Gravitational recoils of supermassive black holes in hydrodynamical simulations of gas rich galaxies

TL;DR

This study uses high-resolution hydrodynamical simulations to explore how gas-rich galaxy environments influence the dynamics, accretion, and observational signatures of recoiling supermassive black holes, revealing complex interactions and implications for galaxy evolution.

Contribution

It demonstrates the impact of gaseous discs on black hole recoil dynamics, accretion rates, and their effects on galaxy-BH relationships, providing new insights into high-redshift galaxy evolution.

Findings

Gaseous discs shorten BH return timescales after recoil.

Recoiled BHs can have high, variable accretion rates detectable as off-centred AGN.

Recoil increases scatter in BH mass–host galaxy relations.

Abstract

We study the evolution of gravitationally recoiled supermassive black holes (BHs) in massive gas-rich galaxies by means of high-resolution hydrodynamical simulations. We find that the presence of a massive gaseous disc allows recoiled BHs to return to the centre on a much shorter timescale than for purely stellar discs. Also, BH accretion and feedback can strongly modify the orbit of recoiled BHs and hence their return timescale, besides affecting the distribution of gas and stars in the galactic centre. However, the dynamical interaction of kicked BHs with the surrounding medium is in general complex and can facilitate both a fast return to the centre as well as a significant delay. The Bondi-Hoyle-Lyttleton accretion rates of the recoiling BHs in our simulated galaxies are favourably high for the detection of off-centred AGN if kicked within gas-rich discs -- up to a few per cent of the Eddington accretion rate -- and are highly variable on timescales of a few 10^7 yrs. In major merger simulations of gas-rich galaxies, we find that gravitational recoils increase the scatter in the BH mass -- host galaxy relationships compared to simulations without kicks, with the BH mass being more sensitive to recoil kicks than the bulge mass. A generic result of our numerical models is that the clumpy massive discs suggested by recent high-redshift observations, as well as the remnants of gas-rich mergers, exhibit a gravitational potential that falls steeply in the central regions, due to the dissipative concentration of baryons. As a result, supermassive BHs should only rarely be able to escape from massive galaxies at high redshifts, which is the epoch where the bulk of BH recoils is expected to occur.[Abridged]