2.1 Observational and Theoretical Evidence for the Accretion Paradigm

There is both observational and theoretical evidence for this picture. Based on optical spectra of AGN, Jackson & Wall (1999) identify two classes of object: Class A (quasars, ``N'' radio galaxies, Seyfert galaxies) with strong narrow emission lines and Class B (most radio galaxies and weak radio cores) with weak or no narrow line emission. Narrow line strength is considered a better parameter than, for example, broad line strength, as it is less affected by orientation effects (see, e.g., Antonucci & Miller 1985). In this scheme Class A objects are identified with high nuclear gas content and, therefore, with high accretion rate, while Class B AGN are identified with low accretion rate. An important point noted by Jackson & Wall is that the Fanaroff & Riley Class I radio sources are an homogeneous class, while FR II sources are not: FR Is are all Class B AGN while most, but not all, FR IIs are Class A. That is, some FR II sources appear to have powerful jets and yet a rather low accretion rate.

In addition, in present theories of accretion, a rapidly-accreting supermassive black hole embedded in an elliptical bulge of stars is predicted to appear as a quasar-like object (1-2 orders of magnitude brighter than its host). For 0.1 the standard disk models of Shakura & Sunyaev (1973) appear to be most appropriate, with the optical luminosity (integrated disk emission from plasma at a temperature of < 10⁵ K) scaling in solar units as

(1)

If the black hole mass is directly related to galactic bulge luminosity, i.e., L^opt_gal 2.4 x 10¹⁰ L m₉^0.8, as suggested by Kormendy & Richstone (1995), then the ratio of accretion disk to bulge luminosity is

(2)

for > 0.1. On the other hand, a drop in accretion rate well below this value produces a much fainter ``advection-dominated'' (Narayan et al. 1998) accretion disk with bolometric luminosity dropping as L^bol_acc ², and optical luminosity dropping even faster than that. (The exact accretion rate at which this drop-off occurs [_cr ~ ²] varies steeply with the value of the viscosity parameter , which is usually taken to be ~ 0.3 for ADAF models.) Such disks are geometrically thick, optically thin, and emit mainly nonthermal radio emission. For the remainder of this paper, we will take = 0.1 to be typical of Class A quasar-like objects and = 0.01 to be typical of Class B radio cores.