The far-infrared continuum of quasars

ISO provides a key new far-infrared window through which to observe the multi-wavelength spectral energy distributions (SEDs) of quasars and active galactic nuclei (AGN). It allows us, for the first time, to observe a substantial fraction of the quasar population in the far-IR, and to obtain simultaneous, multiwavelength observations from 5-200 mu m. With these data we can study the behavior of the LR continuum in comparison with expectations from competing thermal and non-thermal models. A key to determining which mechanism dominates, is the measurement of the peak wavelength of the emission and the shape of the far-IR-mm turnover. Turnovers which are steeper than v(2.5) indicate thermal dust emission in the far-IR. Preliminary results from our ISO data show broad, fairly smooth, IR continuum emission With far-IR turnovers generally too steep to be explained by nonthermal synchrotron emission. Assuming thermal emission throughout leads to a wide inferred temperature range similar to 50 - 1000 K. The hotter material, often called the AGN component, probably originates in dust close to and heated by the central source: e.g. the ubiquitous molecular torus. The cooler emission is too strong to be due purely to cool, host galaxy dust, and so indicates either the presence of a starburst in addition to the AGN or AGN-heated dust covering a wider range of temperatures than present in the standard, optically thick torus models.