5.3.3 Infrared Properties

Models for the obscuring torus suggest it becomes transparent in the far-infrared (Pier and Krolik 1993), where the radiation, possibly thermal emission by dust grains (Sanders et al. 1989), therefore becomes isotropic. To date, the only bulk measurements of AGN at far-infrared wavelengths come from IRAS, which was not sensitive enough to detect most radio galaxies individually. Separate co-adds of IRAS data for radio galaxies and quasars, including many non-detections, do show that quasars are systematically more luminous at all observed wavelengths from 12 µm to 100 µm. This suggests that there is a fundamental difference between the two populations, or there is a significant contribution from beamed far-infrared continuum in the quasars, or the torus is still radiating anisotropically at ~ 50 µm in the rest frame (Heckman et al. 1994). ISO observations will be fundamental in addressing this issue further.

In AGN for which the optical depth to the nucleus is not too high, detection of broad Paschen lines may be possible. To date, Pa has been seen in some broad-line radio galaxies and also in a few narrow-line radio galaxies, and in all cases the reddening to the broad-line region was larger than the reddening to the narrow-line region (Hill et al. 1995). With more sensitive instruments, the expectation is that infrared broad lines will be found in a number of Type 2 radio galaxies. The torus is likely quite thick in the equatorial plane, so it is only at intermediate angles that the extinction would be small enough for Pa photons to escape. Interestingly, radio galaxies appear to have nuclear sources stronger in the infrared than in the optical, exactly as expected for a reddened, hidden nucleus (Dunlop et al. 1993).