Simulations of Recoiling Massive Black Holes in the Via Lactea Halo
  The coalescence of a massive black hole (MBH) binary leads to the
gravitational-wave recoil of the system and its ejection from the galaxy core.
We have carried out N-body simulations of the motion of a MBH = 3.7x10^6 Msun
MBH remnant in the Via Lactea I simulation, a Milky Way sized dark matter halo.
The black hole receives a recoil velocity of Vkick = 80, 120, 200, 300, and 400
km/s at redshift 1.5, and its orbit is followed for over 1 Gyr within a live
host halo, subject only to gravity and dynamical friction against the dark
matter background. We show that, owing to asphericities in the dark matter
potential, the orbit of the MBH is hightly non-radial, resulting in a
significantly increased decay timescale compared to a spherical halo. The
simulations are used to construct a semi-analytic model of the motion of the
MBH in a time-varying triaxial Navarro-Frenk-White dark matter halo plus a
spherical stellar bulge, where the dynamical friction force is calculated
directly from the velocity dispersion tensor. Such a model should offer a
realistic picture of the dynamics of kicked MBHs in situations where gas drag,
friction by disk stars, and the flattening of the central cusp by the returning
black hole are all negligible effects. We find that MBHs ejected with initial
recoil velocities Vkick > 500 km/s do not return to the host center within
Hubble time. In a Milky Way-sized galaxy, a recoiling hole carrying a gaseous
disk of initial mass ~MBH may shine as a quasar for a substantial fraction of
its wandering phase. The long decay timescales of kicked MBHs predicted by this
study may thus be favorable to the detection of off-nuclear quasar activity.
