Studying the ultraviolet spectrum of the first spectroscopically confirmed supernova at Redshift Two

We present observations of DES16C2nm, the first spectroscopically confirmed hydrogen-free superluminous supernova (SLSN-I) at redshift $z\approx 2$. DES16C2nm was discovered by the Dark Energy Survey (DES) Supernova Program, with follow-up photometric data from the Hubble Space Telescope, Gemini, and the European Southern Observatory Very Large Telescope supplementing the DES data. Spectroscopic observations confirm DES16C2nm to be at z = 1.998, and spectroscopically similar to Gaia16apd (a SLSN-I at z = 0.102), with a peak absolute magnitude of $U=-22.26\pm 0.06$. The high redshift of DES16C2nm provides a unique opportunity to study the ultraviolet (UV) properties of SLSNe-I. Combining DES16C2nm with 10 similar events from the literature, we show that there exists a homogeneous class of SLSNe-I in the UV (${\lambda }_{\mathrm{rest}}\approx 2500$ Å), with peak luminosities in the (rest-frame) U band, and increasing absorption to shorter wavelengths. There is no evidence that the mean photometric and spectroscopic properties of SLSNe-I differ between low ($z\lt 1$) and high redshift ($z\gt 1$), but there is clear evidence of diversity in the spectrum at ${\lambda }_{\mathrm{rest}}\lt 2000\,\mathring{\rm A} $, possibly caused by the variations in temperature between events. No significant correlations are observed between spectral line velocities and photometric luminosity. Using these data, we estimate that SLSNe-I can be discovered to z = 3.8 by DES. While SLSNe-I are typically identified from their blue observed colors at low redshift ($z\lt 1$), we highlight that at $z\gt 2$ these events appear optically red, peaking in the observer-frame z-band. Such characteristics are critical to identify these objects with future facilities such as the Large Synoptic Survey Telescope, Euclid, and the Wide-field Infrared Survey Telescope, which should detect such SLSNe-I to z = 3.5, 3.7, and 6.6, respectively