DES15E2mlf: a spectroscopically confirmed superluminous supernova that exploded 3.5 Gyr after the big bang

We present the Dark Energy Survey (DES) discovery of DES15E2mlf, the most distant superluminous supernova (SLSN) spectroscopically confirmed to date. The light curves and Gemini spectroscopy of DES15E2mlf indicate that it is a Type I superluminous supernova (SLSN-I) at z = 1.861 (a lookback time of ∼10 Gyr) and peaking at M_AB = -22.3 ± 0.1 mag. Given the high redshift, our data probe the rest-frame ultraviolet (1400-3500 Å) properties of the SN, finding velocity of the C III feature changes by ∼5600 km s^{- 1} over 14 d around maximum light. We find the host galaxy of DES15E2mlf has a stellar mass of 3.5^{+3.6}_{-2.4} × 10⁹ M_⊙, which is more massive than the typical SLSN-I host galaxy.