2007MNRAS.378..551B

We present a sample of 40 ultrasteep spectrum (USS; α ≤ - 1.3, S_ν∝ ν^α) radio sources selected from the Westerbork in the Southern Hemisphere (WISH) catalogue. The USS sources have been imaged in K band at the Cerro Tololo Inter-American Observatory (CTIO) and with the Very Large Telescope (VLT) at Cerro Paranal. We also present VLT, Keck and William Herschel Telescope (WHT) optical spectroscopy of 14 targets selection from four different USS samples. For 12 sources, we have been able to determine the redshifts, including four new radio galaxies at z > 3. We find that most of our USS sources have predominantly small (<6arcsec) radio sizes and faint magnitudes (K ≳ 18). The mean K-band counterpart magnitude is. The expected redshift distribution estimated using the Hubble K-z diagram has a mean of, which is higher than the predicted redshift obtained for the Sydney University Molonglo Sky Survey-NRAO VLA Sky Survey (SUMSS-NVSS) sample and the expected redshift obtained in the 6C** survey. The compact USS sample analysed here may contain a higher fraction of galaxies which are high redshift and/or are heavily obscured by dust. Using the 74, 352 and 1400MHz flux densities of a subsample, we construct a radio colour-colour diagram. We find that all but one of our USS sources have a strong tendency to flatten below 352MHz. We also find that the highest redshift source from this paper (at z = 3.84) does not show evidence for spectral flattening down to 151MHz. This suggests that very low frequency selected USS samples will likely be more efficient to find high redshift galaxies.