We identify a strong lensing galaxy in the cluster IRC 0218 (also known as XMM-LSS J02182$-$05102) that is spectroscopically confirmed to be at $z=1.62$, making it the highest-redshift strong lens galaxy known. The lens is one of the two brightest cluster galaxies and lenses a background source galaxy into an arc and a counterimage. With Hubble Space Telescope (HST) grism and Keck/LRIS spectroscopy, we measure the source redshift to be $z_{rm S}=2.26$. Using HST imaging in ACS/F475W, ACS/F814W, WFC3/F125W, and WFC3/F160W, we model the lens mass distribution with an elliptical power-law profile and account for the effects of the cluster halo and nearby galaxies. The Einstein radius is $theta_{rm E}=0.38^{+0.02}_{-0.01}$ ($3.2_{-0.1}^{+0.2}$ kpc) and the total enclosed mass is M$_{rm tot} (< theta_{rm E})=1.8^{+0.2}_{-0.1}times10^{11}~{rm M}_{odot}$. We estimate that the cluster environment contributes $sim10$% of this total mass. Assuming a Chabrier IMF, the dark matter fraction within $theta_{{rm E}}$ is $f_{rm DM}^{{rm Chab}} = 0.3_{-0.3}^{+0.1}$, while a Salpeter IMF is marginally inconsistent with the enclosed mass ($f_{rm DM}^{{rm Salp}} = -0.3_{-0.5}^{+0.2}$). The total magnification of the source is $mu_{rm tot}=2.1_{-0.3}^{+0.4}$. The source has at least one bright compact region offset from the source center. Emission from Ly$alpha$ and [O III] are likely to probe different regions in the source.
Download