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We have compiled a sample of 234 ultra‐steep‐spectrum‐selected (USS‐selected) radio sources in order to find high‐redshift radio galaxies (HzRGs). The sample is in the southern sky at  −40° &lt; δ &lt; −30°  which is the overlap region of the 408‐MHz Revised Molonglo Reference Catalogue, 843‐MHz Sydney University Molonglo Sky Survey (the MRCR–SUMSS sample) and the 1400‐MHz National Radio Astronomy Observatory Very Large Array Sky Survey (NVSS). This is the third in a series of papers on the MRCR–SUMSS sample. Here, we present optical spectra from the Australian National University 2.3‐m telescope, European Southern Observatory (ESO) New Technology Telescope and ESO Very Large Telescope for 52 of the identifications from Paper II, yielding redshifts for 36 galaxies, 13 of which have   z  &gt; 2 . We analyse the   K − z   distribution and compare 4‐arcsec‐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 fits in the   K − z   plot support passive evolution of radio galaxy hosts since   z  &gt; 3 . By comparing USS‐selected samples in the literature, we find that the resultant median redshift of the samples shown is not dependent on the flux density distribution or selection frequency of each sample. In addition, our finding 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 HzRGs 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 Lyα line, suggesting that they are surrounded by a dense medium. For the bulk of the sources, spectral line ratios show that photoionization is the primary excitation process.

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