The far-infrared spectral energy distributions of quasars

The origin of the infrared emission in Active Galactic Nuclei (AGN), whose strength is comparable to the optical/ultraviolet (OUV) emission, is genera lly thought to be a combination of thermal emission from dust and non-therm al, synchrotron emission. Although data are sparse, particularly in the far -infrared, the broad wavelength range of this emission suggests a wide rang e of temperatures and a combination of AGN and starburst heating mechanisms . The strength of the non-thermal emission is expected to be related to the radio emission. While this scenario is well established, basic questions, such as the spatial and temperature distribution of the dust, the relative importance of AGN and starburst heating and the significance of the non-the rmal contribution remain largely undetermined. The wide wavelength range of the Infrared Space Observatory (ISO) [Kessler, M.F., 1999. In: Cox, P., Ke ssler, M. (Eds.), The Universe as Seen by ISO, ESA SP-427, ESA, Paris] comb ined with its arcmin spatial resolution and increased sensitivity facilitat ed the observation of a larger subset of the AGN population than previously covered, allowing these questions to be investigated in more detail. This paper will review the spectral energy distributions (SED) of AGN with parti cular emphasis on the infrared emission and on ISO's contributions to our k nowledge. Preliminary results from ISO observations of X-ray selected and h igh-redshift AGN will be described. (C) 2001 Elsevier Science B.V. All righ ts reserved.