Black hole evolution: III. Statistical properties of mass growth and spin evolution using large-scale hydrodynamical cosmological simulations

TL;DR

This study uses large-scale hydrodynamical cosmological simulations to analyze how supermassive black holes grow in mass and evolve in spin over cosmic time, highlighting the roles of gas accretion and mergers.

Contribution

It provides a detailed semi-analytical model of black hole spin evolution and quantifies the relative impact of gas accretion and mergers across different cosmic epochs.

Findings

High-redshift galaxies exhibit large gas accretion rates driving black hole growth.

Cold gas accretion at high redshift aligns black hole spins with gas angular momentum.

At low redshift, mergers dominate mass growth, reducing spin magnitude and altering spin direction.

Abstract

Supermassive black holes (BH) at the centres of galaxies can rapidly change their mass and spin by gas accretion and mergers. Using hydrodynamical cosmological simulations, with prescriptions for BH growth and feedback from Active Galactic Nuclei, we study how the evolution of BH mass growth is driven by gas accretion and mergers. Using a semi-analytical approach to evolve spins, we also highlight the mechanisms responsible for driving the magnitude and the direction of spins as a function of cosmic time. We find that in the high-redshift universe galaxies maintain large values of gas accretion onto BHs, which therefore is the main driver of their mass and spin evolution. Sustained accretion of cold gas at high-redshift tends to align BH spins with the angular momentum of the surrounding gas and maximise their magnitude. Conversely, at low redshift, as BHs get more massive and galaxies more gas-poor, the contribution from binary coalescences to the total BH mass growth increases, especially at the high-mass end, and tends to decrease the magnitude of spins and change their direction.