DISCOVERY OF A STRONG LENSING GALAXY EMBEDDED IN A CLUSTER AT z = 1.62

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 _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 θ_E=0.38_−0.01^+0.02 arcsec (3.2_−0.1^+0.2 kpc) and the total enclosed mass is M _tot(<θ_E)=1.8_−0.1^+0.2×10¹¹ M_⊙. We estimate that the cluster environment contributes ∼10% of this total mass. Assuming a Chabrier initial mass function (IMF), the dark matter fraction within θ_E is f_DM^Chab=0.3_−0.3^+0.1, while a Salpeter IMF is marginally inconsistent with the enclosed mass (f_DM^Salp=−0.3_−0.5^+0.2). The total magnification of the source is μ_tot=2.1_−0.3^+0.4. The source has at least one bright compact region offset from the source center. Emission from Lyα and [O III] are likely to probe different regions in the source