3.3 Observational Advantages of the Spin Paradigm

There are several observational advantages to the spin paradigm for AGN. It lifts the degeneracy of accretion onto black holes and explains the difference between radio loud and radio quiet objects: a given central source type (m₉, ) can have a powerful jet (j -> 1) or little or no jet at all (j -> 0). Maximal Kerr holes produce powerful radio sources while Schwarzschild holes produce ``radio quiet'' objects. Even with only modest efficiency in converting the jet into particles and fields, the spent rotational energy of a black hole (E_rot 10⁶² erg m₉ j²) can easily account for the observed energy in the lobes of the most powerful sources (De Young 1975). Taking the FR I/II break to occur at a jet power of 10⁴⁴ erg s^-1 for a 10⁹ M hole (using a Bicknell flux conversion factor of = 10^-11 Hz^-1) equations (4) and (5) predict correctly that radio quasars (Class A, = 0.1, j = 0.01-1.0) will be predominantly FR II sources (10^45-49 erg s^-1), low-spin radio galaxies (Class B, = 0.01, j = 0.01-0.1) will be FR I sources (10^41-43 erg s^-1), but there should be a population of Class B sources with low accretion rate and high black hole spin ( = 0.01, j = 0.1-1.0) that produce some FR II sources without central quasars (10^43-45 erg s^-1). These are the Class B FR II sources noted by Jackson & Wall (1999).

The spin paradigm even offers an explanation for why present-day giant radio sources occur only in elliptical galaxies and why the distribution of optically-selected quasars may be bi-modal in radio power. The e-folding spindown time (E_rot / L_jet) is very short - much shorter than a cosmic evolutionary time

(7)

even with a jet active duty cycle of only 10%. As a result, all AGN should be radio quiet at the present epoch, their black holes having spun down when the universe was very young. In order to continually produce radio sources up to the present epoch, there must be periodic input of significant amounts of angular momentum from accreting stars and gas, or from a merger with another supermassive black hole (see also Wilson & Colbert 1995). Such activity is triggered most easily by violent events such as galaxy mergers. Since only elliptical galaxies undergo significant mergers (the merging process is believed to be responsible for their elliptical shape), only ellipticals are expected to be giant radio sources in the present epoch. With little merger activity, spiral galaxies are expected to be relatively radio quiet. Since merging and non-merging galaxies will fuel and re-kindle their black hole's spin in very different ways, bi-modality in the radio luminosity distribution is to be expected.