  We study the phenomenon of quantum friction in a system consisting of a
polarizable atom moving at a constant speed parallel to a metallic plate. The
metal is described using a charged hydrodynamic model for the electrons. This
model featuring long-range interactions is appropriate for a clean metal in a
temperature range where scattering due to Coulomb interactions dominates over
the scattering of electron by impurities. We find that a quantum friction force
between the atom and the metal surface exists even in the absence of intrinsic
damping in the metal, but that it only starts once the velocity of the atom
exceeds the effective speed of sound in the metal. We argue that this condition
can be fulfilled most easily in metals with nearly empty or nearly filled
bands. We make quantitative predictions for the friction force to the second
and fourth order in the atomic polarizability, and show that the threshold
behavior persists to all orders of the perturbation theory.
