First Release of High-Redshift Superluminous Supernovae from the Subaru HIgh-Z SUpernova CAmpaign (SHIZUCA). I. Photometric Properties

We report our first discoveries of high-redshift supernovae from the Subaru HIgh-Z sUpernova CAmpaign, the transient survey using Subaru/Hyper Suprime-Cam (HSC). We report the discovery of three supernovae at the spectroscopically confirmed redshifts of 2.399 (HSC16adga), 1.965 (HSC17auzg), and 1.851 (HSC17dbpf), and two supernova candidates with the host galaxy photometric redshifts of 3.2 (HSC16apuo) and 4.2 (HSC17dsid), respectively. In this paper, we present their photometric properties, and the spectroscopic properties of the confirmed high-redshift supernovae are presented in the accompanying paper. The supernovae with the confirmed redshifts of z similar or equal to 2 have the rest-ultraviolet peak magnitudes close to -21 mag, and they are likely superluminous supernovae. The discovery of three supernovae at z similar or equal to 2 roughly corresponds to the approximate event rate of similar to 900 +/- 520 Gpc(-3) yr(-1) with Poisson error, which is consistent with the total superluminous supernova rate estimated by extrapolating the local rate based on the cosmic star formation history. Adding unconfirmed superluminous supernova candidates would increase the event rate. Our superluminous supernova candidates at the redshifts of around 3 and 4 indicate the approximate superluminous supernova rates of similar to 400 +/- 400 Gpc(-3) yr(-1) (z similar or equal to 3) and similar to 500 +/- 500 Gpc(-3 )yr(-1) (z similar to 4) with Poisson errors. Our initial results demonstrate the outstanding capability of HSC to discover high-redshift supernovae.