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 =-1.63, 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.
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Breugel et al. (1999) studied this question.