  The orbital decay of massive black holes in galaxies in the aftermath of
mergers is at the heart of whether massive black holes successfully pair and
merge, leading to emission of low-frequency gravitational waves. The role of
dynamical friction sourced from the gas distribution has been uncertain because
many analytical and numerical studies have either focussed on a homogeneous
medium or have not reached resolutions below the scales relevant to the
problem, namely the Bondi-Hoyle-Lyttleton radius. We performed numerical
simulations of a massive black hole moving in a turbulent medium in order to
study dynamical friction from turbulent gas. We find that the black hole slows
down to the sound speed, rather than the turbulent speed, and that the orbital
decay is well captured if the Bondi-Hoyle-Lyttleton radius is resolved with at
least five resolution elements. We find that the larger the turbulent eddies,
the larger the scatter in dynamical friction magnitude, because of the
stochastic nature of the problem, and also because of the larger over- and
under-densities encountered by the black hole along its trajectory. Compared to
the classic solution in a homogeneous medium, the magnitude of the force
depends more weakly on the Mach number, and dynamical friction is overall more
efficient for high Mach numbers, but less efficient towards and at the
transonic regime.
