Open Access1–5 μ m imaging of 3CRR galaxies: the K–z relation and the geometry of the torus
Author(s) -
Simpson Chris,
Ward Martin,
Wall J. V.
Publication year - 2000
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2000.03920.x
Subject(s) - physics , quasar , astrophysics , radio galaxy , luminous infrared galaxy , redshift , galaxy , torus , luminosity , active galactic nucleus , astronomy , peculiar galaxy , galaxy group , geometry , mathematics
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.