Dust emission from quasars and quasar host galaxies

We test emission models of circumnuclear: dust tori around quasars, at low and high redshifts, by using a large collection of photometric data for an unbiased sample of 120 optically selected objects with millimetric and submillimetric fluxes, including new unpublished data. Under the assumption that the dust is heated by a point-like source with a power-law primary spectrum, as defined by the observed optical-ultraviolet continuum, we infer the basic model parameters, such as dust masses, temperature: distributions and torus sizes, by numerically solving the radiative transfer equation in the dust distribution. In addition to the substantiated statistics, an essential improvement over previous analyses comes from the use of optical-ultraviolet data to constrain the primary illuminating continuum, which is needed to estimate dust temperatures and sizes. The dependences of the best-fitting parameters on luminosity and redshift are studied and the contribution of dust in the host galaxy to the observed fluxes is briefly mentioned. This analysis constrains the properties of the enriched interstellar medium in the galaxies hosting the quasars. The dust abundance does not display appreciable trends as a function of redshift, from z similar or equal to 1 to almost 5, and shows that at these early epochs dust and metals are at least as abundant as, and often more abundant than, they are in local Galactic counterparts. This evolutionary pattern is remarkably at variance with respect to what is expected for disc galaxies, like the Milky Way, slowly building metals during the whole Hubble time. It rather points in favour of a much more active phase of star formation at early epochs, probably related to the formation of massive spheroidal galaxies.