Hydrodynamic Simulations of M51 and the interaction with NGC 5195

The M51 galaxy is one of best examples of grand design structure in galaxies. The most likely origin of the spiral arms in M51 is the ongoing interaction with NGC 5195. Here we report recent calculations which model the orbit of M51 and NGC 5195. These calculations, which for the first time focus on the gas dynamics, are able to reproduce even detailed features in M51, as well as demonstrate the nature of the spiral structure of M51.