Rapidly Spinning Black Holes in Quasars: An Open Question

Wang et al. (2006) estimated an average radiative efficiency of 30%–35% for quasars at moderate redshift. We find that their method is not independent of quasar lifetimes and thus that quasars do not necessarily have such high efficiencies. Nonetheless, it is possible to derive interrelated constraints on quasar lifetimes, Eddington ratios, and radiative efficiencies of supermassive black holes. We derive such constraints using a statistically complete sample of quasars with black hole mass estimates from broad Mg II. For quasars with L/LEdd & 1, lifetimes can range from 60 Myr for the nonrotating case to 460 Myr for the maximally rotating case. Coupled with observed black hole masses, long quasar lifetimes would imply high radiative efficiencies while quasar lifetimes of ≤60 Myr would imply that radiatively inefficient accretion must be important in the accretion history of quasars. Subject headings: black hole physics — quasars: general