The dusty environment of quasars: far-infrared properties of optical quasars

We present Infrared Space Observatory (ISO) far-IR photometry of a complete subsample of optically selected bright quasars belonging to two complete surveys selected through multicolor (U, B, V, R, I) techniques. The ISOPHOT camera on board the ISO satellite was used to target these quasars at wavelengths of 7.3, 11.5, 60, 100, and 160 μm. Almost two-thirds of the objects were detected at least in one ISOPHOT band. The detection rate is independent of the source redshift, very likely because of the negative K-correction of the far-IR thermal emission. More than a half of the optically selected QSOs show significant emission between 4 and 100 μm in the quasar rest frame. These fluxes have a very likely thermal origin, although in a few objects an additional contribution from a nonthermal component is plausible in the long-wavelength bands. In a color-color diagram these objects span a wide range of properties, from AGN-dominated to ULIRG-like. The far-IR composite spectrum of the quasar population presents a broad far-IR bump between 10 and 30 μm and a sharp drop at λ > 100 μm in the quasar rest frame. The amount of energy emitted in the far-IR is on average a few times larger than that emitted in the blue, and the ratio LFIR/LB increases with the bolometric luminosity. Objects with fainter blue magnitudes have larger ratios between the far-IR (λ > 60 μm) fluxes and the blue-band flux, which is attributed to extinction by dust around the central source. No relation between the blue absolute magnitude and the dust color temperature is seen, suggesting that the dominant source of FIR energy could be linked to a concurrent starburst rather than to gravitational energy produced by the central engine.