  The power of jets from black holes are expected to depend on both the spin of
the black hole and the structure of the accretion disk in the region of the
last stable orbit. We investigate these dependencies using two different
physical models for the jet power: the classical Blandford-Znajek (BZ) model
and a hybrid model developed by Meier. In the BZ case, the jets are powered by
magnetic fields directly threading the spinning black hole while in the hybrid
model, the jet energy is extracted from both the accretion disk as well as the
black hole via magnetic fields anchored to the accretion flow inside and
outside the hole's ergosphere. The hybrid model takes advantage of the
strengths of both the Blandford-Payne and BZ mechanisms, while avoiding the
more controversial features of the latter. We develop these models more fully
to account for general relativistic effects and to focus on advection-dominated
accretion flows (ADAF) for which the jet power is expected to be a significant
fraction of the accreted rest mass energy.
  We apply the models to elliptical galaxies, in order to see if these models
can explain the observed correlation between the Bondi accretion rates and the
total jet powers. For typical values of the disk viscosity parameter
alpha~0.04-0.3 and mass accretion rates consistent with ADAF model
expectations, we find that the observed correlation requires j>0.9; i.e., it
implies that the black holes are rapidly spinning. Our results suggest that the
central black holes in the cores of clusters of galaxies must be rapidly
rotating in order to drive jets powerful enough to heat the intracluster medium
and quench cooling flows.
