Wide and Deep Exploration of Radio Galaxies with Subaru HSC (WERGS). III. Discovery of a z = 4.72 Radio Galaxy with Lyman Break Technique

TL;DR

This paper reports the discovery of a high-redshift (z=4.72) radio galaxy using the Lyman break technique with Subaru HSC data, revealing a dusty, massive galaxy missed by ultra-steep spectrum surveys.

Contribution

It presents the first identification of a z=4.72 radio galaxy via the Lyman break method, expanding the search for high-z radio galaxies beyond ultra-steep spectrum criteria.

Findings

Discovered a z=4.72 radio galaxy with spectroscopic confirmation.

The galaxy is massive and dusty, with a stellar mass of about 10^11.4 solar masses.

The radio spectral index indicates it is not part of the ultra-steep spectrum population.

Abstract

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.