A note on the center of mass in non-commutative theories
  The dynamics of a stack of N D-branes is described by U(N) gauge theory of which the central U(1) describes the center of mass motion and the remaining SU(N) describes the internal dynamics. In the non-commutative situation, these two parts are coupled by the *-commutator interaction. We describe here how to identify the correct decoupled U(1) center of mass subsector of U(N) gauge theory for the case of the non-commutative torus. The internal dynamics remainder is not a theory of SU(N) valued fields but has a simple description in momentum space. 
Robert C. Helling