Wideband very large array observations of A2256. I. Continuum, rotation measure, and spectral imaging

Abstract

We report new observations of A2256 with the Karl G. Jansky Very Large Array (VLA) at frequencies between 1 and 8 GHz. These observations take advantage of the 2:1 bandwidths available during a single observation to study the spectral index, polarization, and rotation measure as well as using the associated higher sensitivity per unit time to image total intensity features down to ∼0.″5 resolution. We find that the Large Relic, which dominates the cluster, is made up of a complex of filaments that show correlated distributions in intensity, spectral index, and fractional polarization. The rotation measure varies across the face of the Large Relic but is not well correlated with the other properties of the source. The shape of individual filaments suggests that the Large Relic is at least 25 kpc thick. We detect a low surface brightness arc connecting the Large Relic to the Halo and other radio structures, suggesting a physical connection between these features. The center of the F-complex is dominated by a very steep-spectrum, polarized, ring-like structure, F2, without an obvious optical identification, but the entire F-complex does have interesting morphological similarities to the radio structure of NGC 1265. Source C, the Long Tail, is unresolved in width near the galaxy core and is ≲ 100 pc in diameter there. This morphology suggests either that C is a one-sided jet or that the bending of the tails takes place very near the core, consistent with the parent galaxy having undergone extreme stripping. Overall it seems that many of the unusual phenomena can be understood in the context of A2256 being near the pericenter of a slightly off-axis merger between a cluster and a smaller group. Given the lack of evidence for a strong shock associated with the Large Relic, other models should be considered, such as reconnection between two large-scale magnetic domains.