  We compute the dynamical friction on a small perturber moving through an
inviscid fluid, i.e., a superfluid. Crucially, we account for the tachyonic
gravitational mass for sound waves, reminiscent of the Jeans instability of the
fluid, which results in non-zero dynamical friction even for subsonic
velocities. Moreover, we illustrate that the standard leading order effective
theory in the derivative expansion is in general inadequate for analysing
supersonic processes. We show this in two ways: (i) with a fluid treatment,
where we solve the linearized hydrodynamical equations coupled to Newtonian
gravity; and (ii) with a quasiparticle description, where we study the energy
dissipation of a moving perturber due to phonon radiation. Ordinarily a
subsonic perturber moving through a superfluid is kinematically prohibited from
losing energy, however the Jeans instability modifies the dispersion relation
of the fluid which can result in a small but non-vanishing dynamical friction
force. We also analyse the soft phonon bremsstrahlung by a subsonic perturber
scattered off an external field.
