Rotating Supermassive Black Holes (7) ...

Before leaving this introduction to the unified picture of AGNs and quasars, let us briefly address an unresolved issue. Why are some AGNs radio loud and some radio quiet? Since we believe that RF emission is typically associated with jets, a closely related question is that of why some AGNs have strong jets and some have either no jets or only weak ones. The answers to these questions are not known completely, but we present some ideas that are at least plausible.

Significance of Angular Momentum
It has been proposed that there may be three fundamental parameters that govern the type of AGN that we see:

1. The mass of the central black hole engine
2. The rate of mass accretion
3. The angular momentum of the central source (its magnitude and its orientation relative to the observer)

The significance of the magnitude of the angular momentum is twofold. First, the orbital motion of charged particles around the black hole determines whether electromagnetic effects are large enough to form and collimate relativistic jets of particles. High velocity revolution of plasma would increase the electromagnetic effects and make jets more likely. Second, if jets are formed, rapid rotation of the black hole engine leads to a gyroscopic stabilizing force that keeps the jets pointed in the same direction. (Since some jets are nearly straight over distances of millions of light years, the direction that the jet source points must be almost constant for millions of years.)

An Hypothesis
We may then hypothesize that

  • Lower-mass, higher-spin black holes favor the formation of low luminosity radio sources lying near the galactic nucleus.
  • Higher-mass, lower-spin black holes do not favor the production of jets and lead to radio quiet AGNs.
  • Higher-mass, higher-spin black holes produce strong jets and luminous, radio loud AGNs.

    • Although these are plausible ideas, we stress that it is not yet certain that the understanding of jets and whether AGNs are radio loud can be reduced to such simple terms.