The alignment of disk and black hole spins in active galactic nuclei

The inner parts of an accretion disk around a spinning black hole are forced to align with the spin of the hole by the Bardeen-Petterson effect. Assuming that any jet produced by such a system is aligned with the angular momentum of either the hole or the inner disk, this can, in principle, provide a mechanism for producing steady jets in active galactic nuclei (AGNs) whose direction is independent of the angular momentum of the accreted material. However, the torque that aligns the inner disk with the hole, also by Newton's third law, tends to align the spin of the hole with the outer accretion disk. In this Letter, we calculate this alignment timescale, t(align), for a black hole powering an AGN, and we show that it is relatively short. This timescale is typically much less than the derived ages for jets in radio-loud AGNs and implies that the jet directions, in general, are not controlled by the spin of the black hole. We speculate that the jet directions are most likely controlled either by the angular momentum of the accreted material or by the gravitational potential of the host galaxy.