Episodic Random Accretion and the Cosmological Evolution of Supermassive Black Hole Spins

TL;DR

This paper investigates the evolution of supermassive black hole spins through radiative efficiency, revealing a decline from high to low redshift, and suggests episodic accretion reduces spin over cosmic time, challenging previous assumptions about jet formation.

Contribution

It introduces a formalism linking radiative efficiency to observable quasar properties, demonstrating cosmological evolution of black hole spin and its implications for galaxy evolution.

Findings

Radiative efficiency decreases from ~0.3 at z~2 to ~0.03 at z~0

Black hole spin evolution is driven by episodic, random accretion

Black hole growth history aligns with cosmic star formation history

Abstract

The growth of supermassive black holes (BHs) located at the centers of their host galaxies comes mainly from accretion of gas, but how to fuel them remains an outstanding unsolved problem in quasar evolution. This issue can be elucidated by quantifying the radiative efficiency parameter ($\eta$) as a function of redshift, which also provides constraints on the average spin of the BHs and its possible evolution with time. We derive a formalism to link $\eta$ with the luminosity density, BH mass density, and duty cycle of quasars, quantities we can estimate from existing quasar and galaxy survey data. We find that $\eta$ has a strong cosmological evolution: at z~2, $\eta \approx 0.3$, and by $z\approx 0$ it has decreased by an order of magnitude, to $\eta\approx 0.03$. We interpret this trend as evolution in BH spin, and we appeal to episodic, random accretion as the mechanism for reducing the spin. The observation that the fraction of radio-loud quasars decreases with increasing redshift is inconsistent with the popular notion that BH spin is a critical factor for generating strong radio jets. In agreement with previous studies, we show that the derived history of BH accretion closely follows the cosmic history of star formation, consistent with other evidence that BHs and their host galaxies coevolve.