On the far-infrared emission of quasars

Infrared spectral energy distributions between 4.8 and 200 mu m are present ed for a subsample of 10 quasars and radio galaxies from the European Centr al Quasar Programme observed with the Infrared Space Observatory. For three of the sources, this represents the first far-infrared detection. The spec tral energy distributions (SEDs) reveal signatures of thermal dust as well as synchrotron emission. In most cases, one of the two components is so dom inant that the other remains hidden. The SEDs of the radio-quiet and steep- spectrum quasars show a bump around 60 mu m and a decline longward of 100 m u m-strong evidence for thermal emission. It can be described as a superpos ition of several modified blackbodies, showing the broad variety of tempera tures from hot (approximate to 600 K) to cool (approximate to 30 K) dust pr esent in these objects. The infrared emission of the flat-spectrum radio qu asars is in smooth continuation of the radio spectrum, supporting the inter pretation as synchrotron emission. For one of these sources (3C 279), a bum p is prying above the synchrotron spectrum, and we interpret it as thermal emission. For comparison within the framework of unified schemes, the radio galaxies Cyg A (3C 405) and 3C 20 are observed. While 3C 20 has not been d etected, Cyg A reveals an infrared spectrum surprisingly similar to that of 3C 48 and the radio-quiet quasars, which strongly supports the interpretat ion of the radio galaxy Cyg A as a hidden quasar.