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.
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.