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

Abstract

We have compiled a sample of 234 ultra-steep-spectrum(USS)-selected radio sources in order to find high-redshift radio galaxies (HzRGs). The sample is in the southern sky at -40 deg 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} solar mass galaxies. Dispersions about the fits in the K-z plot support passive evolution of radio galaxy hosts since z>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 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-alpha line, suggesting they are surrounded by a dense medium...abridged.