  We analyze the peculiar infrared instability that characterizes stationary
inhomogeneous flows when their velocity crosses the sound speed by decreasing
values. For definiteness, we work in the context of one dimensional atomic Bose
condensates. These flows are unstable under ultra low real frequency
perturbations because of the unbounded mode amplification near the sonic
horizon. This results in a condensation of low frequency phonons which produces
a spatially structured flow in the supersonic domain. Numerical simulations
reveal that this zero-frequency undulation suppresses the instability when its
spatial extension is infinite, and when its phase is near that of a "shadow
soliton" solution attached to the sonic horizon. These phenomena are akin to
the condensation of rotons in flowing superfluid helium-4 when exceeding the
Landau velocity. They also pertain to shallow water waves propagating on
transcritical flows.
