2.3. Infrared

The infrared (IR) emission of PG quasars has been systematically studied with the Infrared Astronomical Satellite (IRAS; Sanders et al. 1989), and, more recently, with the ISO satellite (Haas et al. 2003). The latter work confirms the basic results of the former, while adding further details. The basic characteristics of the IR emission of quasars are the following:

The integrated IR emission (2 - 200 µm) is, on average, ~ 30% of the bolometric luminosity, with values in individual objects ranging from ~ 15% to ~ 50%. The spectral shape is characterized by (i) a minimum at ~ 1 - 2 µm, corresponding to the sublimation temperature of the most refractary dust (between 1000 and 2000 K, depending on the composition of the dust grains), (ii) an "IR bump", typically at 10 - 30 µm (but there are examples of flat spectra, or peaks anywhere between 2 and 100 µm), due to the thermal emission of dust, with a temperature range between 50 and 1000 K, and (iii) a steep decline (f , > 3) at large wavelengths, typical of the low energy spectrum of a gray emitter (Chini et al. 1989).

The spectral shape of most of the sources in the sample is better reproduced, according to Haas et al. (2003), by reprocessing of the quasar primary emission, with the contribution of a starburst being negligible. However, this is still a controversial point, since the IR continuum expected from a quasar or a starburst is strongly dependent on the geometric and physical properties of the reprocessing medium, and the same observed continuum can often be successfully explained with more than one model (Elitzur, Nenkova, & Ivezic 2004).

The far-IR emission of radio-loud quasars is quite different than that of radio-quiet quasars. The spectrum between the IR bump and the submillimeter range is well reproduced by a power law with spectral index close to = 2.5, as expected from self-synchrotron absorption. The main emission mechanism here is not reprocessing by dust, but synchrotron emission by relativistic electrons.