A Radio Galaxy at z=5.19

TL;DR

This paper reports the discovery of the most distant known AGN, a radio galaxy at redshift 5.19, identified through ultra-steep spectrum selection and confirmed by spectroscopic Ly-alpha emission, extending the K-z relation to higher redshifts.

Contribution

It presents the identification and characterization of the highest-redshift radio galaxy, demonstrating the effectiveness of ultra-steep spectrum selection for finding distant AGN.

Findings

Discovered a radio galaxy at z=5.19, the most distant known AGN.

Extended the K-z relation for radio galaxies to z > 5.

Observed a multi-component, radio-aligned morphology typical of lower-redshift radio galaxies.

Abstract

We report the discovery of the most distant known AGN, the radio galaxy TN J0924-2201 at z = 5.19. The radio source was selected from a new sample of ultra-steep spectrum (USS) sources, has an extreme radio spectral index alpha_365MHz^1.4GHz = -1.63, and is identified at near-IR wavelengths with a very faint, K = 21.3 +- 0.3 object. Spectroscopic observations show a single emission line at lambda ~ 7530A, which we identify as Ly-alpha. The K-band image, sampling rest frame U-band, shows a multi-component, radio-aligned morphology, typical of lower-redshift radio galaxies. TN J0924-2201 extends the near-IR Hubble, or K-z, relation for powerful radio galaxies to z > 5, and is consistent with models of massive galaxies forming at even higher redshifts.