Four extreme relic radio sources in clusters of galaxies

We describe the results of the highest-resolution radio observations yet ma de of four relic radio sources in the Abell clusters A13, A85, A133, and A4 038. Our Very Large Array images at 1.4 GHz with 4" resolution and a noise level of 1 sigma similar to 20 mu Jy beam(-1) (1.1 K) show a remarkable var iety of fine structure in the form of spectacular arcs, wisps, plumes, and loops. Their integrated radio flux densities fall very rapidly with frequen cy, with power-law slopes, alpha, between 2.1 and 4.4 near 1.4 GHz [where S -v proportional to (v/v(o))(-alpha)]. The relics possess linear polarizatio n 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 optic al identification of a host galaxy formerly associated with a relic remains problematic. In A85, A133, and A4038 the travel times for the brightest cl uster galaxies are significantly longer than the modeled ages of the relics ; there is always at least one nearby relatively bright elliptical galaxy t hat 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 exce llent fits to the broadband radio spectra using the anisotropic model of sp ectral aging, and we have extended the model to include diffusion of partic les between regions of different field strength. The steep radio spectra im ply ages for the relics of similar to 10(8) yr, at the start of which perio d their radio luminosities would have been similar to 10(25) 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 surro unding intracluster gas, which has probably limited their free expansion an d prevented them from fading by adiabatic cooling.