Hubble Space Telescope Observations of Gravitationally Lensed Features in the Rich Cluster AC 114

Abstract

Deep Hubble Space Telescope (HST) images of superlative resolution obtained for the distant rich cluster AC 114 (z = 0.31) reveal a variety of gravitational lensing phenomena for which ground-based spectroscopy is available. We present a luminous arc which is clearly resolved by HST and appears to be a lensed z = 0.64 sub-L^* spiral galaxy with a detected rotation curve. Of greatest interest is a remarkably symmetrical pair of compact blue images separated by to" and lying close to the cluster cD. We propose that these images arise from a single very faint background source gravitationally lensed by the cluster core. Deep ground-based spectroscopy confirms the lensing hypothesis and suggests that the source is a compact star-forming system at a redshift z = 1.86. Taking advantage of the resolved structure around each image and their very blue colors, we have identified a candidate third image of the same source ∼50" away. The angular separation of the three images is much larger than previous multiply imaged systems and indicates a deep gravitational potential in the cluster center. Resolved multiply imaged systems, readily recognized with HST, promise to provide unique constraints on the mass distribution in the cores of intermediate-redshift clusters.