F. Govoni

BOW SHOCK AND RADIO HALO IN THE MERGING CLUSTER A520

Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M = 2.1. This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structure related to the shock, where the latter may provide preenergized electrons for subsequent turbulent reacceleration. The radio edge may be caused by electron acceleration by the shock. If so, the synchrotron spectrum should have a slope of α 1.2 right behind the edge, with quick steepening farther away from the edge. Alternatively, if shocks are inefficient accelerators, the radio edge may be explained by an increase in the magnetic field and density of preexisting relativistic electrons due to gas compression. In the latter model, there should be radio emission in front of the shock with the same spectrum as that behind it, but 10-20 times fainter. If future sensitive radio measurements do not find such preshock emission, then the electrons are indeed accelerated (or reaccelerated) by the shock, and one will be able to determine its acceleration efficiency. We also propose a method to estimate the magnetic field strength behind the shock, based on measuring the dependence of the radio spectral slope upon the distance from the shock. In addition, the radio edge provides a way to constrain the diffusion speed of the relativistic electrons.

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.

Deep images of cluster radio halos

New radio data are presented for the clusters A401, A545, A754, A1914, A2219 and A2390, where the presence of diuse radio emission was suggested from the images of the NRAO VLA Sky Survey. Sensitive images of these clusters, obtained with the Very Large Array (VLA) at 20 cm confirm the existence of the diuse sources and allow us to derive their fluxes and intrinsic parameters. The correlation between the halo radio power and cluster X-ray luminosity is derived for a large sample of halo clusters, and is briefly discussed.New radio data are presented for the clusters A401, A545, A754, A1914, A2219 and A2390, where the presence of diffuse radio emission was suggested from the images of the NRAO VLA Sky Survey. Sensitive images of these clusters, obtained with the Very Large Array (VLA)at 20 cm confirm the existence of the diffuse sources and allow us to derive their fluxes and intrinsic parameters.The correlation between the halo radio power and cluster X-ray luminosity is derived for a large sample of halo clusters, and is briefly discussed.

A comparison of radio and X-ray morphologies of four clusters of galaxies containing radio halos

Clusters of galaxies may contain cluster-wide, centrally located, diuse radio sources, called halos. They have been found to show morphologies similar to those of the X-ray emission. To quantify this qualitative statement we performed a point-to-point comparison of the radio and the X-ray emission for four clusters of galaxies containing radio halos: Coma, Abell 2255, Abell 2319, Abell 2744. Our study leads to a linear relation between the radio and the X-ray surface brightness in two clusters, namely Abell 2255 and Abell 2744. In Coma and A2319 the radio and the X-ray brightnesses seem to be related with a sub-linear power law. Implications of these ndings within simple radio halo formation models are briefly discussed.

The black hole mass of low redshift radiogalaxies

We make use of two empirical relations between the black hole mass and the global properties (bulge luminosity and stellar velocity dispersion) of nearby elliptical galaxies, to infer the mass of the central black hole (MBH) in low redshift radiogalaxies. Using the most recent determinations of black hole masses for inactive early type galaxies we show that the bulge luminosity and the central velocity dispersion are almost equally correlated (similar scatter) with the central black-hole mass. Applying these relations to two large and homogeneous datasets of radiogalaxies we find that they host black-holes whose mass ranges between � 5×10 7 to � 6×10 9 M⊙ (average �8.9). MBH is found to be proportional to the mass of the bulge (Mbulge). The distribution of the ratio MBH/Mbulge has a mean value of 8×10 −4 and shows a scatter that is consistent with that expected from the associated errors. At variance with previous claims no significant correlation is instead found between MBH (or Mbulge) and the radio power at 5 GHz.

The giant radio halo in Abell 2163

New radio data is presented for the rich cluster Abell 2163. The cluster radio emission is characterized by the presence of a radio halo, which is one of the most powerful and extended halos known so far. In the NE peripheral cluster region, we also detect diuse elongated emission, which we classify as a cluster relic. The cluster A2163 is very hot and luminous in X-ray. Its central region is probably in a highly non relaxed state, suggesting that this cluster is likely to be a recent merger. The existence of a radio halo in this cluster conrms that halos are associated with hot massive clusters, and conrms the connection between radio halos and cluster merger processes. The comparison between the radio emission of the halo and the cluster X-ray emission shows a close structural similarity. A power law correlation is found between the radio and X-ray brightness, with index =0.64. We also report the upper limit to the hard X-ray emission, obtained from a BeppoSAX observation. We discuss the implications of our results.

Correlation of the magnetic field and the intra-cluster gas density in galaxy clusters

We present a correlation between X-ray surface brightness and Faraday rotation measure in galaxy clusters, both, from radio and X-ray observations as well as from modeling of the intra-cluster medium. The ob- served correlation rules out a magnetic eld of constant strength throughout the cluster. Cosmological, magneto- hydrodynamic simulations of galaxy clusters are used to show that for a magnetic eld of cosmic origin this correlation is expected and excellently reproduces the observations showing that the rms scatter of the Faraday rotation increases linearly with the X-ray surface brightness. From the correlation between the observable quanti- ties, rotation measure and X-ray surface brightness, we infer a relation between the physical quantities: magnetic eld and gas density. For the best available observations, those of A119, we nd B/ n 0:9 e .

The fundamental plane of radio galaxies

We collected photometrical and dynamical data for 73 low red-shift ( z< 0:2) Radio Galaxies (LzRG) in order to study their Fundamental Plane (FP). For 22 sources we also present new velocity dispersion data that complement the photometric data given in our previous study of LzRG (Govoni et al. 2000a). It is found that the FP of LzRG is similar to the one dened by non-active elliptical galaxies, with LzRG representing the brightest end of the population of early type galaxies. Since the FP mainly reflects the virial equilibrium condition, our result implies that the global properties of early{type galaxies (dening the FP) are not influenced by the presence of gas accretion in the central black hole. This is fully in agreement with the recent results in black hole demography, showing that virtually all luminous spheroidal galaxies host a massive black hole and therefore may potentially become active. We conrm and extend to giant ellipticals the systematic increase of the mass-to-light ratio with galaxy luminosity.