that seen in more relaxed cool core clusters.

Cosmic ray streaming in clusters of galaxies

The observed bimodality in radio luminosity in galaxy clusters is puzzling. We investigate the possibility that cosmic-ray (CR) streaming in the intra-cluster medium can 'switch off' hadronically induced radio and gamma-ray emission. For self-confined CRs, this depends on the source of MHD wave damping: if only non-linear Landau damping operates, then CRs stream on the slow Alfvenic timescale, but if turbulent wave damping operates, super-Alfvenic streaming is possible. As turbulence increases, it promotes outward streaming more than it enables inward turbulent advection. Curiously, the CR flux is independent of $\nabla f$ (as long as it is non-zero) and depends only on plasma parameters; this enables radio halos with flat inferred CR profiles to turn off. We perform 1D time-dependent calculations of a radio mini-halo (Perseus) and giant radio halo (Coma) and find that both diminish in radio luminosity by an order of magnitude in several hundred Myr, given plausible estimates for the magnetic field in the outskirts of the cluster. Due to the energy dependence of CR streaming, spectral curvature develops, and radio halos turn off more slowly at low frequencies -- properties consistent with observations. Similarly, CR streaming rapidly turns off gamma-ray emission at the high-energies probed by Cherenkov telescopes, but not at the low energies probed by Fermi. CR mediated wave-heating of the ICM is unaffected, as it is dominated by ~GeV CRs which stream Alfvenically.

Abell 2142 at large scales: An extreme case for sloshing?

We present results obtained with a new XMM-Newton observation of A2142, a famous textbook example of cluster with multiple cold fronts, which has been studied in detail with Chandra but whose large scale properties are presented here for the first time. We report the discovery of a a new cold front, the most distant one ever detected in a galaxy cluster, at about one Mpc from the center to the SE. Residual images, thermodynamics and metal abundance maps are qualitatively in agreement with predictions from numerical simulations of the sloshing phenomenon. However, the scales involved are much larger, similarly to what recently observed in the Perseus cluster. These results show that sloshing is a cluster-wide phenomenon, not confined in the cores, which extends well beyond the cooling region involving a large fraction of the ICM up to almost half of the virial radius. The absence of a cool core and a newly discovered giant radio halo in A2142, in spite of its relaxed X-ray morphology, suggest that large scale sloshing, or the intermediate merger which caused it, may trigger Mpc-scale radio emission and may lead to the disruption of the cluster cool core

Modelling giant radio halos Doctoral Thesis Award Lecture 2012

AbstractWe review models for giant radio halos in clusters of galaxies, with a focus on numerical and theoretical work. After summarising the most important observations of these objects, we present an introduction to the theoretical aspects of hadronic models. We compare these models with observations using simulations and find severe problems for hadronic models. We give a short introduction to reacceleration models and show results from the first simulation of CRe reacceleration in cluster mergers. We find that in‐line with previous theoretical work, reacceleration models are able to elegantly explain main observables of giant radio halos. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

MAGNETOHYDRODYNAMIC TURBULENCE AND COSMIC-RAY REACCELERATION IN GALAXY CLUSTERS

Cosmological MHD simulations of galaxy cluster formation show a significant amplification of seed magnetic fields. We developed a novel method to decompose cluster magnetized turbulence into modes and showed that the fraction of the fast mode is fairly large, around 1/4 in terms of energy. This is larger than that was estimated before, which implies that cluster turbulence interacts with cosmic rays rather efficiently. We propose a framework to deal with electron and proton reacceleration in galaxy clusters that includes feedback on turbulence. In particular, we establish a new upper limit on proton and electron fluxes based on turbulence intensity. These findings, along with detailed modeling of reacceleration, will help to reconcile the observed giant radio haloes and the unobserved diffuse gamma-ray emission from these clusters.

Challenges to our understanding of radio relics: X-ray observations of the Toothbrush cluster

The cluster 1RXS J0603.3+4214 is a merging galaxy cluster that hosts three radio relics and a giant radio halo. The northern relic, the Toothbrush, is 1.9-Mpc long and has an unusual linear morphology. According to simple diffusive shock acceleration theory, its radio spectral index indicates a Mach number of 3.3-4.6. Here, we present results from a deep XMM-Newton observation of the cluster. We observe two distinct cluster cores that have survived the merger. The presence of three shocks at or near the locations of the radio relics is confirmed by density and temperature discontinuities. However, the observation poses several puzzles that challenge our understanding of radio relics: (i) at the Toothbrush, the shock Mach number is not larger than 2, in apparent conflict with the shock strength predicted from the radio spectrum; (ii) at the Toothbrush, the shock front is, in part, spatially offset from the radio emission; (iii) at the eastern relic, we detect a temperature jump corresponding to a Mach number of approximately 2.5, but there is no associated surface brightness discontinuity. We discuss possible explanations for these findings.

Radio observations of Planck clusters

AbstractRecently, a number of new galaxy clusters have been detected by the ESA‐Planck satellite, the South Pole Telescope, and the Atacama Cosmology Telescope using the Sunyaev‐Zel'dovich effect. Several of the newly detected clusters are massive, merging systems with disturbed morphology in the X‐ray surface brightness. Diffuse radio sources in clusters, called giant radio halos and relics, are direct probes of cosmic rays and magnetic fields in the intra‐cluster medium. These radio sources are found to occur mainly in massive merging clusters. Thus, the new SZ‐discovered clusters are good candidates to search for new radio halos and relics. We have initiated radio observations of the clusters detected by Planck with the Giant Metrewave Radio Telescope. These observations have already led to the detection of a radio halo in PLCKG171.9–40.7, the first giant halo discovered in one of the new Planck clusters. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

DISCOVERY OF A GIANT RADIO HALO IN A NEWPLANCKGALAXY CLUSTER PLCKG171.9–40.7

We report the discovery of a giant radio halo in a new, hot, X-ray luminous galaxy cluster recently found by Planck, PLCKG171.9-40.7. The radio halo was found using Giant Metrewave Radio Telescope observations at 235 MHz and 610 MHz, and in the 1.4 GHz data from a NRAO Very Large Array Sky Survey pointing that we have reanalyzed. The diffuse radio emission is coincident with the cluster X-ray emission, has an extent of ~1 Mpc and a radio power of ~5x 10^24 W/Hz at 1.4 GHz. Its integrated radio spectrum has a slope of alpha~1.8 between 235 MHz and 1.4 GHz, steeper than that of a typical giant halo. The analysis of the archival XMM-Newton X-ray data shows that the cluster is hot (~10 keV) and disturbed, consistent with X-ray selected clusters hosting radio halos. This is the first giant radio halo discovered in one of the new clusters found by Planck.

XMM–Newton observations of the merging galaxy cluster CIZA J2242.8+5301

We studied the intracluster medium of the galaxy cluster CIZA J2242.8+5301 using deep XMM-Newton observations. The cluster hosts a remarkable 2-Mpc long, ~50-kpc wide radio relic that has been nicknamed the "Sausage". A smaller, more irregular counter-relic is also present, along with a faint giant radio halo. We analysed the distribution of the ICM physical properties, and searched for shocks by trying to identify density and temperature discontinuities. East of the southern relic, we find evidence of shock compression corresponding to a Mach number of 1.3, and speculate that the shock extends beyond the length of the radio structure. The ICM temperature increases at the northern relic. More puzzling, we find a "wall" of hot gas east of the cluster centre. A partial elliptical ring of hot plasma appears to be present around the merger. While radio observations and numerical simulations predict a simple merger geometry, the X-ray results point towards a more complex merger scenario.

Radio halos in future surveys in the radio continuum

Giant radio halos (RH) are Mpc-scale synchrotron sources detected in a significant fraction of massive and merging galaxy clusters.Their statistical properties can be used to discriminate among various models for their origin. Theoretical predictions are important as new radio telescopes are about to begin to survey the sky at low and high frequencies with unprecedented sensitivity. We carry out Monte Carlo simulations to model the formation and evolution of RH in a cosmological framework by assuming that RH are either generated in turbulent merging clusters, or are purely hadronic sources generated in more relaxed clusters, "off-state" halos. The models predict that the luminosity function of RH at high radio luminosities is dominated by the contribution of RH generated in turbulent clusters. The generation of these RH becomes less efficient in less massive systems causing a flattening of the luminosity function at lower luminosities. This flattening is compensated by the contribution of "off-state" RH that dominate at lower luminosities. By restricting to clusters at z<0.6, we show that the planned EMU+WODAN surveys at 1.4 GHz have the potential to detect up to ~200 RH, increasing their number by one order of magnitude. A fraction of these sources will be "off-state" RH that should be found at flux level < 10 mJy, presently accessible only to deep pointed observations. We also explore the synergy between the Tier 1 LOFAR survey at 150 MHz and the EMU+WODAN surveys at 1.4 GHz. We predict a larger number of RH in the LOFAR survey due to the high LOFAR sensitivity, but also due to the existence of RH with very steep spectrum that glow up preferentially at lower frequencies. These RH are only predicted in the framework of turbulent re-acceleration models and should not have counterparts in the EMU+WODAN surveys, thus the combination of the two surveys will test theoretical models.

Probing the origin of giant radio haloes through radio and γ-ray data: the case of the Coma cluster

We combine all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the gamma-ray upper limits obtained by the Fermi-LAT and with the magnetic field strength derived from Faraday rotation measures (RM). We explore the possibility that the radio halo is due to synchrotron emission of secondary electrons generated via p-p collisions in the intra-cluster-medium (ICM). First we investigate the case of pure secondary models. We use the observed spatial distribution of the halo's radio brightness to constrain the amount of cosmic rays (CRs) and their spatial distribution in the cluster that are required by the model. Under the canonical assumption that the spectrum of CRs is a power-law in momentum and that the spectrum of secondaries is stationary, we find that the combination of the steep spectrum of CRs necessary to explain the spectrum of the halo and their very broad spatial distribution (and large energy density) result in a gamma-ray emission in excess of present limits, unless the cluster magnetic field is sufficiently large. However such a field appears inconsistent with constraints from RM. Second we investigate more complex models based on secondary particles in which CR protons and their secondaries are all reaccelerated by MHD turbulence. We show that under these conditions it is possible to reproduce the radio data and to predict gamma-rays in agreement with the Fermi-LAT limits without tension with constraints on the cluster magnetic field. Reacceleration of secondaries by MHD turbulence also requires a spatial distribution of CRs much flatter than that of the ICM, if both the turbulent and magnetic field energy densities scale with that of the ICM. However broader spatial distributions of turbulence and field and/or the reacceleration of additional primary electrons in the ICM greatly alleviate this requirement.