  The prominent interacting galaxy pair NGC 4038/9 contains many active
star-forming regions and is continuously forming new star clusters. We present
a self-consistent n-body model for this system which includes an SPH gas
component. The model qualitatively explains the apparent concentration of gas
in the so-called overlap region between the two nuclei as a bridge of gas
connecting the two galaxies. Projected on the sky, the bridge appears as a
dense spot of gas. We discuss some implications for the evolution of
Ultra-luminous infrared galaxies.
