Black hole spin evolution through hydrodynamical cosmological simulations including AGN feedback. (arXiv:1304.4583v1 [astro-ph.CO])

Black hole spin evolution through hydrodynamical cosmological simulations including AGN feedback. (arXiv:1304.4583v1 [astro-ph.CO]):
Supermassive black holes (BH) at the centres of galaxies can rapidly change
their mass and spin by gas accretion and mergers. Using hydrodynamical
cosmological simulations, with prescriptions for BH growth and feedback from
Active Galactic Nuclei, we study the evolution of BH spins and we highlight the
mechanisms responsible for driving the amplitude and the direction of spins as
a function of cosmic time. We find that in the high-redshift universe, galaxies
maintain large values of gas accretion onto BHs. BHs tend to align their spins
with the angular momentum of the surrounding gas and maximise their amplitude.
On the opposite, at low redshift, as BHs get more massive, the contribution
from binary coalescence to the total BH mass growth increases and tends to
decrease the amplitude of spins and change their direction.