The formation of young massive clusters by colliding flows

Young massive clusters (YMCs) are the most intense regions of star formation in galaxies. Formulating a model for YMC formation while at the same time meeting the constraints from observations is, however, highly challenging. We show that forming YMCs requires clouds with densities greater than or similar to 100 cm(-3) to collide with high velocities (greater than or similar to 20 km s(-1)). We present the first simulations which, starting from moderate cloud densities of similar to 100 cm(3), are able to convert a large amount of mass into stars over a time period of around 1 Myr, to produce dense massive clusters similar to those observed. Such conditions are commonplace in more extreme environments, where YMCs are common, but atypical for our Galaxy, where YMCs are rare.