THE MOST MASSIVE ACTIVE BLACK HOLES AT z ∼ 1.5-3.5 HAVE HIGH SPINS AND RADIATIVE EFFICIENCIES

The radiative efficiencies (eta) of 72 luminous unobscured active galactic nuclei at z similar to 1.5- 3.5, powered by some of the most massive black holes (BHs), are constrained. The analysis is based on accretion disk (AD) models, which link the continuum luminosity at rest- frame optical wavelengths and the BH mass (M-BH) to the accretion rate through the AD, M. AD. The data are gathered from several literature samples with detailed measurements of the H beta emission line complex, observed at near- infrared bands. When coupled with standard estimates of bolometric luminosities (L-bol), the analysis suggests high radiative efficiencies, with most of the sources showing eta > 0.2, that is, higher than the commonly assumed value of 0.1, and the expected value for non- spinning BHs (eta = 0.057). Even under more conservative assumptions regarding L-bol (i.e., L-bol = 3xL(5100)), most of the extremely massive BHs in the sample (i.e., M-BH greater than or similar to 3x10(9) M-circle dot) show radiative efficiencies which correspond to very high BH spins (a(*)), with typical values well above a(*) similar or equal to 0.7. These results stand in contrast to the predictions of a spin- downscenario, in which a series of randomly oriented accretion episodes leads to a(*) similar to 0. Instead, the analysis presented here strongly supports a spin- upscenario, which is driven by either prolonged accretion or a series of anisotropically oriented accretion episodes. Considering the fact that these extreme BHs require long- duration or continuous accretion to account for their high masses, it is argued that the most probable scenario for the super- massive black holes under study is that of an almost continuous sequence of randomly yet not isotropically oriented accretion episodes.