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

摘要

The radiative efficiencies (η) of 72 luminous unobscured active galactic nuclei at z ～ 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 (MBH) to the accretion rate through the AD,M AD. The data are gathered from several literature samples with detailed measurements of the Hβ 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 showingη > 0.2, that is, higher than the commonly assumed value of 0.1, and the expected value for non-spinning BHs (η = 0.057). Even under more conservative assumptions regarding L_(bol) (i.e., L_(bol) = 3×L_(5100)), most of the extremely massive BHs in the sample (i.e.,M_(BH) > 3×10~9M_⊙) show radiative efficiencies which correspond to very high BH spins (a_?), with typical values well above a_? 0.7. These results stand in contrast to the predictions of a "spin-down" scenario, in which a series of randomly oriented accretion episodes leads to a_? ～ 0. Instead, the analysis presented here strongly supports a "spin-up" scenario, 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.