Dynamical friction force as a function of the Mach number, i.e. the velocity in units of the sound speed. The dashed lines indicated the dynamical friction for a collisionless medium according to the Chandrasekhar formula (Chandrasekhar 1943) and the solid lines are for a gaseous medium as given by Ostriker (1999). We note that for the plot, we assume r_max = ℳ c_s t and ln(r_max/r_min) = ln(ℳ)+C. The line colours correspond to different values of C ∈ {4.0, 6.0, 8.0, 10.0, 12.0}. In addition, the red dotted lines indicate the velocity of a secondary BH moving on a circular orbit for different adiabatic indices.