Discovery of a high-redshift Einstein ring

We report the discovery of a partial Einstein ring of radius 1$\farcs$48 produced by a massive (and seemingly isolated) elliptical galaxy. The spectroscopic follow-up at the VLT reveals a 2$L_\star$ 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 \pm 0.4) \times 10^{11}$$h_{70}^{-1}$$M_{\odot}$ . The fundamental plane relation indicates an evolution rate of ${\rm d}$$\log (M/L)_{B} / {\rm d}z = -0.57\pm0.04$, similar to other massive ellipticals at this redshift. The source galaxy shows strong interstellar absorption lines indicative of large gas-phase 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.