New scaling relations in cluster radio haloes and the re-acceleration model (original) (raw)

New scaling relations in cluster radio halos and the re-acceleration model

Monthly Notices of The Royal Astronomical Society, 2007

In this paper we derive new expected scaling relations for clusters with giant radio halos in the framework of the re-acceleration scenario in a simplified, but physically motivated, form, namely: radio power (P R ) vs size of the radio emitting region (R H ), and P R vs total cluster mass (M H ) contained in the emitting region and cluster velocity dispersion (σ H ) in this region.

On the evolution of giant radio halos and their connection with cluster mergers

Astronomy & Astrophysics, 2009

Giant radio halos are diffuse, Mpc-scale, synchrotron sources located in the central regions of galaxy clusters and provide the most relevant example of cluster non-thermal activity. Radio and X-ray surveys allow to investigate the statistics of halos and may contribute to constrain their origin and evolution. We investigate the distribution of clusters in the plane X-ray (thermal, L_X) vs synchrotron (P_{1.4})luminosity, where clusters hosting giant radio halos trace the P_{1.4}--L_X correlation and clusters without radio halos populate a region that is well separated from that spanned by the above correlation. The connection between radio halos and cluster mergers suggests that the cluster Mpc-scale synchrotron emission is amplified during these mergers and then suppressed when clusters become more dynamically relaxed. In this context, by analysing the distribution in the P_{1.4}--L_X plane of clusters from X-ray selected samples with adequate radio follow up, we constrain the typical time-scale of evolution of diffuse radio emission in clusters and discuss the implications for the origin of radio halos. We conclude that cluster synchrotron emission is suppressed (and amplified) in a time-scale significantly smaller than 1 Gyr. We show that this constraint appears difficult to reconcile with the hypothesis that the halo's radio power is suppressed due to dissipation of magnetic field in galaxy clusters. On the other hand, in agreement with models where turbulent acceleration plays a role, present constraints suggest that relativistic electrons are accelerated in Mpc-scale regions, in connection with cluster mergers and for a time-interval of about 1 Gyr, and then they cool in a relatively small time-scale, when the hosting cluster becomes more dynamically relaxed.

On the Connection Between Giant Radio Halos and Cluster Mergers

Astrophysical Journal, 2010

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<z< 0.32. Using several methods to characterize cluster substructures, namely the power ratios, centroid shift and X-ray brightness concentration parameter, we show that clusters with and without radio halo can be quantitatively differentiated in terms of their dynamical properties. In particular, we confirm that radio halos are associated to dynamically disturbed clusters and cluster without radio halo are more "relaxed", with only a couple of exceptions where a disturbed cluster does not exhibit a halo.

A morphological comparison between giant radio halos and radio mini-halos in galaxy clusters

Astronomy & Astrophysics, 2008

In this letter we present a morphological comparison between giant radio halos and radio mini-halos in galaxy clusters based on radio--X-ray luminosity, P_{1.4}-L_X, and radio luminosity-size, P_{1.4}-R_H, correlations. We report evidence that P_{1.4}-L_X and P_{1.4}-R_H trends may also exist for mini--halos: mini--halo clusters share the same region of giant halo clusters in the (P_{1.4},L_X) plane, whereas they are clearly separated in the (P_{1.4},R_H) plane. The synchrotron emissivity of mini-halos is found to be more than 50 times larger than that of giant halos, implying a very efficient process for their origins. By assuming a scenario of sporadical turbulent particle re-acceleration for both giant and mini halos, we discuss basic physical differences between these sources. Regardless of the origin of the turbulence, a more efficient source of injection of particles, which eventually takes part in the re-acceleration process, is required in mini-halos, and this may result from the central radio galaxy or from proton-proton collisions in the dense cool core regions.

Constraining B in galaxy clusters from statistics of giant radio halos

Astronomische Nachrichten, 2006

There are several possibilities to constrain the value of the magnetic field in the ICM, the most direct ones being the combination of inverse Compton and synchrotron observations, and the Faraday rotation measures. Here we discuss on the possibility to provide constraints on the magnetic field in the ICM from the analysis of the statistical properties of the giant radio halos, Mpc--scale diffuse radio emission in galaxy clusters. Present observations of a few well studied radio halos can be interpreted under the hypothesis that the emitting relativistic electrons are re-accelerated on their way out. By using statistical calculations carried out in the framework of the re-acceleration model we show that the observed radio--power vs cluster mass correlation in radio halos can be reproduced only by assuming mu\mumuG fields in the ICM and a scaling of the magnetic field with cluster mass BproptoMvbB \propto M_v^bBproptoMvb, with bgeq0.6b \geq 0.6bgeq0.6. We also show that the expected occurrence of radio halos with mass and redshift, and their number counts are sensitive to the magnetic field intensity in massive galaxy clusters and to the scaling of B with cluster mass. Thus future deep surveys of radio halos would provide constraints on B in galaxy clusters.

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

Monthly Notices of the Royal Astronomical Society, 2012

We combine for the first time all available information about the spectral shape and morphology of the radio halo of the Coma cluster with the recent γ-ray upper limits obtained by the Fermi-LAT and with the magnetic field strength derived from Faraday rotation measures. We explore the possibility that the radio emission is due to synchrotron emission of secondary electrons. First we investigate the case of pure secondary models that are merely based on the mechanism of continuous injection of secondary electrons via proton-proton collisions in the intra-cluster medium. We use the observed spatial distribution of the halo's radio brightness to constrain the amount of cosmic ray protons and their spatial distribution in the cluster that are required by the model. Under the canonical assumption that the spectrum of cosmic rays is a power-law in momentum and that the spectrum of secondaries is stationary, we find that the combination of the steep spectrum of cosmic ray protons necessary to explain the spectrum of the halo and the very broad spatial distribution (and large energy density) of cosmic rays result in a γ-ray emission in excess of present limits, unless the cluster magnetic field is relatively large. However this large magnetic field required to not violate present γ-ray limits appears inconsistent with that derived from recent Faraday rotation measures. Second we investigate more complex models in which the cosmic rays confined diffusively in the Coma cluster and their secondary electrons are all reaccelerated by MHD turbulence. We show that under these conditions it is possible to explain the radio spectrum and morphology of the radio halo and to predict γ-ray fluxes in agreement with the Fermi-LAT upper limits without tension with present constraints on the cluster magnetic field. Reacceleration of secondary particles also requires a very broad cosmic ray spatial profile, much flatter than that of the intracluster medium, at least provided that both the turbulent and magnetic field energy densities scale with that of the intracluster medium. However, this requirement can be easily alleviated if we assume that a small amount of (additional) seed primary electrons are reaccelerated in the cluster's external regions, or if we adopt flatter scalings of the turbulent and magnetic field energy densities with distance from the cluster center.

Statistics of giant radio haloes from electron reacceleration models

Monthly Notices of The Royal Astronomical Society, 2006

The most important evidence of non-thermal phenomena in galaxy clusters comes from giant radio haloes (GRHs), spectacular synchrotron radio sources extended over ≥Mpc scales, detected in the central regions of a growing number of massive galaxy clusters. A promising possibility to explain these sources is given by in situ stochastic reacceleration of relativistic electrons by turbulence generated in the cluster volume during merger events. Cassano and Brunetti have recently shown that the expected fraction of clusters with radio haloes and the increase of such a fraction with cluster mass can be reconciled with present observations provided that a fraction of 20–30 per cent of the turbulence in clusters is in the form of compressible modes.In this work, we extend the above-mentioned analysis by including a scaling of the magnetic field strength with cluster mass. We show that, in the framework of the reacceleration model, the observed correlations between the synchrotron radio power of a sample of 17 GRHs and the X-ray properties of the hosting clusters are consistent with, and actually predicted by a magnetic field dependence on the virial mass of the form B∝Mbv, with b≳ 0.5 and typical μG strengths of the average B intensity. The occurrence of GRHs as a function of both cluster mass and redshift is obtained: the evolution of such a probability depends on the interplay between synchrotron and inverse Compton losses in the emitting volume, and it is maximized in clusters for which the two losses are comparable.The most relevant findings are that the predicted luminosity functions of GRHs are peaked around a power P1.4 GHz∼ 1024 W Hz−1, and severely cut off at low radio powers due to the decrease of the electron reacceleration in smaller galaxy clusters, and that the occurrence of GRHs at 1.4 GHz beyond a redshift z∼ 0.7 appears to be negligible. As a related check, we also show that the predicted integral radio source counts within a limited volume (z≤ 0.2) are consistent with present observational constraints. Extending the source counts beyond z= 0.2, we estimate that the total number of GRHs to be discovered at ∼ mJy radio fluxes could be ∼100 at 1.4 GHz. Finally, the occurrence of GRHs and their number counts at 150 MHz are estimated in view of the forthcoming operation of low-frequency observatories (LOFAR, LWA) and compared with those at higher radio frequencies.

Merging clusters and the formation of radio haloes

Computer Physics Communications, 2005

In this paper we present the preliminary result obtained from a set of cosmological numerical simulations run with the goal of describing self-consistently the evolution of the gas diffused in clusters of galaxies (Intra-Cluster Medium-ICM), including hydrodynamical and gravitational processes, in a cosmological framework. We have followed the details of the physical evolution of the ICM with particular attention to shock processes and merging events and their possible feedbacks the relativistic plasma. Furthermore, we have analyzed the details of the ICM dynamics in the most massive clusters, using tracers particles which follows the fluid in its motion.

Radio halos in a mass-selected sample of 75 galaxy clusters

Astronomy & Astrophysics

Context. 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. Aims. 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. Methods. We selected 75 clusters with M ≥ 6 × 1014 M⊙ 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 and/or the Jansky Very Large Array 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. Results. This observational campaign led to the detection of several cluster-scale diffuse rad...

Diffuse Cluster-wide Radio Halos

We will discuss the properties and origins of halos and relics including estimates of the cluster magnetic fields, and present results for a few recently discovered halos and relics. The electrons in the suprathermal high energy tail of the thermal gas distribution are likely to provide the seed particles for acceleration through mergers and turbulences to relativistic energies. These relativistic particles are then responsible for the synchrotron emission of the halos.