A first sample of faint radio sources with virtually complete redshifts -- I. Infrared images, the Hubble diagram and the alignment effect

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Abstract
We have obtained redshifts and infrared images for a sample of faint B2/6C radio sources, the fluxes of which are about six times fainter than those of sources in the bright 3C sample. We now have unambiguous redshifts for 90 per cent of the sources, making this the first faint radio sample with such complete redshift information. We find that the infrared Hubble diagrams (K versus z) of the 3C sample and the B2/6C sample are similar at a low redshift (z < 0.6), but by a redshift of z ~ 1 the 6C/B2 galaxies are on average ≃ 0.6 mag fainter in the K-band than are 3C galaxies at the same redshift. This suggests that the bright K magnitudes of 3C galaxies at z ~ 1 are not the result of stellar evolution, but of a correlation between infrared and radio luminosity. We also find that the infrared structures of B2/6C galaxies at z ~ 1 are less clearly aligned with their radio structures than are 3C galaxies at this redshift, implying that the strength of the alignment effect depends on radio luminosity. Finally, above a redshift of 2 we find that the dispersion in the K-z relation of the B2/6C sample is ≃ 2.7 times greater than that at low redshift, a result which is expected if, at these redshifts, we are beginning to penetrate into the epoch in which radio galaxies formed.
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