The most massive active black holes at z ∼ 1.5-3.5 have high spins and radiative efficiencies

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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, . 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 showing η > 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 × 109 M) 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.

Original languageEnglish
Article numberL9
JournalAstrophysical Journal Letters
Issue number1
StatePublished - 1 Jul 2014
Externally publishedYes


  • black hole physics
  • galaxies: active
  • galaxies: nuclei
  • quasars: general

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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