Studying the Ultraviolet Spectrum of the First Spectroscopically Confirmed Supernova at Redshift Two

Smith, M. and Sullivan, M. and Desai, Shantanu et. al. (2018) Studying the Ultraviolet Spectrum of the First Spectroscopically Confirmed Supernova at Redshift Two. The Astrophysical Journal, 854 (1). p. 37. ISSN 1538-4357

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Abstract

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.

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