Dust tori in radio galaxies
  We investigate the validity of the quasar - radio galaxy unification scenario
and detect dust tori within radio galaxies of various types. Using VISIR on the
VLT, we acquired sub-arcsecond (~0.40") resolution N-band images, at a
wavelength of 11.85 micron, of the nuclei of a sample of 27 radio galaxies of
four types in the redshift range z=0.006-0.156. The sample consists of 8
edge-darkened, low-power Fanaroff-Riley class I (FR-I) radio galaxies, 6
edge-brightened, class II (FR-II) radio galaxies displaying low-excitation
optical emission, 7 FR-IIs displaying high-excitation optical emission, and 6
FR-II broad emission line radio galaxies. Out of the sample of 27 objects, 10
nuclei are detected and several have constraining non-detections at
sensitivities of 7 mJy, the limiting flux a point source has when detected with
a signal-to-noise ratio of 10 in one hour of source integration. On the basis
of the core spectral energy distributions of this sample we find clear
indications that many FR-I and several low-excitation FR-II radio galaxies do
not contain warm dust tori. At least 57+-19 percent of the high-excitation
FR-IIs and almost all broad line radio galaxies display excess infrared
emission, which must be attributed to warm dust reradiating accretion activity.
The FR-I and low-excitation FR-II galaxies all possess low efficiencies,
calculated as the ratio of bolometric and Eddington luminosity log
(L_bol/L_Edd) < -3. This suggests that thick tori are absent at low accretion
rates and/or low efficiencies. We argue that the unification viewing angle
range 0-45 degrees of quasars should be increased to ~60 degrees, at least at
lower luminosities.
