Orbital orientation evolution of massive binary black holes at the centres of non-spherical galaxies

Abstract

At the centre of a spherical and kinematically isotropic galaxy, the orientation of a massive binary black hole (BBH) orbit (i.e. the direction of the BBH orbital angular momentum) undergoes a random walk. If the stars in a spherical system have non-zero total angular momentum, the BBH orbital orientation evolves towards aligning with the total stellar angular momentum direction. In this article, we show that a triaxial galaxy has an alignment-erasing effect, i.e. the alignment of the BBH orientation towards the galaxy rotation axis can be decreased significantly or erased. We also show that in a non-rotating axisymmetric galaxy the BBH orbital orientation evolves towards the axisymmetric axis and precesses about it in a retrograde direction. Our results provide a step towards understanding the spin orientation of the final merged black hole (and hence the probable orientation of any jet produced) within its host galaxy and may help to constrain the recoil velocity of the merged black hole arising from gravitational wave radiation as well.