How the super-Eddington regime affects black hole spin evolution in high-redshift galaxies

TL;DR

This study uses hydrodynamical simulations to explore how super-Eddington accretion phases influence black hole spin evolution in high-redshift galaxies, revealing complex feedback effects and growth patterns.

Contribution

It provides a combined analysis of super- and sub-Eddington accretion impacts on black hole spin, introducing an analytical model for spin evolution under weak super-Eddington feedback.

Findings

Super-Eddington episodes are too infrequent to significantly decrease BH spin.

Thin disc accretion in quasar mode tends to increase BH spin.

Low initial spin BHs can grow to high mass and gain a small boost from retrograde accretion.

Abstract

By performing three-dimensional hydrodynamical simulations of a galaxy in an isolated dark matter halo, we follow the evolution of the spin parameter $a$ of a black hole (BH) undergoing super-Eddington phases throughout its growth. This regime, suspected to be accompanied by powerful jet outflows, is expected to decrease the BH spin magnitude. We combine super-Eddington accretion with sub-Eddington phases (quasar and radio modes) and follow the BH spin evolution. Due to the low frequency of super-Eddington episodes, relativistic jets in this regime are not able to decrease the magnitude of the spin effectively, as thin disc accretion in the quasar mode inevitably increases the BH spin. The combination of super- and sub-Eddington accretion does not lead to a simple explicit expression for the spin evolution because of feedback from super-Eddington events. An analytical expression can be used to calculate the evolution for $a\lesssim0.3$, assuming the super-Eddington feedback is consistently weak. Finally, BHs starting with low spin magnitude are able to grow to the highest mass, and if they initially start misaligned with the galactic disc, they get a small boost of accretion through retrograde accretion.