A new approach to constraining or determining the spin of a massive black hole is proposed. A key parameter in the analysis is the dimensionless ratio, r, of the energy released to the mass of the hole. It is shown that the black hole spin j may be written as a function of the dimensionless ratio of the black hole spin energy to the black hole mass. When extraction of the black hole spin energy powers an outflow from the hole, the ratio r provides an indication of the black hole spin. The method is applied to a sample of 19 very powerful classical double radio galaxies with a range of size and redshift. The sources are found to have remarkably similar values of r and j, implying that the sources have very similar physical conditions at the time the outflow is generated. The weighted mean value of the spins is 0.12 ±0.01. A sample of 29 central dominant galaxies is also studied; most of these sources have amorphous or FRI radio structure. The sources have a broad range of ratios r, and correspondingly broad range of black hole spins j with values from about 0.001 to 0.4, and a median value of about 0.03. The broad range probably results from the fact that each source is observed at a different stage in its lifetime. The intrinsic range of parameters could be as tight as it is for the powerful classical double radio galaxies.
black hole physics - galaxies: active - galaxies: nuclei