  The dispersion law for plasma oscillations in a two-dimensional electron gas
in the hydrodynamic approximation interpolates between $\Omega \propto
\sqrt{q}$ and $\Omega \propto q$ dependences as the wave vector $q$ increases.
As a result, downstream of a charged impurity in the presence of a uniform
supersonic electric current flow, a wake pattern of induced charge density and
potential is formed whose geometry is controlled by the Mach number $M$. For
$1<M\leqslant \sqrt{2}$ the wake consists of transverse wavefronts confined
within a sector whose angle is given by the classic Mach condition. An
additional wake of larger angle resembling the Kelvin ship wake and consistsing
of both transverse and diverging wavefronts is found outside the Mach sector
for $M>\sqrt{2}$. These wakes also trail an external charge traveling
supersonically a fixed distance away from the electron gas.
