Studying the Ultraviolet Spectrum of the First Spectroscopically Confirmed Supernova at Redshift Two
Authors: Smith M. , Sullivan M. , Nichol R. C. , Galbany L. , D’Andrea C. B. , Inserra C. , Lidman C. , Rest A. , Schirmer M. , Filippenko A. V. , Zheng W. , Cenko S. Bradley , Angus C. R. , Brown P. J. , Davis T. M. , Finley D. A. , Foley R. J. , González-Gaitán S. , Gutiérrez C. P. , Kessler R. , Kuhlmann S. , Marriner J. , Möller A. , Nugent P. E. , Prajs S. , Thomas R. , Wolf R. , Zenteno A. , Abbott T. M. C. , Abdalla F. B. , Allam S. , Annis J. , Bechtol K. , Benoit-Lévy A. , Bertin E. , Brooks D. , Burke D. L. , Rosell A. Carnero , Kind M. Carrasco , Carretero J. , Castander F. J. , Crocce M. , Cunha C. E. , Costa L. N. da , Davis C. , Desai S. , Diehl H. T. , Doel P. , Eifler T. F. , Flaugher B. , Fosalba P. , Frieman J. , García-Bellido J. , Gaztanaga E. , Gerdes D. W. , Goldstein D. A. , Gruen D. , Gruendl R. A. , Gschwend J. , Gutierrez G. , Honscheid K. , James D. J. , Johnson M. W. G. , Kuehn K. , Kuropatkin N. , Li T. S. , Lima M. , Maia M. A. G. , Marshall J. L. , Martini P. , Menanteau F. , Miller C. J. , Miquel R. , Ogando R. L. C. , Petravick D. , Plazas A. A. , Romer A. K. , Rykoff E. S. , Sako M. , Sanchez E. , Scarpine V. , Schindler R. , Schubnell M. , Sevilla-Noarbe I. , Smith R. C. , Soares-Santos M. , Sobreira F. , Suchyta E. , Swanson M. E. C. , Tarle G. , Walker A. R. ,   (The DES Collaboration)
Published: February 2018

Abstract
We present observations of DES16C2nm, the first spectroscopically confirmed hydrogen-free superluminous supernova (SLSN-I) at redshift

. 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

. 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 (

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 (

) and high redshift (

), but there is clear evidence of diversity in the spectrum at

, 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 (

), we highlight that at

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.