Secular Dynamical Anti-Friction in Galactic Nuclei
  We identify a gravitational-dynamical process in near-Keplerian potentials of
galactic nuclei that occurs when an intermediate-mass black hole (IMBH) is
migrating on an eccentric orbit through the stellar cluster towards the central
supermassive black hole (SMBH). We find that, apart from conventional dynamical
friction, the IMBH experiences an often much stronger systematic torque due to
the secular (i.e., orbit-averaged) interactions with the cluster's stars. The
force which results in this torque is applied, counterintuitively, in the same
direction as the IMBH's precession and we refer to its action as
"secular-dynamical anti-friction" (SDAF). We argue that SDAF, and not the
gravitational ejection of stars, is responsible for the IMBH's eccentricity
increase seen in the initial stages of previous N-body simulations. Our
numerical experiments, supported by qualitative arguments, demonstrate that (1)
when the IMBH's precession direction is artificially reversed, the torque
changes sign as well, which decreases the orbital eccentricity, (2) the rate of
eccentricity growth is sensitive to the IMBH migration rate, with zero
systematic eccentricity growth for an IMBH whose orbit is artificially
prevented from inward migration, and (3) SDAF is the strongest when the central
star cluster is rapidly rotating. This leads to eccentricity growth/decrease
for the clusters rotating in the opposite/same direction relative to the IMBH's
orbital motion.
