A Radio Galaxy at z = 5.19

We report the discovery of the most distant known active galactic nucleus, the radio galaxy TN J0924-2201 at z=5.19. The radio source was selected from a new sample of ultrasteep spectrum sources, has an extreme radio spectral index α^1.4GHz_365MHz=-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 λ~7530 Å, which we identify as Lyα. The K-band image, sampling rest-frame U band, shows a multicomponent, radio-aligned morphology, which is 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.

Based on observations at the W. M. Keck Observatory, which is operated as a scientific partnership among the University of California, the California Institute of Technology, and NASA. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.