Recoiling Massive Black Holes in Gas-Rich Galaxy Mergers
  The asymmetric emission of gravitational waves produced during the
coalescence of a massive black hole (MBH) binary imparts a velocity "kick" to
the system that can displace the hole from the center of its host. Here we
study the trajectories and observability of MBHs recoiling in three (one major,
two minor) gas-rich galaxy merger remnants that were previously simulated at
high resolution, and in which the pairing of the MBHs had been shown to be
successful. We run new simulations of MBHs recoiling in the major merger
remnant with Mach numbers in the range 1<M<6, and use simulation data to
construct a semi-analytical model for the orbital evolution of MBHs in gas-rich
systems. We show that: 1) in major merger remnants the energy deposited by the
moving hole into the rotationally supported, turbulent medium makes a
negligible contribution to the thermodynamics of the gas. This contribution
becomes significant in minor merger remnants, potentially allowing for an
electromagnetic signature of MBH recoil; 2) in major merger remnants, the
combination of both deeper central potential well and drag from high-density
gas confines even MBHs with kick velocities as high as 1200 km/s within 1 kpc
from the host's center; 3) kinematically offset nuclei may be observable for
timescales of a few Myr in major merger remnants in the case of recoil
velocities in the range 700-1,000 km/s; 4) in minor mergers remnants the effect
of gas drag is weaker, and MBHs with recoil speeds in the range 300-600 km/s
will wander through the host halo for longer timescales. When accounting for
the probability distribution of kick velocities, however, we find that the
likelihood of observing recoiling MBHs in gas-rich galaxy mergers is very low,
typically below 10^-5 - 10^-6.
