Spatially offset black holes in the Horizon-AGN simulation and comparison to observations

TL;DR

This study compares simulated and observed displacements of supermassive black holes from galaxy centers, revealing larger offsets in simulations due to missing stellar and dark matter dynamical friction effects.

Contribution

It introduces a detailed analysis of black hole offsets in Horizon-AGN and highlights the need to incorporate stellar and dark matter dynamical friction in simulations.

Findings

Offset fraction increases over cosmic time

Black holes tend to orbit in the galaxy plane with decaying radius

Simulated offsets are larger than observed, indicating missing physics

Abstract

We study the displacements between the centres of galaxies and their supermassive black holes (BHs) in the cosmological hydrodynamical simulation Horizon-AGN, and in a variety of observations from the literature. The BHs in Horizon-AGN feel a sub-grid dynamical friction force, sourced by the surrounding gas, which prevents recoiling BHs being ejected from the galaxy. We find that i) the fraction of spatially offset BHs increases with cosmic time, ii) BHs live on prograde orbits in the plane of the galaxy with an orbital radius that decays with time but stalls near $z=0$, and iii) the magnitudes of offsets from the galaxy centres are substantially larger in the simulation than in observations. We attribute the stalling of the infall and excessive offset magnitudes to the fact that dynamical friction from stars and dark matter is not modelled in the simulation, and hence provide a way to improve the black hole dynamics of future simulations.