Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS).
  III. Discovery of a z = 4.72 Radio Galaxy with Lyman Break Technique
  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 $\log{M_*/M_{\odot}}
= 11.4$. 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.
