Andreani Paola , Franceschini Alberto , & Granato Gianluigi
Dipartimento di Astronomia, Universita` di Padova Osservatorio Astronomico di Padova, Italy
We compare the global Spectral Energy Distribution (S.E.D.) from the optical through ISO observations to millimeter of two samples of optically selected radio-quiet quasars, one at low redshifts, and one at high redshifts. The addition of ISO (CAM and PHT) observations provide new data in the poorly known far-IR region of the S.E.D., where the dust emission peaks and starts to become optically thick. This information is crucial to test the structure and anisotropy of the obscuring-reprocessing torus and its fundamental geometrical and physical parameters (dimensions, dust temperature and optical depth). We then use these S.E.D.s to test emission models of circum-nuclear dust torii around these objects, as predicted by unification models of AGNs, and to check the existence of a so-called ``quasar 2'' population (in analogy with the Seyfert 2 galaxies).
Under the assumption that the dust is heated only by a point-like non-thermal source, we infer model parameters, such as dust mass, temperature distributions and torus sizes rather independently from the quasar bolometric luminosity. Dependences of best-fit parameters on luminosity and redshift are studied and the contribution of dust in the host galaxy to the observed fluxes is briefly mentioned.
The inference of the physical properties of dust in these objects will have profound impact onto the question of the host galaxy formation and provides crucial information about the enriched interstellar medium in the galaxies hosting the quasars.
In the redshift space, the dust abundance does not display appreciable trends from z 1 to almost 5, and shows that dust and metals are at least as, and often more, abundant at these early epochs than they are in local galactic counterparts.
This evolutionary pattern is remarkably at variance with respect to what is expected for disk galaxies, like the Milky Way, slowly building metals during the whole Hubble time. It rather points in favour of a much more active star-formation phase at early epochs, probably related to the formation of massive spheroidal galaxies.