Motional dressed states in a Bose condensate: Superfluidity at   supersonic speed
  We present an exact analytic solution of a nonlinear Schr\"odinger field interacting with a moving potential (obstacle) at supersonic speed. We show that the field forms a stable shape-invariant structure localized around the obstacle --- a dressing effect that protects the field against excitations by the obstacle. 
C. K. Law, C. M. Chan, P. T. Leung, and M.-C. Chu