  [abridged] How does a star cluster of more than few 10,000 solar masses form?
We present the case of the cluster NGC 346 in the Small Magellanic Cloud, and
its star-forming region N66, and we propose a scenario for its formation, based
on observations of the rich stellar populations in the region. Young massive
clusters (YMCs) host a high fraction of early-type stars, indicating an
extremely high star formation efficiency. The Magellanic Clouds host a wide
range of such clusters with the youngest being still embedded in their giant
HII regions. Hubble Space Telescope imaging of such star-forming complexes
allows the detailed study of star formation at scales typical for molecular
clouds. Our cluster analysis of newly-born stars in N66 shows that star
formation in the region proceeds in a clumpy hierarchical fashion, leading to
the formation of both a dominant YMC, hosting about half of the observed
pre--main-sequence population, and a dispersed self-similar distribution of the
remaining stars. We investigate the correlation between star formation rate
derived from star-counts and molecular gas surface density in order to unravel
the physical conditions that gave birth to NGC 346. We find a steep correlation
between these two parameters with a considerable scatter. The fraction of mass
in stars is found to be systematically higher within the central 15 pc (where
the YMC is located) than outside, which suggests variations in the star
formation efficiency within the same star-forming complex. This trend possibly
reflects a change of star formation efficiency in N66 between clustered and
non-clustered star formation. Our findings suggest that the formation of NGC
346 is the combined result of star formation regulated by turbulence and of
early dynamical evolution induced by the gravitational potential of the dense
interstellar medium.
