L. Feretti

Clusters of galaxies: observational properties of the diffuse radio emission

Clusters of galaxies, as the largest virialized systems in the Universe, are ideal laboratories to study the formation and evolution of cosmic structures. The luminous matter of clusters consists of galaxies and of an embedding intracluster medium (ICM), which has been heated to temperatures of tens of millions degrees, and thus is detected through its thermal emission in the soft X-ray regime. Most of the detailed knowledge of galaxy clusters has been obtained in recent years from the study of ICM through X-ray Astronomy. At the same time, radio observations have proved that the ICM is mixed with non-thermal components, i.e. highly relativistic particles and large-scale magnetic fields, detected through their synchrotron emission.The knowledge of the properties of these non-thermal ICM components has increased significantly, owing to sensitive radio images and to the development of theoretical models. Diffuse synchrotron radio emission in the central and peripheral cluster regions has been found in many clusters. Moreover large-scale magnetic fields appear to be present in all galaxy clusters, as derived from Rotation Measure (RM) studies. Non-thermal components are linked to the cluster X-ray properties, and to the cluster evolutionary stage, and are crucial for a comprehensive physical description of the intracluster medium. They play an important role in the cluster formation and evolution.We review here the observational properties of diffuse non-thermal sources detected in galaxy clusters: halos, relics and mini-halos. We discuss their classification and properties. We report published results up to date and obtain and discuss statistical properties. We present the properties of large-scale magnetic fields in clusters and in even larger structures: filaments connecting galaxy clusters. We summarize the current models of the origin of these cluster components, and outline the improvements that are expected in this area from future developments thanks to the new generation of radio telescopes.

MAGNETIC FIELDS IN CLUSTERS OF GALAXIES

The existence of magnetic fields associated with the intracluster medium in clusters of galaxies is now well established through different methods of analysis. Magnetic fields are investigated in the radio band from studies of the rotation measure of polarized radio galaxies and the synchrotron emission of cluster-wide diffuse sources. Other techniques include X-ray studies of the inverse Compton emission and of cold fronts and magneto hydrodynamic simulations. We review the main issues that have led to our knowledge on magnetic fields in clusters of galaxies. Observations show that cluster fields are at the μG level, with values up to tens of μG at the center of cooling core clusters. Estimates obtained from different observational approaches may differ by about an order of magnitude. However, the discrepancy may be alleviated by considering that the magnetic field is not constant throughout the cluster, and shows a complex structure. In particular, the magnetic field intensity declines with the cluster ...

Radio halo and relic candidates from the NRAO VLA Sky Survey

Abstract We present the first results of the search of new halo and relic candidates in the NRAO VLA Sky Survey. We have inspected a sample of 205 clusters from the X-ray-brightest Abell-type clusters presented by Ebeling et al. (1996) , and found 29 candidates. Out of them, 11 clusters are already known from the literature to contain a diffuse cluster-wide source, while in 18 clusters this is the first indication of the existence of this type of sources. We classify these sources as halos or relics according to their location in the cluster center or periphery, respectively. We find that the occurrence of cluster halos and relics is higher in clusters with high X-ray luminosity and high temperature. We also confirm the correlation between the absence of a cooling flow and the presence of a radio halo at the cluster center.

VLBI Observations of a Complete Sample of Radio Galaxies: 10 Years Later

A complete sample of 27 radio galaxies was selected from the B2 and 3CR catalogs in order to study their properties on the milliarcsecond scale. In the Appendix of this paper we present new radio images for 12 of them. Thanks to the present data, all the sources in this sample have been imaged at milliarcsecond resolution. We discuss the general results. In particular we stress the evidence for high-velocity jets in low-power radio galaxies, compare high- and low-power sources, and discuss the source properties in light of the unified scheme models. We conclude that the properties of parsec-scale jets are similar in sources with different total radio power and kiloparsec-scale morphology. From the core-total radio power correlation, we estimate that relativistic jets with Lorentz factor ? in the range 3-10 are present in high- and low-power radio sources. We discuss also the possible existence of a two-velocity structure (fast spine and lower velocity external shear layer).

HARD X-RAY RADIATION IN THE COMA CLUSTER SPECTRUM

Hard X-ray radiation has been detected for the first time in the Coma Cluster by BeppoSAX. Thanks to the unprecedented sensitivity of the Phoswich Detection System (PDS) instrument, the source has been detected up to ~80 keV. There is clear evidence (4.5 σ) for nonthermal emission in excess of thermal emission above ~25 keV. The hard excess is very unlikely to be the result of X Comae, the Seyfert 1 galaxy that is present in the field of view of the PDS. A hard spectral tail that is due to inverse Compton scattering on cosmic microwave background photons is predicted in clusters, like Coma, with radio halos. Combining the present results with radio observations, a volume-averaged intracluster magnetic field of ~0.15 μG is derived, while the electron energy density of the emitting electrons is ~7×10−14 ergs cm-3.

The Coma cluster magnetic field from Faraday rotation measures

Aims. The aim of the present work is to constrain the Coma cluster magnetic field strength, its radial profile and power spectrum by comparing Faraday rotation measure (RM) images with numerical simulations of the magnetic field. Methods. We have analyzed polarization data for seven radio sources in the Coma cluster field observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday rotation measures with kiloparsec scale resolution. Random three dimensional magnetic field models have been simulated for various values of the central intensity B0 and radial power-law slope η ,w hereη indicates how the field scales with respect to the gas density profile. Results. We derive the central magnetic field strength, and radial profile values that best reproduce the RM observations. We find that the magnetic field power spectrum is well represented by a Kolmogorov power spectrum with minimum scale ∼2 kpc and maximum scale ∼34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B0 = 3.9 μG; η = 0.4) and (B0 = 5.4 μG; η = 0.7) within 1σ. The best agreement between observations and simulations is achieved for B0 = 4.7 μG; η = 0.5. Values of B0 > 7 μ Ga nd 1.0 are incompatible with RM data at 99% confidence level.

Parsec-Scale Properties of Markarian 501

We present the results of a high angular resolution study of the BL Lac object Markarian 501 in the radio band. We consider data taken at 14 different epochs, ranging between 1.6 and 22 GHz in frequency, and including new Space VLBI observations obtained on 2001 March 5 and 6 at 1.6 and 5 GHz. We study the kinematics of the parsec-scale jet and estimate its bulk velocity and orientation with respect to the line of sight. Limb-brightened structure in the jet is clearly visible in our data, and we discuss its possible origin in terms of velocity gradients in the jet. Quasi-simultaneous, multiwavelength observations allow us to map the spectral index distribution and to compare it to the jet morphology. Finally, we estimate the physical parameters of the parsec-scale jet.

Radio and X-ray diffuse emission in six clusters of galaxies

Deep Very Large Array radio observations confirm the presence of halo and relic sources in six clusters of galaxies (A115, A520, A773, A1664, A2254, A2744) where a wide diffuse emission was previously found in the NRAO VLA Sky Survey. New images at 1.4 GHz of these six clusters of galaxies are presented and X-ray data obtained from the ROSAT archive are analyzed. The properties of clusters hosting radio halos and relics are analyzed and discussed. A correlation between the halo radio power and the cluster gravitational mass is presented.

Chandra temperature maps for galaxy clusters with radio halos

We analyze Chandra temperature maps for a sample of clusters with high-quality radio halo data to study the origin of the radio halos. The sample includes A520, A665, A754, A773, A1914, A2163, A2218, A2319, and 1E 0657-56. We present new temperature maps for all but two of them (A520 and A754). All these clusters exhibit distorted X-ray morphology and strong gas temperature variations indicating ongoing mergers. Some clusters, e.g., A520, A665, 1E 0657-56, exhibit the previously reported spatial correlation between the radio halo brightness and the hot gas regions. However, it is not a general feature. While most mergers are too messy to allow us to disentangle the projection effects, we find clear counterexamples (e.g., A754 and A773) in which the hottest gas regions do not exhibit radio emission at the present sensitivity level. This cannot be explained by projection effects and therefore argues against merger shocks?at least those relatively weak ones responsible for the observed temperature structure in most clusters?as the main mechanism for the halo generation. This leaves merger-generated turbulence as a more likely mechanism. The two clusters with the clearest radio brightness-temperature correlation, A520 and 1E 0657-56, are both mergers in which a small dense subcluster has just passed through the main cluster, very likely generating turbulence in its wake. The maximum radio brightness and the hot gas are both seen in these wake regions. On the other hand, the halos in 1E 0657-56 and A665 (both high-velocity mergers) extend into the shock regions in front of the subclusters, where no strong turbulence is expected. Thus, in high-velocity (M 2-3) mergers, both shock and turbulence acceleration mechanisms may be significant.

Magnetic fields and Faraday rotation in clusters of galaxies

We present a numerical approach to investigate the relationship between magnetic fields and Faraday rotation effects in clusters of galaxies. We can infer the structure and strength of intra-cluster magnetic fields by comparing our simulations with the observed polarization properties of extended cluster radio sources such as radio galaxies and halos. We find the observations require a magnetic field which fluctuates over a wide range of spatial scales (at least one order of magnitude). If several polarized radio sources are located at different projected positions in a galaxy cluster, as is the case for A119, detailed Faraday rotation images allow us to constrain both the magnetic field strength and the slope of the power spectrum. Our results show that the standard analytic expressions applied in the literature overestimate the cluster magnetic field strengths by a factor of ∼2. We investigate the possible effects of our models on beam depolarization of radio sources whose radiation traverses the magnetized intracluster medium. Finally, we point out that radio halos may provide important information about the spatial power spectrum of the magnetic field fluctuations on large scales. In particular, different values of the index of the power spectrum produce very different total intensity and polarization brightness distributions.