2.2. Origin of the various components

The physical picture for the origin of these components is based on Unification of the AGN classes (Miller & Antonucci 1983, Barthel 1989). The quasar's central super-massive (10^7-9 M) black hole is surrounded by hot accreting gas, primarily confined to an accretion disk (AD), emitting optical and UV radiation to provide the Big Blue Bump. The IR bump originates in dust with a wide range of temperatures, the hottest being directly associated with the AGN while the cooler dust originates in the host galaxy, again with a disk-like/torus geometry. X-ray and radio emission from the core varies on short timescales (~ 1 - 100 lt. days) and so originates close to the central black hole, interior to the AD. Some AGN (~ 10%) have extended radio structure from jets and lobes on much larger scales which will not be discussed here.

In a simple unification scenario broad-lined (Type 1) AGN are viewed face-on while in narrow-lined (Type 2) AGN the broad emission line region (BELR) the soft X-rays and much of the optical/UV emission from the AD are hidden by the dust. The SEDs of type 2 AGN have less prominent Big Blue Bumps and strong soft X-ray absorption than their type 1 counterparts. Comparisons between type 1 and 2 AGN at low redshift (Seyfert 1 and 2), where most known Sy2s reside, agree with these expectations. Their relatively low luminosities result in their optical continua including a significant contribution from the host galaxy within which the central black hole and AGN reside.

To explain the broad IR continuum (Figure 3) using pure thermal emission, the dust must have a wide range of temperatures (~ 50 - 1000K). Based upon the presence of extended dust emission in nearby AGN (e.g. Cen A, Genzel & Cesarsky 2000), strong correlations between hot mid-IR emission and the presence of an AGN (Heisler & de Robertis 1999) and weaker correlations between far-IR emission and other AGN indicators (Andreani et al. 2003), the IR continuum has been modeled using 2/3 components. A hot, obscuring dusty torus in an AGN (Pier & Krolik 1992, Granato & Danese 1994) produces a narrow continuum feature which is combined with a cooler, starburst component (Efstathiou & Rowan-Robinson 1995, Rowan-Robinson 2000). Cool dust in the host galaxy may also contribute.

Figure 3. mm-optical spectral energy distribution of PG1351+640 showing the wide range of temperature required to explain the full IR emission as thermal emission from dust. The curves show grey bodies at the marked temperatures normalised to "fit" the data.