1–5 μm imaging of 3CRR galaxies: the K–z relation and the geometry of the torus |
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| Authors: | Chris Simpson Martin Ward J V Wall |
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| Institution: | Subaru Telescope, National Astronomical Observatory of Japan, 650 N. A'ohōkūPlace, Hilo, HI 96720, USA;X-ray Astronomy Group, Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH;Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH |
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| Abstract: | 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 owing 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-μm imaging 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 targets, whose broad-band colours are consistent with reddened quasar spectra. In all the 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 determined by Taylor et al. Under the assumption that the nuclear sources in radio galaxies have the same intrinsic near-infrared spectra as quasars, our multiwavelength images allow us to limit any possible shift to less than 0.3 mag. |
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| Keywords: | galaxies: active galaxies: nuclei galaxies: photometry infrared: galaxies radio continuum: galaxies |
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