• DOI:10.1093/mnras/sty1996
• Corpus ID: 118830412

Discovery of a radio galaxy at z = 5.72

@article{Saxena2018DiscoveryOA,
  title={Discovery of a radio galaxy at z = 5.72},
  author={A. Saxena and Murilo Marinello and Roderik Overzier and Philip N. Best and Huub J. A. R{\"o}ttgering and Kenneth J Duncan and Isabella Prandoni and Laura Pentericci and M Magliocchetti and Diego Paris and Felice Cusano and Fabrizio De Marchi and Huib T. Intema and G. K. Miley},
  journal={Monthly Notices of the Royal Astronomical Society},
  year={2018}
}

• A. Saxena, M. Marinello, +11 authors G. Miley
• Published 4 June 2018
• Physics
• Monthly Notices of the Royal Astronomical Society

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.