The Most Massive Active Black-Holes at z~1.5-3.5 Have High Spins and Radiative Efficiencies

TL;DR

This study constrains the radiative efficiencies of luminous, massive AGNs at redshifts 1.5-3.5, revealing high black hole spins and efficiencies that support prolonged, anisotropic accretion scenarios over spin-down models.

Contribution

It provides the first evidence that the most massive black holes at high redshift have high spins and efficiencies, supporting a spin-up accretion model.

Findings

Most sources show radiative efficiencies > 0.2.

High efficiencies imply high black hole spins (> 0.7).

Results favor prolonged, anisotropic accretion over spin-down scenarios.

Abstract

The radiative efficiencies ($\eta$) of 72 luminous unobscured Active Galactic Nuclei (AGNs) at $z\sim1.5-3.5$, 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 black hole mass ($M_{\rm BH}$) to the accretion rate through the AD, $\dot{M}_{\rm AD}$. The data are gathered from several literature samples with detailed measurements of the ${\rm H}\beta$ emission line complex, observed at near-IR bands. When coupled with standard estimates of bolometric luminosities ($L_{\rm bol}$), 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 $L_{\rm bol}$ (i.e., $L_{\rm bol}=3\times \lambda L_{\lambda}$[5100$\AA$]), most of the extremely massive BHs in the sample (i.e., $M_{\rm BH} > 3\times10^9\,M_{\rm \odot}$) show radiative efficiencies which correspond to very high BH spins ($a_{\rm *}$), with typical values well above $a_{\rm *}\simeq0.7$. These results stand in contrast to the predictions of a "spin-down"scenario, in which a series of randomly-oriented accretion episodes lead to $a_{\rm *}\simeq0$. 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 continuous accretion to account for their high masses, it is argued that the most probable scenario for the SMBHs under study is that of an almost continuous sequence of randomly- yet not isotropically-oriented accretion episodes.