Discovery of a radio galaxy at z = 5.72
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This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2018 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
We report the discovery of the most distant radio galaxy to date, TGSS J1530+1049 at a redshift of z = 5.72, close to the presumed end of the Epoch of Reionization. The radio galaxy was selected from the TGSS ADR1 survey at 150 MHz for having an ultra-steep spectral index, a1.4 GHz150 MHz = -1.4 and a compact morphology obtained using VLA imaging at 1.4 GHz. No optical or infrared counterparts for the radio source 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 an Lya luminosity of 5.7 × 1042 erg s-1 and an FWHM of 370 km s-1, TGSS J1530+1049 is comparable to 'non-radio' Lyman alpha emitters (LAEs) at a similar redshift. However, with a radio luminosity of log L150MHz = 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 3s limits of J > 24.4 and K > 22.4 (Vega). The K band limit is consistent withz > 5 from the K-z relation for radio galaxies and helps rule out low redshifts. The stellar mass limit derived using simple stellar population models is Mstars < 1010.5 M?. Its relatively low stellar mass and small radio and Lya sizes suggest that TGSS J1530+1049 may be a radio galaxy in an early phase of its evolution.
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