Evidence for cosmic evolution in the spin of the most massive black holes

TL;DR

This paper investigates the evolution of the spin of the most massive black holes over cosmic time, using simulations and observational data to show that black hole spins tend to be higher at lower redshifts.

Contribution

It introduces a method to estimate the cosmic spin history of supermassive black holes based on jet and accretion power ratios, linking simulation results with observational data.

Findings

Black hole spins increase from z~2 to z~0.

Typical spins are a~0.35-0.95 at z~0 and a~0.0-0.25 at z~2.

The evolution suggests a greater role of accretion in black hole growth at lower redshifts.

Abstract

We use results from simulations of the production of magnetohydrodynamic jets around black holes to derive the cosmic spin history of the most massive black holes. We assume that the efficiency of jet production is a monotonic function of spin a, as given by the simulations, and that the accretion flow geometry is similarly thick for quasars accreting close to the Eddington ratio and for low-excitation radio galaxies accreting at very small Eddington rates. We use the ratio of the comoving densities of the jet power and the radiated accretion power associated with supermassive black holes with Mbh>~10^8 Msol to estimate the cosmic history of the characteristic spin a. The evolution of this ratio, which increases with decreasing z, is consistent with a picture where the z~0 active galactic nuclei have typically higher spins than those at z~2 (with typical values a~0.35-0.95 and a~0.0-0.25 respectively). We discuss the implications in terms of the relative importance of accretion and mergers in the growth of supermassive black holes with Mbh>~10^8 Msol.