Discovery of a radio galaxy at z = 5.72

Abstract

We report the discovery of the most distant radio galaxy to date, TGSS1530 at a redshift of $z=5.72$ close to the presumed end of the Epoch of Reionisation. The radio galaxy was selected from the TGSS ADR1 survey at 150 MHz for having to an ultra-steep spectral index, $\hat{I}{\pm }_{\text{1.4 GHz}}^{\text{150 MHz}}=-1.4$ and a compact morphology obtained using VLA imaging at 1.4 GHz. No optical or infrared counterparts for it were found in publicly available sky surveys. Follow-up optical spectroscopy at the radio position using GMOS on Gemini North revealed the presence of a single emission line. We identify this line as Lyman alpha at $z=5.72$, because of its asymmetric line profile, the absence of other optical/UV lines in the spectrum and a high equivalent width. With a Ly$\hat{I}\pm$ luminosity of $5.7\times 10^{42}$ erg s${}^{-1}$ and a FWHM of $370$ km s${}^{-1}$, TGSS1530 is comparable to `non-radio' Lyman alpha emitters (LAEs) at a similar redshift. However, with a radio luminosity of $\log L_{\text{150 MHz}}=29.1$ W Hz${}^{-1}$ and a deconvolved physical size $3.5$ kpc, its radio properties are similar to other known radio galaxies at $z>4$. Subsequent $J$ and $K$ band imaging using LUCI on the Large Binocular Telescope resulted in non-detection of the host galaxy down to $3\ddot{I}ƒ$ limits of $J>24.4$ and $K>22.4$ (Vega). The $K$ band limit is consistent with $z>5$ from the $K-z$ relation for radio galaxies, suggesting stellar mass limits using simple stellar population models of $M_{\text{stars}}<10^{10.5}$ $M_{\odot }$. Its high redshift coupled with relatively small radio and Ly$\hat{I}\pm$ sizes suggest that TGSS1530 may be a radio galaxy in an early phase of its evolution.

11 pages, 7 figures. Latest published version 10 October 2018