MULTIWAVELENGTH ANALYSIS OF ACTIVE GALAXIES - IMPLICATIONS ON UNIFIEDSEYFERT MODELS

We have analyzed the multiwavelength (radio to X-rays) observational p roperties of a sample of 111 galaxies covering a wide range of activit y (QSO, Seyfert 1 and 2 and star forming galaxies), with the aim of lo oking for similarities and systematic differences between the differen t sub-classes, which could provide some clues about the nature of thei r dominant emission mechanism. Our sample of active galaxies can be di vided into two major subgroups: those objects where the far infrared e mission clearly dominates the spectral energy distribution (Seyfert 2 and star forming galaxies), and those where the high energy emission ( UV to X-rays) is comparable to the far infrared (QSO and Seyfert 1 gal axies). The relative strength of the UV emission provides a further di vision between QSO and Seyfert 1 galaxies. On the other hand, while Se yfert 2 and star forming galaxies share many observational properties, the former are significantly brighter at X-rays than the latter. Seyf ert 2 galaxies are found to show a large spread in properties, overlap ping both with Seyfert 1 and star forming galaxies. We have tested the validity of the ''unified Seyfert models'', which assume that Seyfert 1 and 2 galaxies are intrinsically similar, but seen at different ang les through a dense obscuring torus. Seyfert 1 galaxies are significan tly brighter at UV and soft X-ray energies than Seyfert 2, as predicte d by the unified models, but the measured L(softX)/L(UV) ratios in Sey fert 2 galaxies are in average lower than predicted by the ''occultati on/reflection'' picture. Furthermore, the hard X-ray emission is lower in Seyfert 2 than in Seyfert 1 galaxies. While the data are consisten t with the presence of an obscuring torus around the nuclei of at leas t some Seyfert 2 galaxies (like NGC 1068), it seems that in any case t he ''active'' component in Sy 2 represents a smaller contribution to t he energy budget of the whole galaxy than it does in Sy 1 galaxies. A scheme becomes then evident in which the sequence QSO-Sy 1-Sy 2-star f orming galaxies is determined mainly by the relative strength of some kind of ''active'' nucleus with respect to the global emission of stel lar origin in the galaxy.