Negative group velocity and Kelvin-like wake pattern

Wake patterns due to a uniformly traveling source are result of the resonant emission medium's collective excitations. When there exists frequency range where such excitations possess negative group velocity, their interference leads wake pattern resembling Kelvin ship wake: while "transverse" and "divergent" wavefronts trailing source, they oriented oppositely Kelvin's. This is illustrated by an explicit calculation "roton" in superfluid $^{4}He$ Kelvin-like emerges when speed marginally exceeds Landau critical roton velocity.