The properties of the obscuring torus have been modeled by Pier & Krolik (1992) and Granato & Danese (1994). Both groups conclude that the dust should be optically thin in the far-infrared, at wavelengths larger than ∼ 80 µm. Combining this result with the unified models, one can predict that narrow-line radio galaxies (NLRGs) and radio quasars (QSRs) of comparable radio power should emit comparable far-infrared power. No conclusive result was reached when comparing NLRGs and QSRs in the far-infrared, either with IRAS (Heckman et al. 1992), or with ISO (van Bemmel et al. 2000). However, one would expect the general shape of the far-infrared SEDs to be the same for all objects that belong to one class, which is true for NLRGs and QSRs.
Broad line radio galaxies were discovered and classified first in the 1960's, when the first radio surveys were done. They are typically galaxies with a bright nucleus and extended radio structures (like NLRGs), but a quasar spectrum in the optical. In the light of unified models, there seems to be an easy way to fit them in; BLRGs can simply be intermediate angle quasars. The dust torus is seen at an angle that just obscures the central source (and therefore enables us to see the host galaxy), but part of the broad-line region is un-obscured.
As discussed above, one would expect that the infrared SEDs are the same for NLRGs, BLRGs and QSRs. However, there is a group of BLRGs which have a peak in their SED at 25 µm, instead of 60–100 µm as in NLRGs and QSRs (see Fig. 1). This seems to be in contradiction with expectations of the unification schemes, unless these 25 µm peakers are somehow different. So far no NLRGs or QSRs are know with a clear peak in their SED at 25 µm.
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Figure 1. The SED in restframe for the BLRG 3C 390.3 (peaker), and the SEDs of BLRG 3C 332 (filled circles) and and NLRG 3C 321 (plusses), both non-peakers. The 25 µm points are marked in the plot. |