Bounds on Black Hole Spins
  Beam powers and black hole masses of 48 extended radio sources are combined
to obtain lower bounds on the spins and magnetic field strengths of
supermassive black holes. This is done in the context of the models of
Blandford & Znajek (1977) (the 'BZ' model) and Meier (1999); a parameterization
for bounds in the context of other models is suggested. The bounds obtained for
very powerful classical double radio sources in the BZ model are consistent
with black hole spins of order unity for sources at high redshift. The black
hole spins are largest for the highest redshift sources and decrease for
sources at lower redshift; the sources studied have redshifts between zero and
two. Lower power radio sources associated with central dominant galaxies may
have black hole spins that are significantly less than one. Combining this
analysis with other results suggests that the maximum values of black hole spin
associated with powerful radio galaxies decline from values of order unity at a
redshift of 2 to values of order 0.7 at a redshift of zero, while lower power
radio sources have spin values that range from about 0.1 to 0.8. These black
hole spin values decrease if the data are considered in the context of the
Meier model rather than the BZ model.
