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 similar to 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 angstrom) properties of the SN, finding velocity of the C III feature changes by similar to 5600 km s(-1) over 14 d around maximum light. We find the host galaxy of DES15E2mlf has a stellar mass of 3.5(-2.4)(+3.6) x 10(9) M-circle dot, which is more massive than the typical SLSN-I host galaxy.