We report a discovery of z = 4.72 radio galaxy , HSC J083913.17+011308.1 , by using the Lyman break technique with the Hyper Suprime-Cam Subaru Strategic Survey ( HSC-SSP ) catalog for VLA FIRST radio sources .
The number of known high- z radio galaxies ( HzRGs ) at z > 3 is quite small to constrain the evolution of HzRGs so far .
The deep and wide-area optical survey by HSC-SSP enables us to apply the Lyman break technique to a large search for HzRGs .
For an HzRG candidate among pre-selected r -band dropouts with a radio detection , a follow-up optical spectroscopy with GMOS/Gemini has been performed .
The obtained spectrum presents a clear Ly \alpha emission line redshifted to z = 4.72 .
The SED fitting analysis with the rest-frame UV and optical photometries suggests the massive nature of this HzRG with \log { M _ { * } / M _ { \sun } } = 11.4 .
The small equivalent width of Ly \alpha and the moderately red UV colors indicate its dusty host galaxy , implying a chemically evolved and dusty system .
The radio spectral index does not meet a criterion for an ultra-steep spectrum : \alpha ^ { 325 } _ { 1400 } of -1.1 and \alpha ^ { 150 } _ { 1400 } of -0.9 , demonstrating that the HSC-SSP survey compensates for a sub-population of HzRGs which are missed in surveys focusing on an ultra-steep spectral index .