First Release of High-redshift Superluminous Supernovae from the Subaru HIgh-Z SUpernova CAmpaign (SHIZUCA). II. Spectroscopic Properties

We present Keck spectroscopic observations of three probable high-redshift superluminous supernovae (SLSNe) from the Subaru HIgh-Z sUpernova CAmpaign (SHIZUCA), confirming redshifts of 1.851, 1.965, and 2.399. The host galaxies were selected for transient monitoring from multiband photometric redshifts. The supernovae are detected during their rise, and the classically scheduled spectra are collected near maximum light. The rest-frame far-ultraviolet (similar to 1000-2500 angstrom) spectra include a significant host galaxy flux contribution, and we compare our host-galaxy-subtracted spectra to UV-luminous SNe from the literature. While the signal-to-noise ratios of the spectra presented here are sufficient for redshift confirmation, supernova spectroscopic type confirmation remains inconclusive. The success of the first SHIZUCA Keck spectroscopic follow-up program 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.