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

Abstract

By performing three-dimensional hydrodynamical (3D MHD) simulations of a galaxy in an isolated dark matter halo, we are able to trace 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 combined super-Eddington accretion with sub-Eddington phases (quasar and radio modes) and followed the BH spin evolution. Due to the low frequency of the 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 ≲ 0.3, assuming the super-Eddington feedback is consistently weak. Finally, BHs starting with a low spin magnitude are able to grow to the highest mass and if they initially start out as being misaligned with the galactic disc, they get a small boost of accretion via retrograde accretion.