Discovery of a radio galaxy at z = 5.72
  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, $\alpha^{\textrm{150 MHz}}_{\textrm{1.4 GHz}}
= -1.4$ 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$\alpha$ 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 $\log
L_{\textrm{150 MHz}} = 29.1$ 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 $3\sigma$ 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 $M_{\textrm{stars}}< 10^{10.5}$ $M_\odot$.
Its high redshift coupled with relatively small radio and Ly$\alpha$ sizes
suggest that TGSS1530 may be a radio galaxy in an early phase of its evolution.
