Evidence for cosmic evolution in the spin of the most massive black
  holes
  We use results from simulations of the production of magnetohydrodynamic jets
around black holes to derive the cosmic spin history of the most massive black
holes. We assume that the efficiency of jet production is a monotonic function
of spin a, as given by the simulations, and that the accretion flow geometry is
similarly thick for quasars accreting close to the Eddington ratio and for
low-excitation radio galaxies accreting at very small Eddington rates. We use
the ratio of the comoving densities of the jet power and the radiated accretion
power associated with supermassive black holes with Mbh>~10^8 Msol to estimate
the cosmic history of the characteristic spin a. The evolution of this ratio,
which increases with decreasing z, is consistent with a picture where the z~0
active galactic nuclei have typically higher spins than those at z~2 (with
typical values a~0.35-0.95 and a~0.0-0.25 respectively). We discuss the
implications in terms of the relative importance of accretion and mergers in
the growth of supermassive black holes with Mbh>~10^8 Msol.
