1-5 micron 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 relation for radio galaxies is too bright by about half a magnitude due to contributions from the obscured quasar nuclei. Such a result has major implications for the use of the K-band Hubble diagram in understanding the cosmological evolution of radio galaxies. In this paper we present 1-5 micron imaging data of a nearly-complete sample of low-redshift radio galaxies; this approach allows us to accurately determine the strengths of any unresolved nuclear components in the galaxies. We detect nuclear sources in five targets, whose broad-band colours are consistent with reddened quasar spectra. In all five cases the ratio of the inferred intrinsic near-infrared luminosity to the narrow-line luminosity is typical of quasars. We find a correlation between the inferred nuclear extinction and core-to-lobe ratio, which places constraints on the geometry of the torus. We find evidence for a shift of the K-z relation to fainter magnitudes, but by a much smaller amount (~0.1 mag) than Taylor et al. determined. Under the assumption that the nuclear sources in radio galaxies have the same intrinsic near-infrared spectra as quasars, our multi-wavelength images allow us to limit any possible shift to less than 0.3 magnitudes.