  We investigate the effect of a constant external velocity field on the ground
state of a bosonic quasiparticle Hamiltonian. Below a critical velocity the
ground state is a quasiparticle vacuum, corresponding to a pure superfluid
phase at zero temperature. Beyond the critical velocity energy minimization
leads to a macroscopic condensation of quasiparticles at a nonzero wave vector
k_v parallel to the velocity v. Simultaneously, physical particles also undergo
a condensation at k_v and, to a smaller extent, at -k_v. Together with the BEC
at k=0, the three entangled condensates give rise to density modulations of
wave vectors k_v and 2k_v. For larger |v| our model predicts a bifurcation of
k_v with corresponding two pure condensates and no density modulation.
