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Abstract: We investigate the validity of the quasar - radio galaxy unification scenarioand detect dust tori within radio galaxies of various types. Using VISIR on theVLT, we acquired sub-arcsecond ~0.40- resolution N-band images, at awavelength of 11.85 micron, of the nuclei of a sample of 27 radio galaxies offour types in the redshift range z=0.006-0.156. The sample consists of 8edge-darkened, low-power Fanaroff-Riley class I FR-I radio galaxies, 6edge-brightened, class II FR-II radio galaxies displaying low-excitationoptical emission, 7 FR-IIs displaying high-excitation optical emission, and 6FR-II broad emission line radio galaxies. Out of the sample of 27 objects, 10nuclei are detected and several have constraining non-detections atsensitivities of 7 mJy, the limiting flux a point source has when detected witha signal-to-noise ratio of 10 in one hour of source integration. On the basisof the core spectral energy distributions of this sample we find clearindications that many FR-I and several low-excitation FR-II radio galaxies donot contain warm dust tori. At least 57+-19 percent of the high-excitationFR-IIs and almost all broad line radio galaxies display excess infraredemission, 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 logL bol-L Edd < -3. This suggests that thick tori are absent at low accretionrates and-or low efficiencies. We argue that the unification viewing anglerange 0-45 degrees of quasars should be increased to ~60 degrees, at least atlower luminosities.

Author: G. van der Wolk, P.D. Barthel, R.F. Peletier, J.W. Pel

Source: https://arxiv.org/

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