SIMBAD references

We have analyzed the multiwavelength (radio to X-rays) observational properties of a sample of 111 galaxies covering a wide range of activity (QSO, Seyfert 1 and 2 and star forming galaxies), with the aim of looking for similarities and systematic differences between the different sub-classes, which could provide some clues about the nature of their dominant emission mechanism. Our sample of active galaxies can be divided into two major subgroups: those objects where the far infrared emission 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 galaxies). The relative strength of the UV emission provides a further division between QSO and Seyfert 1 galaxies. On the other hand, while Seyfert 2 and star forming galaxies share many observational properties, the former are significantly brighter at X-rays than the latter. Seyfert 2 galaxies are found to show a large spread in properties, overlapping 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 angles through a dense obscuring torus. Seyfert 1 galaxies are significantly brighter at UV and soft X-ray energies than Seyfert 2, as predicted by the unified models, but the measured L_soft X_/LUV_ ratios in Seyfert 2 galaxies are in average lower than predicted by the "occultation/reflection" picture. Furthermore, the hard X-ray emission is lower in Seyfert 2 than in Seyfert 1 galaxies. While the data are consistent with the presence of an obscuring torus around the nuclei of at least some Seyfert 2 galaxies (like NGC 1068), it seems that in any case the "active" component in Sy 2 represents a smaller contribution to the 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 forming galaxies is determined mainly by the relative strength of some kind of "active" nucleus with respect to the global emission of stellar origin in the galaxy.