2005MNRAS.361..259Y

We report on the discovery of the new gravitational lens system CLASS B0631+519. Imaging with the Very Large Array, the Multi-Element Radio-Linked Interferometer Network (MERLIN) and the Very Long Baseline Array (VLBA) reveals a doubly imaged flat-spectrum radio core, a doubly imaged steep-spectrum radio lobe and possible quadruply imaged emission from a second lobe. The maximum separation between the lensed images is 1.16 arcsec. High-resolution mapping with the VLBA at 5 GHz resolves the most magnified image of the radio core into a number of subcomponents spread across approximately 20 mas. No emission from the lensing galaxy or an odd image is detected down to 0.31 mJy (5σ) at 8.4 GHz. Optical and near-infrared imaging with the Advanced Camera for Surveys and the Near-Infrared Camera and Multi-Object Spectrometer on the Hubble Space Telescope (HST) show that there are two galaxies along the line of sight to the lensed source, as previously discovered by optical spectroscopy. We find that the foreground galaxy at z= 0.0896 is a small irregular, and that the other, at z= 0.6196 is a massive elliptical, which appears to contribute the majority of the lensing effect. The host galaxy of the lensed source is detected in the HST near-infrared imaging as a set of arcs, which form a nearly complete Einstein ring. Mass modelling using non-parametric techniques can reproduce the near-infrared observations and indicates that the small irregular galaxy has a (localized) effect on the flux density distribution in the Einstein ring at the 5-10 per cent level.