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.
Categories: astro-ph.GA astro-ph.SR