Subsonic accretion and dynamical friction for a black hole moving
  through a self-interacting scalar dark matter cloud
  We investigate the flow around a black hole moving through a cloud of
self-interacting scalar dark matter. We focus on the large scalar mass limit,
with quartic self-interactions, and on the subsonic regime. We show how the
scalar field behaves as a perfect gas of adiabatic index $\gamma_{\rm ad}=2$ at
large radii while the accretion rate is governed by the relativistic regime
close to the Schwarzschild radius. We obtain analytical results thanks to
large-radius expansions, which are also related to the small-scale relativistic
accretion rate. We find that the accretion rate is greater than for
collisionless particles, by a factor $c/c_s \gg 1$, but smaller than for a
perfect gas, by a factor $c_s/c \ll 1$, where $c_s$ is the speed of sound. The
dynamical friction is smaller than for a perfect gas, by the same factor $c_s/c
\ll 1$, and also smaller than Chandrasekhar's result for collisionless
particles, by a factor $c_s/(cC)$, where $C$ is the Coulomb logarithm. It is
also smaller than for fuzzy dark matter, by a factor $v_0/c \ll 1$.
