The evolution of SMBH spin and AGN luminosities for $z<6$ within a semi-analytic model of galaxy formation

TL;DR

This study uses an advanced semi-analytic galaxy formation model to predict SMBH spin evolution and AGN luminosity functions from redshift 0 to 6, aligning well with observations and revealing dominant accretion modes at different epochs.

Contribution

It introduces an updated SMBH evolution model integrated with GALFORM, providing new insights into SMBH spin and AGN luminosity evolution across cosmic time.

Findings

AGN luminosity function dominated by ADAF at low redshift and low luminosity.

High-redshift, high-luminosity AGN mainly fueled by thin discs or super-Eddington accretion.

SMBH spins show little evolution from redshift 0 to 6.

Abstract

Understanding how Active Galactic Nuclei (AGN) evolve through cosmic time allows us to probe the physical processes that control their evolution. We use an updated model for the evolution of masses and spins of supermassive black holes (SMBHs), coupled to the latest version of the semi-analytical model of galaxy formation GALFORM using the Planck cosmology and a high resolution Millennium style dark matter simulation to make predictions for AGN and SMBH properties for $0 < z < 6$. We compare the model to the observed black hole mass function and the SMBH versus galaxy bulge mass relation at $z=0$, and compare the predicted bolometric, hard X-ray, soft X-ray and optical AGN luminosity functions to observations at $z < 6$, and find that the model is in good agreement with the observations. The model predicts that at $z<2$ and $L_{\mathrm{bol}} < 10^{43} \mathrm{ergs^{-1}}$, the AGN luminosity function is dominated by objects accreting in an Advection Dominated Accretion Flow (ADAF) disc state, while at higher redshifts and higher luminosities the dominant contribution is from objects accreting via a thin disc or at super-Eddington rates. The model also predicts that the AGN luminosity function at $z<3$ and $L_{\mathrm{bol}} < 10^{44} \mathrm{ergs^{-1}}$ is dominated by the contribution from AGN fuelled by quiescent hot halo accretion, while at higher luminosities and higher redshifts, the AGN luminosity function is dominated by the contribution from AGN fuelled by starbursts triggered by disc instabilities. We employ this model to predict the evolution of SMBH masses, Eddington ratios, and spins, finding that the median SMBH spin evolves very little for $0<z<6$.