The cosmic evolution of massive black holes in the Horizon-AGN simulation

TL;DR

This paper uses the Horizon-AGN simulation to study the evolution and demographics of massive black holes, revealing their growth, feedback effects, and observational properties across cosmic time.

Contribution

It provides a comprehensive analysis of black hole evolution in a large cosmological simulation, matching multiple observational constraints and exploring dual AGN phenomena.

Findings

Black holes follow observed mass functions and redshift evolution.

Dual AGN are more common at higher redshifts and decrease over time.

Accretion rates and Eddington ratios align with observational data.

Abstract

We analyse the demographics of black holes (BHs) in the large-volume cosmological hydrodynamical simulation Horizon-AGN. This simulation statistically models how much gas is accreted onto BHs, traces the energy deposited into their environment and, consequently, the back-reaction of the ambient medium on BH growth. The synthetic BHs reproduce a variety of observational constraints such as the redshift evolution of the BH mass density and the mass function. Strong self-regulation via AGN feedback, weak supernova feedback, and unresolved internal processes result in a tight BH-galaxy mass correlation. Starting at z~2, tidal stripping creates a small population of BHs over-massive with respect to the halo. The fraction of galaxies hosting a central BH or an AGN increases with stellar mass. The AGN fraction agrees better with multi-wavelength studies, than single-wavelength ones, unless obscuration is taken into account. The most massive halos present BH multiplicity, with additional BHs gained by ongoing or past mergers. In some cases, both a central and an off-centre AGN shine concurrently, producing a dual AGN. This dual AGN population dwindles with decreasing redshift, as found in observations. Specific accretion rate and Eddington ratio distributions are in good agreement with observational estimates. The BH population is dominated in turn by fast, slow, and very slow accretors, with transitions occurring at z=3 and z=2 respectively.