8. Comparison with thick tori model predictions
Several torus models have been developed so far to explain the obscuration of the BLR and UV/X-ray continuum sources along some lines of sight (AGN unification scheme). Some of these models are generic, while others have been designed to match the case of NGC 1068 .
Pier & Krolik (1992a, b, 1993) propose a thick, parsec-scale, uniform density torus illuminated by a nuclear source. The dust can be heated up to the effective temperature of the nuclear radiation at the inner edge of the torus. They investigate models with effective temperatures between 500 K and 2000 K. Such a model can explain the unresolved core observed at 2.2, 3.5 and 4.8µm with AO in the particular case of NGC 1068 . Does it explain the extended near infrared emission also revealed by these observations? Indeed, extended emission over 1" to 2" could result from reflected radiation from the torus and/or dust in the NLR. Therefore, this model accounts for most of the features unveiled with high resolution imaging in the near infrared.
Efstathiou & Rowan-Robinson (1995) propose a model with a very thick tapered disk. They assume the melting temperature of all dust grains to be identical (1000 K), but consider a radial distribution of the grain physical parameters (size and chemical composition). In the case of NGC 1068 , Efstathiou et al. (1995) have shown that the torus emission alone cannot account for the total infrared emission. They attribute the excess infrared emission to a distribution of optically thin dust with =2 in the NLR region. Their model is in disagreement with the steep grain temperature gradient across the torus, which we infer to exist close to the central engine. Conversely, the dust postulated to be present in the NLR by their model is indeed detected with the AO data set.
Granato & Danese (1994) and Granato et al. (1996, 1997) developed a simple thick (30) torus model extended over several hundreds pc. To minimize the number of free model parameters, they have adopted a dust density distribution constant with radial distance from the nuclear source. But, they do not rule out the possibility, in the case of smaller values of the optical depth (=1.5), that a more concentrated density distribution exists. Their predicted size and shape for the near infrared emission are compatible with those derived through AO observations at 2.2, 3.5 and 4.8µm.
A revised modeling of the AGN in NGC 1068 is timely, owing to the emergence of sub-arcsec resolution images in the near infrared (AO techniques) and in the millimeter range (interferometric techniques), giving direct access to the dust and molecular environment of the central engine.
© European Southern Observatory (ESO) 2000
Online publication: December 17, 1999