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 whilst at the same time
meeting the constraints from observations is highly challenging however. We
show that forming YMCs requires clouds with densities $\gtrsim$ 100 cm$^{-3}$
to collide with high velocities ($\gtrsim$ 20 km s$^{-1}$). We present the
first simulations which, starting from moderate cloud densities of $\sim100$
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.
