Our understanding of active galaxies has advanced greatly in the recent years, mainly through multiwavelength observations of these objects. The infrared plays a particular role in these studies, for three reasons:
4.1. Unification scheme and ISO observations
Many ISOPHOT results on quasars and radio-galaxies are still pending, but those available tend to support the unification scheme. For radio quiet and steep spectrum QSOs, the SED has a bump at around 60 µm, and declines beyond 100 µm, as expected for multi-temperature dust emission in the range of tens to hundreds of K ([Haas et al. 1998]). Most interestingly, the prototype radiogalaxy CygnusA exhibits a similar SED; but 3C20, with a similar radio flux, is not seen by ISOPHOT. Also, on the whole, QSOs appear to be more luminous in the FIR than radio sources, thus, if the unification scheme is correct, the torus is also optically thick in the FIR. The SED of flat-spectrum QSOs is dominated by synchrotron emission, but for 3C279, a variable quasar, "the (dust) bump pryes above the synchrotron spectrum" when the overall emission is low ([Haas et al. 1998]). In the case of Seyfert galaxies, the synchrotron emission is weak or absent in the IR. In the course of a study with ISOPHOT of 10 CfA galaxies, Pérez Garcia et al. (1998) decompose the emission in two or three phases: warm (150K, corresponding to the nucleus), cold (40 to 50K, star forming regions), very cold (10 to 20K, cirrus). As expected from the unification scheme, the warm phase is colder for Seyferts 2 than for Seyferts 1. The respective extensions of the cold and the warm component in the IR are similar to those of the cold and the hot component in the R band. But, in the FIR, the nuclear emission can be a substantial fraction of the total FIR, and thus of the total Lbol of the galaxy.
Clavel et al. (1998) did a statistical study with ISOPHOT-S encompassing 26 Seyfert 1 and 28 Seyfert 2 galaxies. They found that while Seyfert 2 exhibit a weak continuum and strong infrared bands, Seyfert 1 have 7 times stronger continua and weak or non-existing infrared bands. This also agrees with expectations from MIR optically thick torus models.