First release of high-redshift superluminous supernovae from the Subaru HIgh-Z sUpernova CAmpaign (SHIZUCA). II. Spectroscopic properties

We present Keck spectroscopic confirmation of three superluminous supernovae (SLSNe) at z = 1.851, 1.965 and 2.399 detected as part of the Subaru HIgh-Z sUpernova CAmpaign (SHIZUCA). The host galaxies have multi-band photometric redshifts consistent with the spectroscopic values. The supernovae were detected during their rise, allowing the spectra to be taken near maximum light. The restframe far-ultraviolet (FUV; ~1000--2500A) spectra are made up in flux of approximately equal parts supernova and host galaxy. Weather conditions during observations were not ideal, and while the signal-to-noise ratios of the spectra are sufficient for redshift confirmation, the type of each event remains ambiguous. We compare our spectra to the few low redshift SLSN FUV spectra available to date and offer an interpretation as to the type of each supernova. We prefer SLSN-II classifications for all three events. The success of the first SHIZUCA Keck spectroscopic follow-up program is encouraging. It demonstrates that campaigns such as SHIZUCA are capable of identifying high redshift SLSNe with sufficient accuracy, speed and depth for rapid, well-cadenced and informative follow-up.