  In vacuum, the gravitational recoil of the final black hole from the merger
of two black holes depends exclusively on the mass ratio and spins of the
coalescing black holes, and on the eccentricity of the binary. If matter is
present, accretion by the merging black holes may modify significantly their
masses and spins, altering both the dynamics of the binary and the
gravitational recoil of the remnant black hole. This paper considers such
scenario. We investigate the effects on the kick of the final black hole from
immersing the binary in a scalar field cloud. We consider two types of
configurations: one with non-spinning and unequal-mass black holes, and a
second with equal mass and spinning holes. For both types, we investigate how
the gravitational recoil of the final black hole changes as we vary the energy
density of the scalar field. We find that the accretion of scalar field by the
merging black holes could have a profound effect. For the non-spinning,
unequal-mass binary black holes, the kicks are in general larger than in the
vacuum case, with speeds of approximately 1,200 km/s for binaries with mass
ratio 2:1, one order of magnitude larger than in vacuum. For equal mass,
binaries with black holes with spins aligned with the orbital angular momentum,
kicks larger than in vacuum are also found. For systems with spins in the
super-kick configuration, the scalar field triggers a similar dependence of the
kicks with the entrance angle at merger as in the vacuum case but in this case
depending on the strength of the scalar field.
