Discovery of a high-redshift Einstein ring

We report the discovery of a partial Einstein ring of radius 1. 48 produced by a massive (and seemingly isolated) elliptical galaxy. The spectroscopic follow-up at the VLT reveals a 2L galaxy at z = 0.986, which is lensing a post-starburst galaxy at z = 3.773. This unique configuration yields a very precise measure of the mass of the lens within the Einstein radius, (8.3 0.4) 10 11 h -1 70 M . The fundamental plane relation indicates an evolution rate of dlog(M/L) B /dz = -0.57 0.04, similar to other massive ellipticals at this redshift. The source galaxy shows strong interstellar absorption lines indicative of large gasphase metallicities, with fading stellar populations after a burst. Higher resolution spectra and imaging will allow the detailed study of an unbiased representative of the galaxy population when the universe was just 12% of its current age.