Evidence for Rapidly Spinning Black Holes in Quasars

It has long been believed that accretion onto supermassive black holes powers quasars, but there has been relatively few observational constraints on the spins of the black holes. We address this problem by estimating the average radiative efficiencies of a large sample of quasars selected from the Sloan Digital Sky Survey, by combining their luminosity function and their black hole mass function. Over the redshift interval $0.4<z<2.1$, we find that quasars have average radiative efficiencies of $\sim 30% - 35%$, strongly suggesting that their black holes are rotating very fast, with specific angular momentum $a \approx 1$, which stays roughly constant with redshift. The average radiative efficiency could be reduced by a factor of $\sim$2, depending on the adopted zeropoint for the black hole mass scale. The inferred large spins and their lack of significant evolution are in agreement with the predictions of recent semi-analytical models of hierarchical galaxy formation if black holes gain most of their mass through accretion.