The Most Massive Active Black Holes at z ~ 1.5-3.5 have High Spins and Radiative Efficiencies
Abstract
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 (M BH) to the accretion rate through the AD, \dot{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 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.
- Publication:
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The Astrophysical Journal
- Pub Date:
- July 2014
- DOI:
- 10.1088/2041-8205/789/1/L9
- arXiv:
- arXiv:1405.5877
- Bibcode:
- 2014ApJ...789L...9T
- Keywords:
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- black hole physics;
- galaxies: active;
- galaxies: nuclei;
- quasars: general;
- Astrophysics - Astrophysics of Galaxies;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- Accepted for publication in ApJL. 6 pages, 3 figures. Made minor corrections to reflect the published journal version