3.5. Ingredients which determine the Shape of the SED

Many parameters are important in determining the observed shape of an AGN SED. The relative luminosities of the AGN itself, related to the black hole mass, and of its host galaxy determine the visibility of host galaxy emission. The accretion rate and physical properties of the AD combine with its inclination to our line-of-sight to determine the shape of the Big Blue Bump. The amount, geometry, ionisation and optical depth of absorbing dust and gas and its inclination determines the IR continuum and the absorption of the optical, near-IR and soft X-ray continua. The amount and location of scattering material and the strength of the scattered light is also important in edge-on (type 2) AGN where the primary AGN continuum is partially/fully obscured. Finally the presence and strength of a radio source in the core of the AGN affects the radio, far-IR and hard X-ray continua.

One property which, perhaps surpisingly, has not yet been shown to be related to the SED is evolution. With the central black hole as driver of the energy source, the black hole is expected to grow as material is accreted and so the observable properties of the AGN are expected to change as the source ages. Although observations of high redshift AGN/quasars are still limited, there is currently no convincing evidence for a change in SED with redshift (Silverman et al. 2002, Mathur et al. 2002, Brandt et al. 2002, Vignali et al. 2003). Similarly, as discussed earlier, evolution may unify ULIRGs and AGN predicting changes in the IR SED as the dust first enshrouds and then heats up and disperses, allowing the AGN to shine through. While the OUV and IR SEDs show a wide diversity of shapes, there is no compelling evidence for systematic evolution in their properties as a function of redshift. The much larger number of high-redshift AGN now accessible in many wavebands will provide significant new data on SED evolution.