Four Extreme Relic Radio Sources in Clusters of Galaxies

We describe the results of the highest-resolution radio observations yet made of four relic radio sources in the Abell clusters A13, A85, A133, and A4038. Our Very Large Array images at 1.4 GHz with 4″ resolution and a noise level of 1 σ ∼ 20 μJy beam-1 (1.1 K) show a remarkable variety of fine structure in the form of spectacular arcs, wisps, plumes, and loops. Their integrated radio flux densities fall very rapidly with frequency, with power-law slopes, α, between 2.1 and 4.4 near 1.4 GHz [where Sν ∝ (ν/ν 0)-α]. The relics possess linear polarization levels ranging between 2.3% (A133) and 35% (A85); the higher polarization fractions imply a highly ordered magnetic field in the fine structure and low differential Faraday rotation in the intervening cluster gas. The optical identification of a host galaxy formerly associated with a relic remains problematic. In A85, A133, and A4038 the travel times for the brightest cluster galaxies are significantly longer than the modeled ages of the relics; there is always at least one nearby relatively bright elliptical galaxy that provides a better match. Excess X-ray emission in the 0.5 to 2 keV band was found near the relics in A85 and A133. The surface brightness was too high to be attributed to the inverse Compton mechanism alone. We found excellent fits to the broadband radio spectra using the anisotropic model of spectral aging, and we have extended the model to include diffusion of particles between regions of different field strength. The steep radio spectra imply ages for the relics of ∼108 yr, at the start of which period their radio luminosities would have been ∼1025 W Hz-1 at 1.4 GHz, and so their progenitors were on the boundary between FR I and FR II radio galaxies and hence among the most luminous 7% of radio galaxies. We find that the relics are in approximate pressure equilibrium with the surrounding intracluster gas, which has probably limited their free expansion and prevented them from fading by adiabatic cooling.