ALMA survey of massive cluster progenitors from ATLASGAL. Limited
  fragmentation at the early evolutionary stage of massive clumps
  The early evolution of massive cluster progenitors is poorly understood. We
investigate the fragmentation properties from 0.3 pc to 0.06 pc scales of a
homogenous sample of infrared-quiet massive clumps within 4.5 kpc selected from
the ATLASGAL survey. Using the ALMA 7m array we detect compact dust continuum
emission towards all targets, and find that fragmentation, at these scales, is
limited. The mass distribution of the fragments uncovers a large fraction of
cores above 40 $M_\odot$, corresponding to massive dense cores (MDCs) with
masses up to ~400 $M_\odot$. 77 % of the clumps contain at most 3 MDCs per
clump, and we also reveal single clumps/MDCs. The most massive cores are formed
within the more massive clumps, and a high concentration of mass on small
scales reveals a high core formation efficiency. The mass of MDCs highly
exceeds the local thermal Jeans-mass, and observational evidence is lacking for
a sufficiently high level of turbulence or strong enough magnetic fields to
keep the most massive MDCs in equilibrium. If already collapsing, the observed
fragmentation properties with a high core formation efficiency are consistent
with the collapse setting in at parsec scales.
