  Studies of black hole spin and other parameters as a function of redshift
provide information about the physical state and merger and accretion histories
of the systems. One way that black hole spin may be estimated is through
observations of extended radio sources. These sources, powered by outflows from
an AGN, allow the beam power and total outflow energy to be studied. In a broad
class of models, the beam power of the outflow is related to the spin of the
black hole. This relationship is used to estimate black hole spins for 55 radio
sources. The samples studied include 7 FRII quasars and 19 FRII radio galaxies
with redshifts between 0.056 and 1.79, and 29 radio sources associated with CD
galaxies with redshifts between 0.0035 and 0.291. The FRII sources studied have
estimated spin values of between about 0.2 and 1; there is a range of values at
a given redshift, and the values tend to increase with increasing redshift.
Results obtained for FRII quasars are very similar to those obtained for FRII
galaxies. A broader range of spin values are obtained for the sample of radio
sources associated with CD galaxies studied. The fraction of the spin energy
extracted per outflow event is estimated and ranges from about 0.03 to 0.5 for
FRII sources and 0.002 to about 1 for radio sources associated with CD
galaxies; the data are consistent with this fraction being independent of
redshift though the uncertainties are large. The results obtained are
consistent with those predicted by numerical simulations that track the merger
and accretion history of AGN, supporting the idea that, for AGN with powerful
large-scale outflows, beam power is directly related to black hole spin.
