The evolution of active galactic nuclei and their spins

TL;DR

This paper presents a simple yet comprehensive model for the joint evolution of massive black hole masses and spins over cosmic time, incorporating mergers, accretion, and stochastic fueling, to better understand their properties and observational signatures.

Contribution

It introduces a unified model that tracks MBH mass and spin evolution, including various growth mechanisms, and matches observed trends from redshift 6 to today.

Findings

MBH spins and radiative efficiencies decrease over cosmic time.

At z=0, spins in gas-poor galaxies peak at 0.4-0.8, with little mass dependence.

High luminosity AGN at z>1 have higher average spins.

Abstract

Massive black holes (MBHs) in contrast to stellar mass black holes are expected to substantially change their properties over their lifetime. MBH masses increase by several order of magnitude over the Hubble time, as illustrated by Soltan's argument. MBH spins also must evolve through the series of accretion and mergers events that grow the MBH's masses. We present a simple model that traces the joint evolution of MBH masses and spins across cosmic time. Our model includes MBH-MBH mergers, merger-driven gas accretion, stochastic fueling of MBHs through molecular cloud capture, and a basic implementation of accretion of recycled gas. This approach aims at improving the modeling of low-redshift MBHs and AGN, whose properties can be more easily estimated observationally. Despite the simplicity of the model, it captures well the global evolution of the MBH population from z\sim6 to today. Under our assumptions, we find that the typical spin and radiative efficiency of MBHs decrease with cosmic time because of the higher incidence of stochastic processes in gas-rich galaxies and MBH-MBH mergers in gas-poor galaxies. At z=0 the spin distribution in gas-poor galaxies peaks at spins 0.4-0.8, and it is not strongly mass dependent. MBHs in gas-rich galaxies have a more complex evolution, with low-mass MBHs at low redshift having low spins, and spins increasing at larger masses and redshifts. We also find that at z>1 MBH spins are on average highest in high luminosity AGN, while at lower redshifts these differences disappear.