Active Galactic Nuclei

Abstract

The presence of obscuring material (or a dusty 'torus') in active galactic nuclei (AGN) is central to the unification model for AGN. Two models, the multi-population model for radio sources and the receding torus model, are capable of describing observational properties of powerful radio galaxies and radio quasars. Aiming to discriminate between two working models, I study the changes of the half opening angle (or critical angle, θ c ) of the obscuring torus with radio luminosity at 151 MHz, [O III] emission-line luminosity and cosmic epoch for 206 FRII radio sources from the combined sample of 3CRR, 6CE and 7CR complete samples (Arshakian 2005, accepted by A&A, astro-ph/0411636). An equation for estimating the opening angle of the torus by means of the mean projected linear sizes of FRII radio galaxies and quasars has been derived. The small errors involved in estimation of the opening angle makes the linear size statistics a powerful tool for investigating the orientation-dependent structures in AGN. A statistically significant correlation is found only between the half opening angle of the torus and [O III] emission-line luminosity (Fig. 1). The opening angle increases from 20• to 60• with increasing [O III] emission-line luminosity (tan θ c ∝ L 0.35 [O III] ). This empirical relation is interpreted as direct evidence of the receding torus around central engines of powerful FRII double radio sources. In this model the opening angle of the torus increases with increasing luminosity of the ionizing radiation from the AGN, L phot ∝ L O III ∝ tan 2 θ c , which is assumed to be independent of the height of the torus. This result apparently breaks the degeneracy (multi-population or receding torus) in the interpretation of data in favor of the receding torus model. Infrared emission from a clumpy and dusty torus around AGN D 02THOMAS Based on a model for a clumpy and dusty torus surrounding supermassive black holes (Beckert & Duschl 2004, A&A 426, 445), we discuss the near to mid-infrared emission of AGN. Both dynamical arguments and the weakness of spectral features in the infrared suggest that dust in the torus is organized in distinct clouds. The optical depth of individual clouds is so large that the clumpiness of the torus is important for the shape of the broad-band spectrum and the appearance of AGN in high-resolution speckle imaging and interferometric experiments with MIDI and AMBER at the VLTI. The solution of the radiative transfer problem uses a statistical approach for a large number of ≥ 10 4 clouds. In a two step process a database of cloud models is generated first. The emission from a distribution of clouds in the torus model is then calculated in a separate second step. The results show that the optical depth of clouds must decrease from the center outwards. An optical depth τ V ∼ 20 for individual clouds is reached at positions along a line of sight from the observer at which the mean number of clouds is about one. The simulated spectra are in agreement with recent high resolution observations...