The Fate of Intracluster Radio Plasma

Radio plasma injected by active radio galaxies into clusters of galaxies quickly becomes invisible due to radiative losses of the relativistic electrons. In this talk, the fate of radio plasma and its role for the galaxy cluster is discussed: buoyancy removes it from the central regions and allows to transfer its energy into the ambient gas. The remaining low energy electron populations are still able to emit a low luminosity glow of observable radiation via synchrotron-self Comptonized emission. Shock waves in the ambient gas can re-ignite the radio emission.Comment: Invited Talk at `Matter and Energy in Clusters of Galaxies', Taipei 2002, 8 pages, 6 figures, uses newpasp.sty (includued

Inference with minimal Gibbs free energy in information field theory

Non-linear and non-Gaussian signal inference problems are difficult to tackle. Renormalization techniques permit us to construct good estimators for the posterior signal mean within information field theory (IFT), but the approximations and assumptions made are not very obvious. Here we introduce the simple concept of minimal Gibbs free energy to IFT, and show that previous renormalization results emerge naturally. They can be understood as being the Gaussian approximation to the full posterior probability, which has maximal cross information with it. We derive optimized estimators for three applications, to illustrate the usage of the framework: (i) reconstruction of a log-normal signal from Poissonian data with background counts and point spread function, as it is needed for gamma ray astronomy and for cosmography using photometric galaxy redshifts, (ii) inference of a Gaussian signal with unknown spectrum and (iii) inference of a Poissonian log-normal signal with unknown spectrum, the combination of (i) and (ii). Finally we explain how Gaussian knowledge states constructed by the minimal Gibbs free energy principle at different temperatures can be combined into a more accurate surrogate of the non-Gaussian posterior.Comment: 14 page

On the Formation of Cluster Radio Relics

(abridged) We present detailed 3-dimensional magneto-hydrodynamical simulations of the passage of a radio plasma cocoon filled with turbulent magnetic fields through a shock wave. Taking into account synchrotron, inverse Compton and adiabatic energy losses and gains we evolved the relativistic electron population to produce synthetic polarisation radio maps. On contact with the shock wave the radio cocoons are first compressed and finally torn into filamentary structures, as is observed in several cluster radio relics. In the synthetic radio maps the electric polarisation vectors are mostly perpendicular to the filamentary radio structures. If the magnetic field inside the cocoon is not too strong, the initially spherical radio cocoon is transformed into a torus after the passage of the shock wave. Very recent, high-resolution radio maps of cluster radio relics seem to exhibit such toroidal geometries in some cases. This supports the hypothesis that cluster radio relics are fossil radio cocoons that have been revived by a shock wave. For a late-stage relic the ratio of its global diameter to the filament diameter should correlate with the shock strength. Finally, we argue that the total radio polarisation of radio relic should be well correlated with the 3-dimensional orientation of the shock wave that produced the relic.Comment: accepted by MNRAS, 10 pages, 13 figures, some modifications due to comments of a refere

The Radio Luminosity Function of Cluster Radio Halos

A significant fraction of galaxy clusters exhibits cluster wide radio halos. We give a simple prediction of the local and higher redshift radio halo luminosity function (RHLF) on the basis of (i) an observed and a theoretical X-ray cluster luminosity function (XCLF) (ii) the observed radio--X-ray luminosity correlation (RXLC) of galaxy clusters with radio halos (iii) an assumed fraction of 1/3 galaxy clusters to have radio halos as supported by observations. We then find 300-700 radio halos with S_1.4GHz > 1 mJy, and 10^5 - 10^6 radio halos with S_1.4GHz > 1 muJy should be visible on the sky. 14% of the S_1.4GHz > 1 mJy and 56% of the S_1.4GHz > 1 muJy halos are located at z>0.3. Subsequently, we give more realistic predictions taking into account (iv) a refined estimate of the radio halo fraction as a function of redshift and cluster mass, and (v) a decrease in intrinsic radio halo luminosity with redshift due to increased inverse Compton electron energy losses on the Cosmic Microwave Background (CMB). We find that this reduces the radio halo counts from the simple prediction by only 30 % totally, but the high redshift (z>0.3) counts are more strongly reduced by 50-70%. These calculations show that the new generation of sensitive radio telescopes like LOFAR, ATA, EVLA, SKA and the already operating GMRT should be able to detect large numbers of radio halos and will provide unique information for studies of galaxy cluster merger rates and associated non-thermal processes.Comment: Accepted by A&A, 8 pages, 7 figure