Astron. Astrophys. 362, 75-96 (2000)

6. Conclusions

Continuum observations from radio to soft X-ray energies have been presented for a sample of 22 quasars characterized by different degrees of radio emission. The IR data were obtained with ISOPHOT and IRAC1, and the mm data with IRAM, SEST and SCUBA. Further IR and mm data, and some radio and soft X-ray fluxes, were drawn from the literature. The availability of a broad band SED for several types of quasars allows us to separate the dominant spectral components in the radio and IR energy bands, and thus to compare their spectral properties in different types of quasars. The spectral analysis and the comparison of luminosities emitted in the radio, IR, and soft X-ray energy bands yields the following results:

1. What is the dominant mechanisms emitting at IR energies in RLQ and RQQ?

   In our quasar sample the dominant mechanism emitting in the far/mid-IR is thermal emission from dust heated by the optical and UV radiation produced in the outer regions of the AGN. A starburst contributes to the IR emission at different levels, but always less than the AGN (27%). The presence of thermal IR emission in FSRQ remains rather uncertain. Among the three FSRQ of the sample we cannot derive any conclusion for two of them, and for a third one (B2 2201+31A) the observed data suggest a dominant thermal component at m.

2. Do RLQ and RQQ have the same dust properties (temperature, source size, mass, and luminosity)?

   The equilibrium temperature of dust grains, the size and the mass of the dust distribution, and the emitted luminosity have been evaluated for all quasars. The estimated sizes of the observed dust components lie between 0.06 pc and 9.0 kpc, and the temperatures between 43 K and 1900 K. The total luminosity observed in the IR, obtained by integrating the grey body components, varies over a wide range: 2-76010¹¹ . The amount of emitting dust in all types of quasars also varies in a broad range: 610⁴-410⁷ . The distribution of any of these parameters does not differ significantly among the different types of quasars.

3. Does an interplay between the radio and the IR components exist?

   A bright and flat non-thermal component can be sufficiently strong in the IR to mask the dust emission in some sources, particularly FSRQ. However, this does not mean that the dust emission is absent. After subtracting an IR synchrotron component extrapolated from the radio, the residual IR emission had similar spectral shape and luminosity, regardless of the radio properties.

These results are based on the analysis of a small sample, and have to be confirmed by the study of larger samples. ISOPHOT has doubled the number of quasars with IR detections in the sample presented here. A great deal of additional progress on understanding the IR properties of quasars is expected when all of the quasar data available in the ISO archive will be fully analyzed and studied.

© European Southern Observatory (ESO) 2000

Online publication: October 30, 19100
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