Program: GS-2017B-FT-17

Radio galaxies (RGs) are massive galaxies with mature super-massive black holes. Studying high-z RGs is thus a key for constraining the timescales of formation and evolution of massive galaxies and super-massive black holes. However, due to their faintness in optical wavelength, only <10 RGs have been identified at redshift z>4. In order to build a statistical RG sample at z>4 and investigate their formation and evolution, we are carrying out an exploration of z~4-5 RGs using a new deep and wide optical-imaging data by the Subaru Hyper Suprime-Cam and the radio survey FIRST. We found 34 RG candidates at z>4 by color selections of Lyman break technique. In this proposal, we will determine spectroscopic redshifts of two candidates at z~5 as a pilot study of this project. This proposed observation will achieve the discover the most distant known RG and demonstrate the feasibility of our search for z>4 RGs using the combination of the Hyper Suprime-Cam and the FIRST catalogs. This is a re-submission of an accepted FT proposal (GS-2017B-FT-1) due to bad weather and scheduling constraints.