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 alpha(365 MHz)(1.4 GHz) = -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 similar to 7530 Angstrom, which we identify as Ly alpha. 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.