Figure 3 summarizes our description of reradiation from gas and dust in the host galaxy of a quasar. Provided quasars are located in galaxies like those of their lower luminosity cousins, it is hard to imagine how thermal reradiation can fail to make a significant contribution to the infrared luminosity of quasars. As we have outlined above, assuming that this dominates provides attractively natural explanations for the shape of the far infrared and submillimeter spectrum, for the high-frequency radio emission, for the ``3 - 5 µm'' bump, and for the universal minimum in L at = 1014.5 Hz. Some support for the latter can be adduced from the elegant observations of near-infrared variability in Fairall 9 by Clavel et al. (1989). The general absence of variability at longer infrared wavelengths (Neugebauer et al. 1989) is at least consistent with a thermal interpretation. Still, objections can be raised: e.g., the general absence of emission features associated with polycyclic aromatic hydrocarbons and silicates (Moorwood 1989). Nonthermal models can be contrived (and the author must confess to some involvement!) to explain many of the same features, though perhaps less naturally.
This debate will ultimately be resolved by a measurement of the brightness temperatures. Nonthermal models of the submillimeter spectrum (de Kool & Begelman 1989) require brightness temperatures TB > 1010 K, while thermal models require TB < 103 K. The two models thus predict angular sizes for the emitting region differing by a factor 104. A submillimeter interferometer with a few kilometers baseline would determine the nature of the emission. Continued monitoring of infrared variability will also discriminate between thermal and non-thermal models (though one should beware of sources like 3C273, where a weak Blazar component seems occasionally to wobble into the line of sight). The model outlined above predicts hysteresis in the near infrared flux: when the central source increases in brightness, the near infrared flux can rise with it (with only a mean light-travel time delay). But the rise will sublimate the dust in a larger area than before, and if the central luminosity subsequently falls faster than grains can reform and grow, the dust-free ``hole'' will produce a large dip in the near infrared L, at the frequencies corresponding to the missing temperatures. If the emitting dust grains are aligned by shear or a globally anisotropic magnetic field, their thermal radiation could be significantly polarized.
Figure 3. Cartoon illustrating the state of and frequencies of reradiation from gas in a quasar's host galaxy. Note the sudden transition in the gas temperature at ~ 1018 cm caused by the drop in opacity when dust sublimates.
It is devoutly to be hoped that we will soon be freed from our embarrassing ignorance (after 20 years of observations and theoretical activity) of whether the infrared emission from quasars arises from a source 1014 cm across, or from one 1022 cm across.
ACKNOWLEDGEMENTS. I thank: Liz for typing, Gerry Neugebauer, Dave Sanders, Tom Soifer and Ski Antonucci for making this subject impossible to ignore, and the Irvine Foundation, the Boeing Corporation, and the NSF for support under Presidential Young Investigator grant AST 84-51725.