1-5 mu m imaging of 3CRR galaxies: the K-z relation and the geometry of the torus

It has been claimed by Taylor et al. that the low-redshift end of the K-z r elation for radio galaxies is too bright by about half a magnitude owing to contributions from the obscured quasar nuclei. Such a result has major imp lications for the use of the K-band Hubble diagram in understanding the cos mological evolution of radio galaxies. In this paper we present 1-5-mum ima ging data of a nearly complete sample of low-redshift radio galaxies; this approach allows us to determine accurately the strengths of any unresolved nuclear components in the galaxies. We detect nuclear sources in five targe ts, whose broad-band colours are consistent with reddened quasar spectra. I n all the five cases the ratio of the inferred intrinsic near-infrared lumi nosity to the narrow-line luminosity is typical of quasars. We find a corre lation between the inferred nuclear extinction and core-to-lobe ratio, whic h places constraints on the geometry of the torus. We find evidence for a s hift of the K-z relation to fainter magnitudes, but by a much smaller amoun t (similar to0.1 mag) than determined by Taylor et al. Under the assumption that the nuclear sources in radio galaxies have the same intrinsic near-in frared spectra as quasars, our multiwavelength images allow us to limit any possible shift to less than 0.3 mag.