Letter to the Editor

Chemical properties in the most distant radio galaxy

¹ Graduate School of Science and Engineering, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan
e-mail: kenta@cosmos.phys.sci.ehime-u.ac.jp
² Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
³ The Hakubi Project, Kyoto University, Yoshida-Ushinomiya-cho, Sakyo-ku, Kyoto 606-8302, Japan
⁴ INAF – Osservatorio Astrofisico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ Dipartimento di Fisica e Astronomia, Universitá degli Studi di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
⁶ Research Center for Space and Cosmic Evolution, Ehime University, 2-5 Bunkyo-cho, Matsuyama 790-8577, Japan

Received: 6 July 2011
Accepted: 19 July 2011

Abstract

We present a deep optical spectrum of TN J0924−2201, the most distant radio galaxy at z = 5.19, obtained with FOCAS on the Subaru Telescope. We successfully detect, for the first time, the C ivλ1549 emission line from the narrow-line region. In addition to the emission-line fluxes of Lyα and C iv, we set upper limits on the N v and He ii emissions. We use these line detections and upper limits to constrain the chemical properties of TN J0924−2201. By comparing the observed emission-line flux ratios with photoionization models, we infer that the carbon-to-oxygen relative abundance is already [C/O]  > −0.5 at a cosmic age of  ~1.1 Gyr. This lower limit on [C/O] is higher than the ratio expected at the earliest phases of the galaxy chemical evolution, indicating that TN J0924−2201 has already experienced significant chemical evolution at z = 5.19.