Massive black hole binary plane reorientation in rotating stellar systems

We study the evolution of the orientation of the orbital plane of massive black hole binaries (BHBs) in rotating stellar systems in which the total angular momentum of the stellar cusp is misaligned with respect to that of the binary. We compare results from direct summation N-body simulations with predictions from a simple theoretical model. We find that the same encounters between cusp stars and the BHBs that are responsible for the hardening and eccentricity evolution of the binary lead to a reorientation of the binary orbital plane. In particular, binaries whose angular momentum is initially misaligned with respect to that of the stellar cusp tend to realign their orbital planes with the angular momentum of the cusp on a time-scale of a few hardening times. This is due to angular momentum exchange between stars and the BHBs during close encounters, and may have important implications for the relative orientation of host galaxies and radio jets