Program: GN-2017A-Q-8

Radio galaxies (RGs) are the most luminous radio sources at high redshift (z > 2), and are important for studying the formation of massive galaxies and supermassive black holes in massive dark matter halos. Despite their importance, they are very rare (~200 sources known at z>2, 6 at z>4 and 1 at z>5). Almost all high redshift RGs were found by selecting so-called Ultra-Steep-Spectrum (USS) sources from radio surveys, followed by optical spectroscopy to target their extremely bright Lya emission line to obtain a precise redshift. The USS selection technique is a very efficient method based on the fact that the slope of the low-frequency radio spectrum correlates strongly with redshift. We have recently used the 150 MHz GMRT and the 1.4 GHz FIRST sky surveys covering 10,000 deg^2 to select a new sample of the best USS sources that maximizes the probability that the sources are at z>~4. Using GMOS-N, we will measure the precise redshifts and study the luminous (extended) Lya emission line halos of a sample of 15 USS sources. By significantly expanding the sample of known RGs at z>4 we will contribute to the study of this important high redshift population and their role in (massive) galaxy formation. Furthermore, because of our selection criteria, there is a good probability that we will discover the most distant radio source to date (z>5.2). These very distant sources will furthermore be important probes of the epoch of reionization, as they are bright enough that they can be followed up with LOFAR/GMRT for HI 21 cm spectroscopy.