Search for Radio Arcs in Aco Clusters

Gravitational lensing (GL) of distant radio sources by galaxy clusters should produce radio arc(let)s. We extracted radio sources from the FIRST survey near Abell cluster cores and found their radio position angles to be uniformly distributed with respect to the cluster centres. This result holds even when we restrict the sample to the richest or most centrally condensed clusters, and to sources with high S/N and large axial ratio. Our failure to detect GL with statistical methods may be due to poor cluster centre positions. We did not find convincing candidates for arcs either. Our result agrees with theoretical estimates predicting that surveys much deeper than FIRST are required to detect the effect. This is in apparent conflict with the detection of such an effect claimed by Bagchi & Kapahi (1995). The first gravitational lens (GL) was found upon identification of the " double " quasar 0957+561 [14]. Later, the first lensed images of extended optical objects were discovered as the " giant arcs " [11], and since then CCD imaging of lensed galaxies behind galaxy clusters has turned into an " industry ". This allowed both mapping of the clusters' gravitational potential as well as unprecedented insight into high-z galaxies, due to the clusters acting as natural gravitational " telescopes " [13]. Up to now, no radio arcs in clusters have yet been identified, although radio searches have yielded more than half of all confirmed cases of " strong lensing " [6]. Compared to optical galaxies, radio sources are rare (their sky densities are comparable for S 1.5 ∼0.1 mJy and B J ∼24.0 mag), and they are more extended and complex, making it difficult to distinguish lensed from unlensed sources. We have the highest chance to see radio arcs in massive, centrally condensed clusters at z∼0.15–0.6. These tend to be very rich, X-ray luminous, and of mor