Cosmological Evolution of Supermassive Black Holes: Mass Functions and Spins

TL;DR

This paper models the evolution of supermassive black holes' mass functions and spins over cosmic time, revealing a transition from spin-up to spin-down phases driven by different accretion regimes.

Contribution

It introduces a novel framework linking SMBH mass functions, luminosity functions, and spin evolution, highlighting a mass-dependent transition in accretion modes.

Findings

SMBH mass function derived for 0<z<2.

Identifies two distinct accretion regimes affecting spins.

Shows transition epoch varies with black hole mass.

Abstract

We derive the mass function of supermassive black holes (SMBHs) over the redshift range 0<z<2, using the latest deep luminosity and mass functions of field galaxies. Applying this mass function, combined with the bolometric luminosity function of active galactic nuclei (AGNs), into the the continuity equation of SMBH number density, we explicitly obtain the mass-dependent cosmological evolution of the radiative efficiency for accretion. We suggest that the accretion history of SMBHs and their spins evolve in two distinct regimes: an early phase of prolonged accretion, plausibly driven by major mergers, during which the black hole spins up, then switching to a period of random, episodic accretion, governed by minor mergers and internal secular processes, during which the hole spins down. The transition epoch depends on mass, mirroring other evidence for "cosmic downsizing" in the AGN population.