The quest for high-redshift radio galaxies II. Discovery of the most powerful known radio galaxy at z=4.946

As part of our quest for high-redshift radio galaxies (HzRGs), we present the results obtained from optical spectroscopy leading to the confirmation of one HzRG candidate selected using the Lyman break technique. The optical emission, associated with the radio source TXS 2354+015, exhibits a prominent drop in the r band, which is characteristic of sources at z~5. The optical spectrum shows a bright asymmetric emission line, with a large equivalent width of ~900 Angstrom. Its identification with the Ly$\alpha$ line is confirmed by the detection of the NV$\lambda$1640, leading to a redshift estimate of z=4.946. Its broadband radio spectrum is well reproduced using a power law with a slope of 0.94. The radio luminosity at the rest frame frequency of 500 MHz is 6.2e29 W/Hz. This result makes this source the most powerful radio galaxy currently known.