SIMBAD references

We present basic observational data on the 6C** sample. This is a new sample of radio sources drawn from the 151-MHz 6C survey, which was filtered with radio criteria chosen to optimize the chances of finding radio galaxies at z > 4. The filtering criteria are a steep-spectral index and a small angular size. The final sample consists of 68 sources from a region of sky covering 0.421 sr. We present Very Large Array radio maps, and the results of K-band imaging and optical spectroscopy.

Near-infrared counterparts are identified for 66 of the 68 sources, down to a 3σ limiting magnitude of K ∼ 22 mag in a 3-arcsec aperture. Eight of these identifications are spatially compact, implying an unresolved nuclear source. The K-magnitude distribution peaks at a median K ~ 18.7 mag, and is found to be statistically indistinguishable from that of the similarly selected 6C* sample, implying that the redshift distribution could extend to z ≳ 4.

Redshifts determined from spectroscopy are available for 22 (32 per cent) of the sources, over the range of 0.2 ≲ z ≲ 3.3. We measure 15 of these, whereas the other seven were previously known. Six sources are at z > 2.5. Four sources show broad emission lines in their spectra and are classified as quasars. Three of these show also an unresolved K-band identification. 11 sources fail to show any distinctive emission and/or absorption features in their spectra. We suggest that these could be (i) in the so-called `redshift desert' region of 1.2 < z < 1.8 or (ii) at a greater redshift, but feature weak emission-line spectra.