Resonant Dynamical Friction in Nuclear Star Clusters: Rapid Alignment of
  an Intermediate-mass Black Hole with a Stellar Disk
  We investigate the dynamical evolution of an intermediate-mass black hole
(IMBH) in a nuclear star cluster hosting a supermassive black hole (SMBH) and
both a spherical and a flattened disk-like distribution of stellar-mass
objects. We use a direct N-body (phiGPU) and an orbit-averaged (N-ring)
numerical integrator to simulate the orbital evolution of stars and the IMBH.
We find that the IMBH's orbit gradually aligns with the stellar disk if their
mutual initial inclination is less than 90 degree. If it is larger than 90
degree, i.e. counterrotating, the IMBH does not align. Initially, the rate of
orbital reorientation increases linearly with the ratio of the mass of the IMBH
over the SMBH mass and it is orders of magnitude faster than ordinary (i.e.
Chandrasekhar) dynamical friction, particularly for high SMBH masses. The
semimajor axes of the IMBH and the stars are approximately conserved. This
suggests that the alignment is predominantly driven by orbit-averaged
gravitational torques of the stars, a process which may be called resonant
dynamical friction. The stellar disk is warped by the IMBH, and ultimately
increases its thickness. This process may offer a test for the viability of
IMBH candidates in the Galactic Center. Resonant dynamical friction is not
limited to IMBHs; any object much more massive than disk particles may
ultimately align with the disk. This may have implications for the formation
and evolution of black hole disks in dense stellar systems and gravitational
wave source populations for LIGO, VIRGO, KAGRA, and LISA.
