Discovery of a radio galaxy at z = 5.72. Issue 2 (6th August 2018)

• Record Type: Journal Article
• Authors: Saxena, A; Marinello, M; Overzier, R A; Best, P N; Röttgering, H J A; Duncan, K J; Prandoni, I; Pentericci, L; Magliocchetti, M; Paris, D; Cusano, F; Marchi, F; Intema, H T; Miley, GK
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 480:Issue 2(2018)
• Page Start: 2733
• Page End: 2742
• Publication Date: 2018-08-06
• Subjects: galaxies: high-redshift -- galaxies: individual: TGSS J1530+1049; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/sty1996
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: 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, $\alpha ^{\text{{150 MHz}}}_{\text{{1.4 GHz}}} = -1.4$ 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 Lyα luminosity of 5.7 × 10 42 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 L 150MHz = 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 3σ 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 and helps rule out low redshifts. The stellar mass limit derived using simple stellar population models is M stars < 10 10.5 M⊙ . Its relatively low stellar mass and small radio and Lyα sizes suggest that TGSS J1530+1049 may be a radio galaxy in an early phase of its evolution.