Oral Presentation

Discovery of a z = 4.7 Radio Galaxy without an Ultra-steep Spectrum

Author(s): Takuji Yamashita (Ehime U.) and the WERGS collaborators

Presenter: Takuji Yamashita (Ehime University)

We will present the discovery of z = 4.7 radio galaxies without using the ultra-steep spectrum (USS) method, but using the Lyman break technique with Subaru HSC images and the VLA FIRST catalog. HzRGs are essential objects to study radio luminosity functions, which allow us to understand the cosmic evolution of AGNs and galaxies. However, HzRGs are declined to high-z, and thus the numbers of known HzRGs are quite small. Moreover, the HzRGs were identified by the USS method, and thus they are strongly biased toward steep spectral indices.
Our survey for HzRGs, part of a Wide and Deep Exploration of Radio Galaxies with the Subaru HSC (WERGS) project (Yamashita et al. 2018), employed the Lyman break technique for HSC counterparts of FIRST radio sources. The follow-up optical spectroscopy with Gemini/GMOS presents a clear Lyman alpha emission line and a Lyman break feature at z = 4.7. Because this HzRG has a non-steep radio spectrum, it is never found using the classical USS method. This discovery demonstrates the high feasibility of our ongoing survey for HzRGs without the bias of radio spectral indices.