A new search for distant radio galaxies in the Southern hemisphere - III. Optical spectroscopy and analysis of the MRCR-SUMSS sample

We have compiled a sample of 234 ultra-steep-spectrum(USS)-selected radio sources in order to nd high-redshift radio galaxies (HzRGs). The sample is in the southern sky at 40 2. We analyse the K{z distribution and compare 4arcsec-aperture magnitudes with 64-kpc aperture magnitudes in several surveys from the literature; the MRCR{SUMSS sample is found to be consistent with models for 10 11 {10 12 M galaxies. Dispersions about the ts in the K{z plot support passive evolution of radio galaxy hosts since z > 3. By comparing USS-selected samples in the literature, we nd that the resultant median redshift of the samples shown is not dependent on the ux density distribution or selection frequency of each sample. In addition, our nding that the majority of the radio spectral energy distributions remain straight over a wide frequency range suggests that a k-correction is not responsible for the success of USS-selection in identifying high redshift radio galaxies and therefore the steep radio spectra may be intrinsic to the source or a product of the environment. Two galaxies have been found to have both compact radio structures and strong self-absorption in the Ly line, suggesting they are surrounded by a dense medium. For the bulk of the sources, spectral line ratios show that photoionisation is the primary excitation process.