Recent observations and hydrodynamical simulations of star formation inside a giant molecular cloud have revealed that, within a star forming region, stars do not form evenly distributed throughout this region, but rather in small sub-clumps. It is generally believed that these sub-clumps merge and form a young star cluster. The time-scale of this merging process is crucial for the evolution and the possible survival of the final star cluster. The key issue is whether this merging process happens faster than the time needed to remove the residual gas of the cloud. A merging time-scale shorter than the gas-removal time would enhance the survival chances of the resulting star cluster. In this paper we show by means of numerical simulations that the time-scale of the merging is indeed very fast. Depending on the details of the initial sub-clump distribution, the merging may occur before the gas is expelled from the newly-formed cluster either via supernovae or the winds from massive stars. Our simulations further show that the resulting merger-objects have a higher effective star formation efficiency than the overall star forming region and confirm the results that mass-segregated sub-clumps form mass-segregated merger-objects.
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