2.1 The IR Continuum
As can be seen in Fig. 2, the IR continuum contains in general two turnovers, one in the far infrared (FIR) at about 60 µm and another at ~ 1 µm. Observations by IRAS indicated no IR variability of radio-quiet AGN, at least on timescales less than a year. This may have ruled out Comptonized, self-absorbed thermal cyclotron radiation and self-absorbed synchrotron radiation 11 as the cause of the first turnover, since these mechanisms require a rather small source size. An increasing number of indications points towards dust heated by the central UV source as the origin of the IR continuum, even though a contribution from stars and cool dust in the host galaxy may be significant, especially in low-luminosity objects. 12 The combination of dust emission from the nucleus and the host galaxy emission can thus give rise to the quasi power law observed in some sources, without invoking any non-thermal mechanism. The turnovers could correspond to the minimum and maximum temperatures attainable by the dust grains. For instance, the short-wavelength one may be due to the onset of evaporation at about 1500 K. The dust hypothesis has gained additional support from studies of, e.g., ultraluminous IRAS galaxies 13 and flux ratios in IR. 14
In the case of Seyfert 2s, the ionizing radiation seems to escape anisotropically. 15 An explanation due to a dusty obscuring torus seems preferable 16 to the anisotropic emission from the funnel of a thick accretion disk, 17 and is also consistent with the unified scheme for Seyfert galaxies. 18 One unexplained feature of this picture is the geometrical thickness of the torus. 19 Bregman 20 found that the IR-emission from essentially all radio-quiet AGN should have a thermal origin.
One particular example of reprocessing could be the radio-galaxy Cygnus A, where most of the total luminosity seems to be contained in a single spectral feature 21 with a temperature ~ 75 K. The absence of a strong UV-optical continuum in this and other similar sources may be due to dust absorption. The inferred intrinsic luminosity is in the quasar range, and the rest-frame extinction amounts to AV 50 ± 30 magnitudes. 22, 23
An alternative obscuration scenario 24 involves a warped disk of gas and dust, possibly giving rise to most of the FIR and sub-mm continuum emission from radio-quiet AGN. However, the warp itself seems unexplained. 19